The Field Guide to Electronic Components

A plain-language encyclopedia for hobbyists, makers, and anyone who wants to understand what the parts in an electronic circuit actually do. There's no math beyond multiplication and division, and no prior training is assumed.

The component chapters (4–31) follow the same pattern, so once you've read one you know where to look in the others. Each opens with what the part is and how it works, usually with an everyday analogy, then covers:

• Types: the family members you'll run into and how they differ.
• Reading it: markings, codes, and the specs that matter.
• Where it's used: the common circuits and jobs.

Every component chapter then ends with the same three sections:

• Did you know?: history, trivia, and surprising facts.
• Testing: how to check it with a multimeter (and other tools where a meter can't).
• How it fails: failure modes, their causes, and the symptoms they produce.

Chapters 1–3 (foundations) and 32–34 (practice) are organized by topic instead.

Contents

Part I — Foundations

Part II — Connections, switching and protection

Part III — Passive components

Part IV — Semiconductors

Part V — Power

Part VI — Things that move, sound and show

Part VII — Input and sensing

Part VIII — Chips and computers

Part IX — Practice

Appendices

Where to start

• New to electronics? Read chapters 1–3, then whichever component you're holding.
• Have a part and don't know what it is? Appendix C for its looks, then chapter 2 for part numbers and the "Reading it" section of the likely chapter.
• Need a pinout? Appendix D.
• Something's broken? Chapter 34, then the "How it fails" section of the suspect part.
• Just need a number? Appendix A.

1 — Electricity basics

Everything in this guide comes back to a handful of ideas: voltage, current, resistance, power, and the rule that current only flows around a complete loop. Get comfortable with these and every component chapter becomes a variation on a theme.

1.1 Charge, and what's actually moving

All matter is made of atoms, and atoms contain electrons, tiny particles with a negative electric charge. In metals like copper, some electrons aren't tied to any one atom. They drift freely through the metal like a crowd in a hallway. Electric current is those electrons being pushed along in a common direction.

• Charge is measured in coulombs (C). One coulomb is about 6.24 × 10¹⁸ electrons.
• The electrons themselves move surprisingly slowly, often less than a millimeter per second in a wire carrying a few amps. What travels at nearly the speed of light is the push. Every electron nudges its neighbor, the way water leaves the far end of a full hose the instant you open the tap.

💡 Conventional current vs electron flow. Early experimenters (Benjamin Franklin among them) guessed that current flows from + to −. Much later, we learned that electrons actually move from − to +. By then the convention was everywhere, so it stuck. Schematics, arrows on diode symbols, and nearly every textbook use conventional current (+ → −). It makes no practical difference as long as you're consistent.

1.2 The three quantities

The most common way to picture electricity is water in pipes:

Three rules follow directly:

• Voltage is always between two points. There's no such thing as "the voltage at this wire" by itself, only "the voltage between this wire and that one." When people quote a single voltage, they mean "relative to ground" (§1.9).
• Current flows through things. To measure it, you have to put the meter in the path of the flow.
• Resistance belongs to a path or part. A long thin wire has more than a short thick one. An insulator has so much that almost nothing flows.

Unit prefixes

Engineers often put the prefix where the decimal point would go: 4k7 = 4.7 kΩ, 2R2 = 2.2 Ω, 3V3 = 3.3 V. It avoids a tiny dot that can vanish on a printed page.

1.3 Ohm's law

The relationship between the three quantities is the most useful equation in hobby electronics:

V = I × R (voltage = current × resistance)

Rearranged:

• I = V ÷ R. More push, or less resistance, gives more current.
• R = V ÷ I. Resistance is how much voltage it takes to drive a given current.

Examples:

• 9 V across a 1 kΩ resistor: I = 9 ÷ 1000 = 9 mA.
• A lamp draws 0.5 A from 12 V: R = 12 ÷ 0.5 = 24 Ω (while it's hot and lit).
• 20 mA through 150 Ω: V = 0.02 × 150 = 3 V across the resistor.

Memory aid: write V on top of a triangle with I and R side by side underneath. Cover the quantity you want and the other two show you the formula.

Where Ohm's law doesn't apply directly: parts like diodes, LEDs, and transistors are non-linear, meaning their resistance changes with the voltage across them. Resistors, wires, and heating elements behave "ohmically." For the others, you use their datasheet curves (later chapters show how).

1.4 Power and energy

Power is the rate at which energy is used or delivered, measured in watts (W):

P = V × I

Combining with Ohm's law gives two more handy forms:

• P = I² × R: the heat in a resistor or wire. Double the current, quadruple the heat.
• P = V² ÷ R: the power in a resistor when you know the voltage across it.

Examples: a 12 V motor drawing 2 A uses 24 W. 100 mA through a 100 Ω resistor turns 0.1² × 100 = 1 W into heat.

Energy is power over time, the "amount in the tank":

• Joule (J) = 1 watt for 1 second.
• Watt-hour (Wh) = 1 watt for 1 hour = 3,600 J. Batteries are best compared in Wh.
• Kilowatt-hour (kWh) = 1,000 Wh, the unit on an electricity bill.
• Amp-hour (Ah / mAh) is a charge rating. Multiply by voltage to get energy: 3,000 mAh at 3.7 V = 3 Ah × 3.7 V ≈ 11 Wh.

💡 Almost all wasted electrical power ends up as heat. A part's power rating really says how much heat it can get rid of before it's damaged. That's why power ratings go down when parts are packed into a hot, enclosed box.

1.5 DC and AC

• DC (direct current) pushes in one direction at a steady voltage. Batteries, USB, solar cells, and the inside of nearly every electronic device run on DC.
• AC (alternating current) reverses direction many times a second. The voltage swings smoothly positive and negative in a sine wave. Wall outlets supply AC: 120 V at 60 Hz in North America, 230 V at 50 Hz in most of the rest of the world.

Why the grid uses AC: transformers (chapter 11) can step AC voltage up or down efficiently. Power stations send electricity long distances at hundreds of thousands of volts, where the current, and so the loss in the wires (I²R), is small. Then it's stepped down near your house.

Frequency and period

Frequency (hertz, Hz) is how many cycles happen per second. The period is the time for one cycle: period = 1 ÷ frequency. 60 Hz has a period of about 16.7 ms.

Peak vs RMS

An AC voltage is constantly changing, so which number do you quote? By convention, the RMS ("root mean square") value. That's the steady DC voltage that would heat a resistor by the same amount. For a sine wave, peak = RMS × 1.414. So "120 V" mains actually peaks at about 170 V, and "230 V" peaks at about 325 V. Meters labelled True RMS measure this correctly even for non-sine waveforms.

💡 The "War of the Currents" in the 1880s–90s pitted Thomas Edison's DC system against Nikola Tesla's and George Westinghouse's AC. AC won for long-distance distribution. Today, though, the very longest power links (undersea cables, links between countries) use high-voltage DC, because modern electronics can convert efficiently at the ends.

1.6 Pulsed DC and PWM

A lot of "DC" in electronics isn't steady:

• PWM (pulse-width modulation) switches a DC voltage fully on and fully off very quickly. The duty cycle (the percentage of time it's on) sets the average power. 25 % duty delivers about a quarter of full power. PWM dims LEDs, controls motor speed, positions hobby servos, and is at the heart of every switching power supply. A DC voltmeter on a PWM signal shows only the average.
• Digital signals are rapid on/off patterns that carry data. A multimeter can't show them. You need an oscilloscope or logic analyzer (chapter 3).
• Ripple is a small unwanted wobble on top of a DC supply, left over from rectifying AC or from a switching regulator.

1.7 Circuits: the loop rule

Current flows only around a complete, closed loop, out of the source's positive terminal, through the load, and back to the source's negative terminal. Break the loop anywhere, on either side, and all current stops.

The four states a path can be in:

The return path matters as much as the supply path. The ground or negative wire carries exactly the same current back as the positive wire carries out.

1.8 Series and parallel

Every connection between two parts is one or the other (or a combination).

Series: end to end, one path

• The same current flows through every part.
• The voltage divides among them, and the individual drops add up to the total.
• Resistances add: R_total = R1 + R2 + R3…
• Batteries in series: voltages add (two 1.5 V cells = 3 V), and capacity stays the same.
• A break anywhere stops everything. Old-style Christmas light strings went entirely dark when one bulb failed for exactly this reason.

Parallel: side by side, several paths

• The same voltage is across every part.
• The current divides, and the branch currents add up to the total.
• Combined resistance is less than the smallest branch: 1/R_total = 1/R1 + 1/R2 + … For two resistors: R1 × R2 ÷ (R1 + R2). Two equal resistors in parallel = half of one.
• Batteries in parallel: voltage stays the same, and capacity adds.
• Household outlets and lights are wired in parallel, which is why one bulb failing doesn't turn off the rest.

Kirchhoff's two laws (the formal version)

• Current law: the current flowing into any junction equals the current flowing out. Charge doesn't pile up or vanish.
• Voltage law: going around any closed loop, the voltage rises (sources) equal the voltage drops (loads). The push gets used up exactly.

These two statements, plus Ohm's law, are enough to analyze any circuit made of resistors and sources.

1.9 Ground and reference

Ground (GND, 0 V, common, −) is simply the point in a circuit that everyone agrees to call zero volts. Every other voltage is measured relative to it. "The 5 V rail" means "5 V above ground."

Kinds of "ground" you'll see:

The one rule to remember: two devices that exchange signals must share a ground. A signal is a voltage relative to ground. If the devices' grounds aren't connected, the receiver has no reference and the signal is meaningless.

1.10 Voltage drop

Any current flowing through any resistance loses voltage across it: V_drop = I × R. That includes things you might think of as "just wire":

• Long or thin wires
• Connectors, switch contacts, fuses
• Meter leads and test clips
• PCB traces

Example: 2 A through a cable with 0.5 Ω of total (out-and-back) resistance loses 1 V. On a 5 V circuit, that leaves only 4 V at the far end. Low-voltage, high-current circuits are the most sensitive, because a small loss is a large percentage of the supply. The remedies are thicker wire, shorter runs, fewer connectors, or sending a higher voltage and converting it down near the load.

1.11 Conductors, insulators and semiconductors

Semiconductors are the basis of diodes, transistors, LEDs, and all integrated circuits (chapter 13 explains how).

💡 Silver conducts slightly better than copper, but copper is the practical choice. It's nearly as good and far cheaper. Gold is worse than both. It's used on connector contacts because it never corrodes, not because it conducts better.

💡 Insulators have limits. Push enough voltage across air and it breaks down and conducts. That's a spark, or lightning. Dry air withstands roughly 3,000 V per millimeter.

1.12 Quick reference

2 — Reading schematics and datasheets

A schematic is a map of connections, not a picture of what a circuit looks like physically. A datasheet is the manufacturer's manual for a part. Together they tell you almost everything. This chapter covers how to read both, and how to decode part numbers and markings.

2.1 Schematic basics

• Lines are wires. Their length and route don't matter, only what they connect.
• A dot where lines meet = connected. Lines that cross without a dot are not connected. Older drawings sometimes show a little hump ("hop-over") where one wire crosses another.
• Good practice avoids four-way junctions, because it's too easy to miss a dot. Many schematics stagger junctions into two T's.
• Power and ground are usually drawn as symbols, not wires. Every ground symbol on a page is the same node, and so is every "+5V" flag. The wires between them are implied.
• Net labels: two wires ending in the same text label (e.g. SDA, MOTOR_EN) are connected, even on opposite sides of the page or on different sheets.
• Signals usually flow left to right, and voltage runs high at the top to low at the bottom. That's a convention that makes schematics easier to read, not a rule.

2.2 Common symbols (described)

2.3 Reference designators

Every part on a schematic and circuit board gets a letter-plus-number label. The letters are standard:

The number is just a label (R1, R2, R47…). It doesn't indicate value or importance.

2.4 Naming conventions you'll see on pins

• VCC / VDD / V+ / VIN: positive supply. (VCC is historically bipolar logic, and VDD is historically MOSFET/CMOS logic. Today they're used interchangeably.)
• GND / VSS / VEE / 0V / COM: ground or negative supply.
• Active-low signals do their job when pulled low. They're marked with an overbar, or a /, #, n, _N, or _B prefix or suffix: /RESET, CS#, nSLEEP, OE_N.
• NC means "no connection" on a pin table, but "normally closed" on a switch or relay.
• DNP / DNF ("do not populate/fit") means the footprint exists on the board but the part is deliberately left off.
• Values are often written with the prefix as the decimal point: 4k7, 2R2, 10n, 3V3.

2.5 Block diagrams, wiring diagrams and PCB views

• Block diagram: boxes for major functions (power supply, microcontroller, sensor) with arrows showing signal or power flow. It shows the idea, not the parts.
• Schematic: every component and connection.
• Wiring diagram / harness drawing: the physical wires between connectors, with colors and pin numbers.
• PCB layout / silkscreen: where each part physically sits on the board. The silkscreen (the printed white text) shows reference designators and outlines, and often the polarity marks.
• Pinout: a drawing of a part or connector with every pin labelled. Always check whether it's drawn from the top or the bottom (and for connectors, which side you're looking into). Getting that backwards is a classic way to mirror every pin.

2.6 Part numbers and markings

Part numbers look cryptic but follow patterns:

• A manufacturer prefix: LM (originally National Semiconductor), MC (Motorola/ON), SN (Texas Instruments), CD (RCA/TI CMOS), NE/SA (Signetics/Philips), AD (Analog Devices), MAX (Maxim), TL (TI), IRF (International Rectifier).
• The core number identifies the function. It's often shared across manufacturers, so an LM358, KA358, and TS358 are all the same dual op-amp.
• Suffixes give package, temperature grade, and packing: e.g. N or P = DIP, D = SOIC, -TR = tape and reel, /NOPB = lead-free.
• Transistor and diode families: 1Nxxxx (diodes, US JEDEC), 2Nxxxx (transistors, JEDEC), BC/BD/BF (European Pro Electron transistors), 2SC/2SA/2SD/2SB (Japanese JIS transistors, often printed without the "2S": "C1815" = 2SC1815).
• Tiny SMD parts often carry a 2–4 character marking code instead of the part number (e.g. "1AM" on an onsemi MMBT3904; Diodes Inc. marks the same part "K1N"). You look these up in SMD marking-code databases.
• Date codes like 2331 usually mean year 2023, week 31.

To recognize a part from its shape before you can read its marking, see Appendix C. Pinouts of common parts are in Appendix D.

2.7 How to read a datasheet

Datasheets can run to hundreds of pages, but most follow the same order. Read these sections first:

1. Front page / features: the headline specs (voltage range, current, speed, interface) and a one-paragraph description. Often enough to decide if a part fits.
2. Pin configuration and functions: which pin is which, and which signals are active-low.
3. Absolute maximum ratings: limits that must never be exceeded, even briefly. These are not operating conditions. A part run at its absolute maximum isn't "within spec"; it's on the edge of damage.
4. Recommended operating conditions: the range you should design within.
5. Electrical characteristics: the real behavior (input thresholds, output current, leakage, timing), each with min / typ / max and the test conditions. Design to min and max, not typical. "Typical" is what an average part does at 25 °C.
6. Typical characteristics (graphs): how the part behaves across temperature, voltage, and load. Useful for non-linear parts like diodes and MOSFETs.
7. Application information / typical application circuit: the manufacturer's own example circuit. Copy it as your starting point. Most "new chip doesn't work" problems come from a missing capacitor or resistor shown here.
8. Layout guidelines: where to put capacitors and how to route high-current paths.
9. Package / mechanical drawings: dimensions, pin 1 location, recommended PCB footprint.

Tips:

• Check the footnotes and test conditions. A MOSFET's "Rds(on) = 10 mΩ" may be at 10 V on the gate. At 3.3 V it could be many times worse.
• Look for errata (known silicon bugs) and application notes (longer how-to documents) for complex chips.
• Module listings aren't datasheets. A hobby-module listing may round, exaggerate, or misstate specs. Find the datasheet for the chip on the module.
• Beware "equivalent" parts. Two parts with the same core number from different makers usually match on pinout and function, but can differ in speed, noise, or limits.

3 — Tools and measurement

You can't see electricity, so every diagnosis depends on instruments. This chapter covers the multimeter in depth, the other tools worth knowing about, and the measurement habits that stop you from fooling yourself.

3.1 The multimeter

A digital multimeter (DMM) combines a voltmeter, ammeter, and ohmmeter, usually with extras. It's the one essential tool.

Terminals

Habit: after measuring current, move the red lead straight back to VΩ. Leaving it in a current jack and then touching a battery "to check voltage" is the most common way to blow a meter's fuse, or worse.

Functions

Key specs when buying

• Counts (e.g. 6,000 or 20,000): the display resolution. More counts = more digits.
• Auto-ranging: picks the scale for you. Manual-ranging meters make you choose.
• True RMS: reads non-sine AC correctly.
• CAT rating (CAT II / III / IV plus a voltage): how much surge energy the meter survives safely on mains circuits. Cheap meters without real CAT ratings shouldn't go near mains.
• Input impedance: usually 10 MΩ on DC volts, so it barely disturbs most circuits.
• Burden voltage: the voltage the meter's internal shunt drops in current mode (a few mV to hundreds of mV). It can starve a low-voltage circuit you're measuring.

3.2 Measuring well

Voltage (parallel)

Touch black to the reference (usually ground) and red to the point of interest. A negative reading just means the probes are reversed, which is itself useful information. Measure under real load. A weak battery or bad connection can read fine with nothing drawing current and collapse when something does.

Current (series)

1. Estimate the current. If unsure, start with the high-current jack.
2. Turn off the circuit, open the path (e.g. unplug the positive lead), and bridge the gap with the meter.
3. Power up, read, power down, remove the meter, restore the circuit.
4. Return the red lead to VΩ.

Never connect a meter in current mode across a voltage source. In current mode it's nearly a short circuit.

Resistance, continuity, diode, capacitance

These modes push a small test current from the meter itself, so the circuit must be unpowered (and capacitors discharged). Measuring in-circuit is often misleading, because other parts in parallel change the reading. Lift one leg of a part for a true value.

The voltage-drop test

To find a bad connection, measure the voltage across the connection (a switch, connector, crimp, or fuse) while current flows. A good joint shows a few millivolts. A bad one shows tenths of a volt or more. This finds faults continuity testing misses, because continuity uses a tiny current that a marginal contact can still pass.

Sanity-checking the meter

Before trusting a surprising reading: touch the probes together (≈ 0 Ω, beep), measure a known battery, and check the meter's own battery and fuses. Cracked leads and blown fuses cause most "my meter is wrong" moments.

3.3 Other test instruments

3.4 Hand tools for electronics

• Wire strippers (sized by AWG), flush cutters (flat side leaves a clean cut), needle-nose pliers, precision screwdrivers, tweezers (for SMD).
• Crimpers: each connector family needs the matching die. Ratcheting types won't release until the crimp is complete.
• Soldering iron and accessories: see chapter 32.
• Heat gun for heat-shrink tubing.
• Helping hands / PCB holder, magnifier or microscope.
• ESD wrist strap and mat for handling static-sensitive parts.
• Label maker or tags: unlabelled wires are a future troubleshooting session.

3.5 Measurement habits

1. Know what you expect before you measure. "This should be 3.3 V." A reading only means something against an expectation.
2. Check the mode, range, and unit on the display every time. mV vs V and kΩ vs MΩ are the classic misreads.
3. Measure at the part, not just at the supply. Voltage lost on the way is often the fault.
4. Measure under load and during the problem. Use MIN/MAX capture for dips too fast to see.
5. Keep one hand clear on anything above about 50 V, so current can't flow across your chest.
6. Write readings down. Comparisons across time or between a good and a bad unit are the fastest diagnostic you have.

4 — Wire and cable

Wire is the most overlooked component in a circuit. It has resistance, a current limit, a voltage rating, and a temperature rating, and it fails in predictable ways. Choosing the right wire is as much a design decision as choosing a resistor.

4.1 What a wire is

A conductor (almost always copper) inside insulation. A cable is several insulated conductors in a common outer jacket, sometimes with a shield.

Conductor materials

4.2 Wire size: gauge

AWG (American Wire Gauge)

AWG numbers run backwards: a smaller number is thicker wire. Every 3 gauge steps roughly doubles (or halves) the copper cross-section, and every 6 steps doubles the diameter.

* Rough figures for short runs in open air. Bundled, enclosed, or hot wiring must be derated (run at less current). Building wiring follows electrical codes, not this table.

Metric sizes

Outside North America, wire is sized by cross-sectional area in mm². 0.5, 0.75, 1.0, 1.5 and 2.5 mm² are common. 1.5 mm² is roughly 16 AWG, and 2.5 mm² roughly 14 AWG.

Choosing a size

Wire size is limited by two things:

1. Heat: too much current and the insulation softens or melts.
2. Voltage drop: at low voltages, this usually bites first. V_drop = I × (Ω per meter) × length × 2 (×2 for out and back). For example, 3 A over 3 m of 22 AWG loses 3 × 0.053 × 6 ≈ 0.95 V. That's almost 20 % of a 5 V supply.

A good target for low-voltage DC is to lose under 3–5 % of the supply voltage in the wiring.

4.3 Solid vs stranded

Strand count is written like "7/0.2" (7 strands of 0.2 mm) or "19/32" (19 strands of 32 AWG). More, finer strands = more flexible. Very fine, highly flexible wire is often labelled "silicone" or "test lead" wire.

💡 The skin effect: at high frequencies, current crowds toward the surface of a conductor. Litz wire, made of many individually insulated fine strands woven together, fights this. It's used in induction cookers and wireless chargers.

4.4 Insulation

Insulation also has a voltage rating (commonly 300 V or 600 V for hookup wire). Thin signal wire may be rated for very little. Don't use it where mains-level voltages could appear.

4.5 Types of wire and cable

4.6 Color conventions

Colors are conventions, not guarantees. Always verify with a meter.

Low-voltage DC

• Red = positive, black = negative/ground is near-universal.
• Some kits use other colors for positive (orange, white) or even red for data. Check.

Mains wiring

⚠️ In North America, black = hot on mains, while on DC, black = ground. Never assume.

Ethernet (T568B)

White-orange, orange, white-green, blue, white-blue, green, white-brown, brown (pins 1–8). T568A swaps the orange and green pairs.

Hobby servos

Brown/black = ground, red = power, orange/yellow/white = signal.

4.7 Terminating and joining wire

• Strip cleanly: don't nick the copper. A nicked solid wire breaks at the nick.
• Crimping: a mechanical cold-weld. It's the professional choice for anything that vibrates (see chapter 5).
• Soldering: great for PCBs and fixed joints. The solder wicks up stranded wire and creates a stiff spot where it can fatigue and break, so add strain relief.
• Twist-on connectors ("wire nuts"), lever connectors (e.g. Wago), butt splices: common in building and automotive work.
• Ferrules: metal sleeves crimped onto stranded wire so it seats cleanly in screw terminals.
• Heat-shrink tubing: insulates and strain-relieves joints. It usually shrinks to half (2:1) or a third (3:1) of its diameter. Adhesive-lined types seal against water.
• Strain relief: secure cables before the connection (cable ties, clamps, grommets, boots) so flexing never loads the joint itself.

4.8 Did you know?

• AWG was defined in 1857 and is based on the number of drawing dies a wire passed through. More dies make thinner wire, which is why the numbers run backwards.
• Copper's resistance rises about 0.4 % per °C. A hot wire resists more, gets hotter, and resists more still.
• "Oxygen-free" and exotic audio cables make no measurable difference at audio frequencies over home distances. Gauge and good connections do.
• PCB traces are wires too. A standard 1 oz copper trace 1 mm wide can carry roughly 2–3 A with modest heating.

4.9 Testing wire and cable

4.10 How wire fails

5 — Connectors

A connector lets you join and separate circuits without soldering. Every connector is a pair of metal contacts held together by spring pressure. That makes each one a small resistance and a potential failure point. Connectors are where a large share of real-world faults live.

5.1 How a connector works

• Contacts: usually a pin (male) and a socket or receptacle (female). The socket's spring grips the pin.
• Housing: the plastic body that holds contacts in position, polarizes the connector (so it only fits one way), and often latches it in place.
• Plating: gold (doesn't corrode, best for low-voltage signals), tin (cheap, fine for power and moderate use), silver (high current). Don't mate gold with tin long-term: dissimilar metals corrode ("fretting").
• Termination: how wire attaches: crimp, solder, IDC (insulation displacement), screw, spring clamp, or press-fit into a PCB.

Key specs

Gender and naming

"Male" = pins, "female" = sockets. Some families call the free (cable) half the plug and the fixed (panel/board) half the jack or receptacle. Be careful: some families put pins in the plug, others sockets in the plug.

5.2 Board and wire-to-board connectors

⚠️ JST is a manufacturer, not a connector. "JST connector" can mean any of a dozen incompatible families. Always identify by pitch and series.

5.3 Terminals and wire joins

5.4 Power connectors

5.5 Signal, data and audio connectors

5.6 Waterproof and outdoor connectors

• IP ratings: the first digit is dust (6 = dust-tight), the second is water (4 = splashes, 5 = jets, 7 = immersion to 1 m, 8 = beyond 1 m).
• Common types: circular screw-lock (M12, GX-series), automotive sealed (Deutsch, Superseal, Weatherpack), and LED-strip waterproof 2/3/4-pin inline connectors.
• Seals only work when the wire diameter matches the seal's range and the connector is fully mated.
• Cable glands seal a cable where it passes through an enclosure wall. They're sized by thread (PG7, PG9, M12, M16…) and the cable diameter they grip.

5.7 Crimping, the right way

1. Match the contact to the wire gauge, and the crimper die to the contact family.
2. Strip the right length. The copper should fill the wire barrel with a hair protruding. The insulation should sit under the insulation barrel.
3. Open-barrel contacts have two crimps: the conductor crimp (on bare copper) and the insulation crimp (gripping insulation, for strain relief).
4. Crimp with a proper tool. Pliers make loose, unreliable crimps.
5. Tug-test every crimp. It shouldn't pull out.
6. Insert the contact into the housing until it clicks. The small locking tang should hold it. A contact that pushes back out when mated is a classic intermittent fault.

A good crimp is a gas-tight cold weld and is more reliable than solder in a vibrating environment. Don't solder a crimped contact. The solder wicks up and makes a stiff, fatigue-prone spot.

Removing a contact from a housing: press down the locking tang (through the small window in the housing) with a pin or extraction tool, then pull the wire gently.

5.8 Did you know?

• The 2.54 mm (0.1") pitch dates from the earliest integrated circuits in the 1960s, and still defines breadboards and hobby modules today.
• USB-C plugs have a symmetrical pin layout, so the connector works both ways. The device uses the CC pins to work out the plug's orientation.
• Gold plating on connectors is typically under 1 micron thick. It's there to prevent oxide films, which matter most for tiny signal currents.
• Many "waterproof" failures come from water wicking inside the cable jacket, not through the connector.
• The humble RCA connector was introduced by RCA in the 1940s for phonographs, hence its other name, "phono plug."

5.9 Testing connectors

5.10 How connectors fail

6 — Switches and relays

A switch opens or closes a circuit by moving metal contacts together or apart. A relay is a switch operated by an electromagnet instead of a finger. They're the simplest components in electronics, yet ratings, contact bounce, and arcing make them more interesting than they first appear.

6.1 Poles, throws and states

Momentary switches return when released. Latching (maintained) switches stay put.

6.2 Types of switch

6.3 Ratings

Every switch has voltage and current ratings, often separate for AC and DC:

• DC is harder to switch than AC. When contacts open, an arc forms. AC crosses zero volts 100–120 times a second, which snuffs the arc. DC doesn't, so the arc can sustain itself and burn the contacts. A switch rated "10 A 250 V AC" may be good for only a few amps at 30 V DC. Check the DC rating.
• Inductive and motor loads (and incandescent lamps, whose cold filaments draw several times their running current) are harder on contacts than resistive loads. Ratings may list "resistive" and "inductive" separately.
• Tactile and signal switches are rated in milliamps (~50 mA at 12 V is typical). Use them to signal a microcontroller, not to switch power.
• Minimum current (wetting current): big power contacts can oxidize and fail to conduct reliably when switching tiny signal currents. Gold-plated contacts are best for low-level signals.

6.4 Contact bounce

When metal contacts close, they bounce several times over roughly 1–20 ms before settling. A person sees one press, but a fast microcontroller may see several.

• Software debouncing: ignore further changes for 10–50 ms after the first change, or require the new state to hold that long.
• Hardware debouncing: a resistor–capacitor filter (e.g. 10 kΩ + 100 nF), often followed by a Schmitt-trigger input (chapter 27) to sharpen the edge.
• SR latch debouncing: a changeover (SPDT) switch feeding a set-reset latch gives perfectly clean output. It's the classic textbook method.

Chapter 25 covers wiring buttons to microcontrollers.

6.5 Relays

How a relay works

A coil wound around an iron core becomes an electromagnet when current flows. It pulls a hinged iron armature, which moves one or more switch contacts. When the coil is switched off, a spring returns the armature. The coil and the contacts are electrically isolated, so a small, safe signal can control a large or dangerous circuit.

Reading a relay

Terminal labels: COM (common), NO (normally open), NC (normally closed), and two coil terminals (sometimes marked + and − if the relay has a built-in diode or LED).

Driving a relay from a microcontroller

A relay coil draws too much current for a logic pin, and it's an inductor, so switching it off produces a high-voltage spike (chapter 11). The standard driver:

• An NPN transistor or N-channel MOSFET switches the coil's low side (chapter 16/17).
• A flyback diode across the coil (cathode to the positive side) absorbs the switch-off spike. Never omit it. The spike can destroy the transistor.
• Ready-made relay modules include the transistor, diode, an indicator LED, and often an optocoupler. Many modules are active-low: the relay turns on when the input is pulled low.

Types of relay

6.6 Did you know?

• Relays powered early computers. The Harvard Mark I (1944) used thousands of them, and the famous "first computer bug" was a moth found stuck in a relay of the Harvard Mark II in 1947.
• Telephone exchanges routed calls with vast banks of relays and stepping switches well into the 1980s.
• The satisfying click of a mechanical keyboard comes from a small switch in every key. Different colored "switches" have different spring weights and tactile bumps.
• Mercury tilt switches were once common in thermostats. They're largely banned now because mercury is toxic.
• Reed switches can last billions of operations, because their contacts are sealed in inert gas.

6.7 Testing switches and relays

Switches (power off):

• Continuity across the contacts: open = no beep, actuated = beep. Test every position and pole.
• For a 4-pin tactile switch, find which legs are permanently joined (they beep without pressing). Those pairs form the two sides of the switch.
• Wiggle or tap while holding continuity. Crackle means worn or dirty contacts.
• Under load: measure voltage across the closed switch. It should be near zero (tens of mV at most). A few tenths of a volt means burned contacts.

Relays:

• Coil: Ω across the coil terminals. It should match the datasheet (tens to hundreds of Ω). OL = open coil.
• Contacts, unpowered: COM–NC beeps, COM–NO open.
• Energize the coil at its rated voltage (from a bench supply): you should hear a click, and now COM–NO beeps and COM–NC opens.
• Contact resistance under load: voltage across the closed contacts with current flowing.

6.8 How they fail

To reduce contact wear on inductive loads, add a snubber (a resistor + capacitor in series across the contacts) for AC loads, or a flyback diode across the load for DC loads.

7 — Fuses and circuit protection

Protection components sit quietly doing nothing until something goes wrong: a short, a surge, a reversed battery, a static spark. Then they sacrifice themselves, or divert the energy, so that wiring, equipment, and people aren't harmed.

The single most important idea: a fuse protects the wiring (and prevents fire). It doesn't make a faulty device work, and it isn't sized to the device's needs.

7.1 Fuses

How a fuse works

A fuse contains a thin metal element that heats as current passes (heat = I² × R). At its rated current it stays cool enough. At a significant overload it melts ("blows") and opens the circuit. The time it takes depends on how large the overload is: a dead short blows it in milliseconds, and a mild 50 % overload may take minutes or hours.

Fuse ratings

Reading a glass fuse cap: T3.15AL250V = time-lag, 3.15 A, low breaking capacity, 250 V.

Types of fuse

Sizing a fuse

1. Find the wire's safe current (chapter 4). The fuse must blow below this.
2. Find the circuit's normal maximum current and any inrush. The fuse must hold above this.
3. Pick a rating between the two, choose slow-blow if there's inrush, and check the voltage and breaking capacity.
4. Place the fuse as close to the power source as possible, on the positive (or live) side. Everything upstream of the fuse is unprotected.
5. When one source feeds several wires of different sizes, fuse each branch for its own wire.

💡 A large power supply's own short-circuit protection doesn't protect thin wiring. A 20 A supply can happily push 15 A through a thin wire, and that's well within the supply's rating, while the wire overheats. Branch fuses solve this.

7.2 Resettable fuses (PTC / polyfuse)

A polymeric PTC is a small component (often a yellow disc or an SMD chip) containing a polymer loaded with carbon. At normal current it conducts. When overloaded it heats, the polymer expands, the carbon particles separate, and resistance jumps to a high value, limiting current to a trickle. Remove the fault and let it cool, and it resets.

• Specs: hold current (always OK), trip current (will trip), max voltage.
• It doesn't fully open. A small leakage current keeps it warm and tripped.
• It resets slowly (seconds to minutes) and its resistance creeps up after each trip.
• Common on USB ports, dev boards, battery packs, and speaker protection.

7.3 Circuit breakers and mains protection

💡 A breaker or fuse protects wiring from fire. A GFCI/RCD protects people from shock. A person can be fatally shocked by a current far too small to blow a fuse.

7.4 Overvoltage and surge protection

These are often layered: a GDT for the huge energy, a MOV for the mid-range, and a TVS diode for the fast final clamp.

7.5 Reverse-polarity protection

Connecting a battery backwards destroys many circuits in an instant. Options:

7.6 Inrush current limiting

When a device with large input capacitors is switched on, they briefly act like a short circuit, drawing a big current spike that can blow fuses, weld switch contacts, or trip breakers.

• NTC thermistor: high resistance when cold (limits the surge), heats up and drops to low resistance in operation. Common in power supplies. It needs cooling-off time between power cycles to work again.
• Soft-start circuits and hot-swap controllers: ramp the voltage up gradually.
• Pre-charge resistors: used in big battery systems, with a relay to bypass the resistor once the capacitors are charged.

7.7 Electronic protection (eFuses and load switches)

Modern boards often use ICs instead of fuses:

• eFuse: a MOSFET plus a controller that limits current, cuts off on over-current, over-voltage, or over-temperature, and resets electronically.
• Load switch: turns power to a subsystem on and off, often with current limiting and a controlled slew rate.
• Battery protection ICs: disconnect a lithium cell on over-charge, over-discharge, or short (chapter 19).
• Thermal shutdown: most regulators and motor drivers turn themselves off when overheating.

7.8 Did you know?

• Thomas Edison patented a fuse in 1890, but crude fusible links protected telegraph lines before that.
• Some fuses are filled with sand (silica). When the element vaporizes, the sand quenches the arc and absorbs energy, giving a much higher breaking capacity.
• A car's fuse box can hold 40+ fuses, because every circuit's wiring is sized differently.
• Static electricity from walking across carpet can reach 10,000–25,000 V. It's harmless to you, but enough to damage sensitive chips, even when you don't feel the spark (below about 3,000 V).

7.9 Testing protection components

7.10 How protection fails (and what it tells you)

8 — Resistors

The resistor is the most common component in electronics. It resists current by a fixed, known amount, and that simple behavior does a remarkable number of jobs: limiting current, setting voltages, holding signals steady, measuring current, and setting the behavior of chips.

8.1 How it works

Resistance comes from electrons bumping into the atoms of the material they flow through. Every collision turns a little of their energy into heat. A resistor is a carefully controlled amount of a material that resists flow more than copper does.

A conductor's resistance depends on three things:

• The material (its resistivity): carbon and metal alloys resist more than copper.
• Length: longer means more resistance.
• Cross-section: thinner means more resistance.

That's why resistors are made from thin films, fine wire, or carbon mixtures. It's also why wires themselves have (small) resistance.

A resistor obeys Ohm's law exactly: V = I × R. It has no polarity and works the same both ways round, with AC or DC. Whatever power it absorbs, P = I² × R, becomes heat.

8.2 Types of resistor

8.3 Reading resistor values

Color code (through-hole)

Mnemonic for 0–9: Bad Beer Rots Our Young Guts But Vodka Goes Well.

• 4 bands: digit, digit, multiplier, tolerance. Yellow–violet–red–gold = 47 × 100 = 4.7 kΩ ±5 %.
• 5 bands (precision): digit, digit, digit, multiplier, tolerance. Brown–black–black–red–brown = 100 × 100 = 10 kΩ ±1 %.
• 6 bands: as 5 bands, plus a temperature coefficient band.
• Which end to start from? The tolerance band is often spaced slightly apart, and gold/silver never start a value. When in doubt, measure it. Reds, oranges, and browns are hard to tell apart under poor light.

SMD codes

SMD sizes

Named by length × width in hundredths of an inch (imperial): 0201, 0402, 0603, 0805, 1206, 2512. 0805 (2.0 × 1.25 mm) and 1206 are hand-solderable. 0402 and smaller need a steady hand and magnification. Bigger packages handle more power (0603 ≈ 0.1 W, 1206 ≈ 0.25 W, 2512 ≈ 1 W).

8.4 The specs that matter

Standard values (E-series)

Resistors are made in logarithmically spaced "preferred values," so every value is within tolerance of its neighbors:

• E6 (±20 %): 10, 15, 22, 33, 47, 68
• E12 (±10 %): 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82
• E24 (±5 %): adds 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91
• E96 (±1 %): 96 values per decade (100, 102, 105, 107, 110…)

Each is multiplied by powers of 10: 4.7 Ω, 47 Ω, 470 Ω, 4.7 kΩ, 47 kΩ, 470 kΩ, 4.7 MΩ.

8.5 Combining resistors

• Series: R = R1 + R2 + … Use this to make a value you don't have (e.g. 220 + 100 = 320 Ω) or to split power and voltage across several parts.
• Parallel: 1/R = 1/R1 + 1/R2 + … For two: R1 × R2 ÷ (R1 + R2). Two equal resistors = half the value, double the power rating.
• Ten 1 kΩ, ¼ W resistors in parallel make a 100 Ω, 2.5 W resistor.

8.6 What resistors are used for

Current limiting

Protects parts that would otherwise draw too much current, most famously LEDs: R = (V_supply − V_LED) ÷ I_LED (chapter 14). Also used on transistor bases (chapter 16), and in series with inputs to limit fault currents.

Voltage divider

Two resistors in series across a voltage produce a fraction of it at their junction:

V_out = V_in × R2 ÷ (R1 + R2) (R2 is the one to ground)

• Used to scale a high voltage down to what an analog input can measure (e.g. 12 V → 3 V with 30 kΩ over 10 kΩ).
• Used to set the output of adjustable regulators (chapter 20) and to bias transistors.
• A divider is not a power supply. Drawing current from the midpoint drags its voltage down. The load must draw much less current than flows through the divider (a factor of 10 or more), or use a regulator instead.
• Higher resistor values waste less current but become sensitive to the input they feed. A small capacitor (e.g. 100 nF) from the midpoint to ground steadies readings.

Pull-up and pull-down resistors

A digital input connected to nothing floats: it picks up stray charge and reads at random. A pull-up resistor ties it gently to the supply (reads HIGH by default), and a pull-down ties it to ground (reads LOW by default). A switch or open-drain output can then overpower the resistor to change the state.

• Typical values: 4.7 kΩ–47 kΩ for switches, 1–10 kΩ for buses like I²C (chapter 30).
• Smaller values give faster, more noise-immune signals but more current. Larger values give lower current but slower, more fragile signals.
• Many microcontrollers have built-in pull-ups (typically 20–50 kΩ) that can be turned on in software.

Current sensing (shunts)

A small, known resistance in the current path develops a voltage proportional to the current: I = V ÷ R. A 0.1 Ω shunt carrying 2 A drops 0.2 V. Current-sense amplifier chips (e.g. INA219, INA226) read such shunts precisely. Keep shunts small so they don't waste voltage or power. Place them in the ground ("low side") or supply ("high side") path.

Setting chip behavior

Many ICs read a resistor to set a value: a charger's charge current, an LED driver's output current, a regulator's output voltage, a timer's period, or the USB-C "I'm a device" signal (5.1 kΩ on the CC pins).

Termination and damping

Small series resistors (22–470 Ω) on fast signal lines absorb reflections and ringing. Long transmission lines are terminated with a resistor matching the cable impedance (e.g. 120 Ω across the ends of an RS-485 or CAN bus).

Bleeder resistors

A high-value resistor across a large capacitor slowly discharges it after power-off, for safety.

Heaters and loads

Resistors are deliberately used as heaters (in 3D-printer beds, defrosters, incubators) and as dummy loads for testing power supplies.

8.7 Did you know?

• The ohm is named after Georg Ohm, who published his law in 1827. It was initially ridiculed by his peers.
• The color code dates to the 1920s, created by the Radio Manufacturers Association so that values could be read regardless of how the part was mounted.
• Resistors generate a tiny noise voltage just from being warm (thermal or Johnson–Nyquist noise). It sets a fundamental limit on how quietly any circuit can amplify.
• The "zero-ohm resistor" isn't quite zero: it's typically under 50 mΩ, with a current rating.
• Standard resistor values come from the "Renard series", a geometric spacing proposed in the 1870s for balloon ropes, later adopted for many preferred-number systems.

8.8 Testing resistors

1. Power off, and discharge any capacitors in the circuit.
2. Multimeter on Ω. Probe across the resistor.
3. Read the value and the unit (Ω, kΩ, MΩ).
4. Compare with the marking, allowing for tolerance.

Tips:

• In-circuit readings are usually low, because other parts are in parallel. Lift one leg for an exact reading.
• Don't hold both probe tips with your fingers above ~100 kΩ. Your body resistance in parallel lowers the reading.
• Very low values (below ~1 Ω) are dominated by the probe lead resistance. Short the probes first and subtract that reading (or use the meter's "relative" mode).
• Quick in-circuit check under power: voltage across the resistor ÷ its value = current through it.

8.9 How resistors fail

A burned resistor is almost always a symptom. Replacing it without finding the shorted transistor, capacitor, or diode that overloaded it usually burns the new one too.

9 — Potentiometers and resistive sensors

Some resistors are meant to change. Potentiometers change when you turn a knob. Thermistors, photoresistors, force sensors, and strain gauges change with temperature, light, pressure, or bending. They're among the simplest ways to connect the physical world to a circuit.

9.1 Potentiometers

How a pot works

A potentiometer ("pot") is a resistive track with a terminal at each end and a third terminal, the wiper, that slides along the track. Turning the shaft moves the wiper.

• As a voltage divider (three terminals): connect the ends to supply and ground, and the wiper voltage varies smoothly from 0 to the full supply. This is how most volume and control knobs work.
• As a variable resistor (rheostat, two terminals): use one end and the wiper. The resistance between them changes as you turn. Tie the unused end to the wiper so an open-circuit wiper (from wear) fails safe to full resistance.

Pins are usually numbered 1, 2 (wiper, middle), 3. Swapping 1 and 3 reverses the direction of rotation.

Types of potentiometer

Taper

• Linear (B, or "LIN"): resistance changes evenly with rotation. Used for general controls.
• Logarithmic / audio (A, or "LOG"): changes slowly at first, then quickly. It matches how our ears perceive loudness, so volume controls feel even.
• Reverse log (C): the opposite, less common.
• Markings like B10K = linear 10 kΩ, A50K = audio 50 kΩ. (Some Japanese and older parts swap the A/B letters. Measure when it matters.)

9.2 Thermistors (temperature-dependent resistors)

A thermistor's resistance changes strongly with temperature.

• NTC thermistors are specified by R25 (resistance at 25 °C) and a B (beta) value (e.g. 3950) describing the curve.
• The response isn't linear. Microcontrollers convert the reading using the beta equation or the more accurate Steinhart–Hart equation, or a lookup table.
• Typical circuit: a voltage divider with a fixed resistor equal to R25, read by an ADC.

Related temperature-sensing resistors:

• RTD (resistance temperature detector): usually platinum (Pt100 = 100 Ω at 0 °C, Pt1000). Very accurate and linear-ish, but a small change per degree (~0.39 Ω/°C for Pt100). Needs a special amplifier or converter chip.
• Silicon temperature-sense resistors (KTY series): a middle ground.

9.3 Photoresistors (LDRs)

A light-dependent resistor is made of cadmium sulfide (CdS). Its resistance drops as light increases, from mega-ohms in darkness to a few hundred ohms or kilo-ohms in bright light.

• Slow: tens of milliseconds to respond, and even slower to recover from bright to dark.
• Imprecise: great for "is it dark?" (automatic night lights, street lamps), poor for measuring light levels.
• Being phased out in many regions because of cadmium (RoHS restrictions). Phototransistors, photodiodes, and digital light sensors replace them (chapters 14, 26).
• Classic circuit: an LDR plus a fixed resistor as a voltage divider, read by an ADC or a comparator.

9.4 Force, flex and strain sensors

The Wheatstone bridge

Four resistors in a diamond, fed with voltage across two opposite corners. The voltage between the other two corners is zero when the bridge is balanced. It swings positive or negative as one resistor (the sensor) changes. The bridge turns tiny resistance changes into a small voltage that an amplifier can boost. It's the basis of load cells, many pressure sensors, and precision measurement generally.

9.5 Other resistive sensors

• Soil moisture (resistive probes): two exposed conductors. Wetter soil conducts better. The probes corrode quickly when powered continuously, so power them only during a reading, or use capacitive sensors instead.
• Water / leak detection: the same principle as soil moisture.
• Gas sensors (MQ series): a heated tin-dioxide element whose resistance changes in the presence of certain gases (chapter 26).
• Hot-wire anemometers: airflow cools a heated wire, changing its resistance.
• Magnetoresistors: resistance changes with magnetic field. Used in compasses and old hard-disk read heads.

9.6 Did you know?

• The word "potentiometer" originally described a 19th-century instrument for measuring voltage ("potential") by balancing it against a known voltage along a wire.
• A joystick is usually two pots at right angles, one per axis.
• "Scratchy" volume knobs are caused by dirt and wear on the track. Contact cleaner sprayed into the pot and a few turns often fixes it temporarily.
• Michael Faraday noticed the first thermistor effect in 1833: silver sulfide conducted better as it warmed, the opposite of metals.
• The bonded strain gauge was invented twice in 1938, independently by Edward Simmons at Caltech and Arthur Ruge at MIT.
• A car's fuel gauge has traditionally been read by a float on an arm that sweeps a wiper along a resistor inside the tank.

9.7 Testing

9.8 How they fail

10 — Capacitors

A capacitor stores electrical energy in an electric field, and gives it back very quickly. It's a tiny, fast-acting reservoir of charge. Capacitors smooth power supplies, filter noise, set timing, pass audio signals while blocking DC, and supply sudden bursts of current. After resistors, they're the most common component.

10.1 How it works

A capacitor is two conductive plates separated by an insulator called the dielectric. Current can't flow through the dielectric. But when you apply a voltage, electrons pile up on one plate and are pushed off the other. The plates become charged, and energy is stored in the field between them.

Water analogy: a pipe blocked by a stretchy rubber membrane. Push water in and the membrane stretches, storing the push. No water passes through, but water moves on both sides while it stretches. Release the pressure and the membrane pushes the water back.

Consequences:

1. A capacitor resists sudden changes in voltage. It has to charge or discharge first, which smooths out dips and spikes.
2. It blocks steady DC once charged, but passes changes (AC, pulses, noise).
3. It can deliver a burst of current faster than a distant power supply can.

Capacitance

Measured in farads (F). One farad is enormous, so practical values are:

• pF (picofarads, 10⁻¹²): tiny RF and timing caps
• nF (nanofarads, 10⁻⁹): filtering and decoupling
• µF (microfarads, 10⁻⁶): power supply and audio
• F (farads): supercapacitors

1 µF = 1,000 nF = 1,000,000 pF.

Capacitance increases with larger plate area, thinner dielectric, and a dielectric with a higher permittivity. That's why high-value capacitors roll up long foils, or stack hundreds of thin ceramic layers.

Charging and the RC time constant

Charged through a resistor, a capacitor's voltage rises quickly at first and then more slowly, following a curve. The time constant τ = R × C (seconds, with R in ohms and C in farads):

• After 1 τ: ~63 % charged
• After 3 τ: ~95 %
• After 5 τ: ~99 % (considered full)

Example: 10 kΩ × 100 µF = 1 second. Discharge follows the mirror-image curve.

Energy stored

E = ½ × C × V² (joules). A 1,000 µF capacitor at 50 V stores 1.25 J, enough to make a startling spark. A 1 F supercap at 5 V stores 12.5 J.

Capacitors and AC (reactance)

A capacitor "resists" AC less as frequency rises. Its reactance is X_C = 1 ÷ (2π × f × C). A 100 nF cap is about 1.6 kΩ at 1 kHz, but only 1.6 Ω at 1 MHz. That's why small ceramic caps are so good at shunting high-frequency noise to ground.

10.2 Types of capacitor

Ceramic dielectric classes

💡 DC-bias derating: a small 10 µF X5R capacitor rated 6.3 V might provide only 3–4 µF when it's actually sitting at 5 V. For critical values, choose a larger package and a higher voltage rating, or check the maker's DC-bias curves.

10.3 Reading capacitors

Ceramic codes (value in pF)

Two significant digits, then the number of zeros:

A letter after the code is the tolerance: J = ±5 %, K = ±10 %, M = ±20 %, Z = +80/−20 %. So 104K = 100 nF ±10 %.

Electrolytics

• Value and voltage printed on the can: 470µF 25V, plus a temperature rating (85 °C or 105 °C).
• The stripe (usually with minus signs) marks the negative lead. On new parts, the longer lead is positive.
• SMD aluminum "cans" have a black-painted segment on top marking negative.

Tantalum

• The stripe or bar marks the POSITIVE end, the opposite of aluminum electrolytics. A frequent source of mistakes.
• Often a code like 107 (100 µF) and a voltage letter or number.

SMD ceramics

Usually unmarked. Measure them, or keep them in labelled reels.

10.4 The specs that matter

10.5 Combining capacitors

Capacitors combine the opposite way to resistors:

• Parallel: capacitances add (C = C1 + C2). The common way to get more capacitance, or to combine a big slow cap with a small fast one.
• Series: 1/C = 1/C1 + 1/C2. Total is less than the smallest. Series raises the total voltage rating (with balancing resistors for electrolytics).

10.6 What capacitors are used for

Decoupling (bypass) capacitors

Digital chips draw current in tiny, sharp spikes every time their transistors switch. The supply wiring can't respond fast enough, so the local voltage wobbles. A 100 nF ceramic placed right at the chip's power pins supplies those spikes locally. Every IC needs one, as close to its pins as possible. Many designs add a 1–10 µF cap nearby too.

Bulk capacitance

A larger electrolytic (10–1,000 µF+) near loads that draw sudden big currents (motors, LED strips, radio transmitters, amplifiers) covers the dip until the supply catches up. It prevents brownout resets and glitches.

Smoothing and filtering

• After a rectifier, a large capacitor fills in the gaps between AC peaks, turning bumpy DC into smooth DC. More capacitance (or higher frequency) means less ripple.
• Low-pass RC filter: a resistor then a capacitor to ground. Passes slow changes, blocks fast ones. The cutoff frequency is f = 1 ÷ (2π × R × C).
• High-pass filter: swap the two positions. It passes changes and blocks DC.
• LC filters (inductor + capacitor) are sharper, and used in power supplies and radio.

Coupling (DC blocking)

A capacitor in series with a signal passes the AC part (audio, for example) while blocking any DC offset. That lets you connect amplifier stages sitting at different DC levels.

Timing

RC charging times set the frequency of oscillators, the length of 555 timer pulses (chapter 28), and power-on reset delays.

Energy storage and pulses

Camera flashes, defibrillators, spot welders, and coilguns dump a charged capacitor's energy in milliseconds. Supercapacitors keep clocks and memory alive during power cuts.

Snubbers and noise suppression

A capacitor (often with a resistor) across relay contacts, switches, or motor terminals absorbs arcs and electrical noise. A 100 nF ceramic across a brushed DC motor's terminals is standard practice.

Motor start and run capacitors

Single-phase AC motors (fans, compressors, pumps) use film or special electrolytic capacitors to create a phase-shifted current that makes them start and run. A failed start capacitor is a classic "motor hums but won't turn" fault.

Sensing

Capacitive sensing measures how a nearby finger, liquid, or object changes a capacitance. It's used in touchscreens, touch buttons, proximity sensors, and soil moisture and liquid level sensors (chapters 25, 26).

10.7 Did you know?

• The first capacitor was the Leyden jar (1745): a glass jar coated inside and out with metal foil. Experimenters gave each other painful shocks with them.
• The farad is named after Michael Faraday.
• Some ceramic capacitors "sing": the piezoelectric effect makes them physically vibrate with changing voltage, producing an audible whine in laptops and power adapters.
• A charged capacitor can hold voltage for days. Large capacitors in TVs, microwave ovens, and camera flashes can be dangerous long after unplugging.
• The "capacitor plague" (early 2000s): a batch of faulty electrolyte caused millions of motherboard capacitors to bulge and fail within a few years.

10.8 Testing capacitors

Safety first: discharge any capacitor over a few volts before handling. Use a resistor (e.g. 1 kΩ, 1–5 W for large caps) across the terminals for several seconds, not a screwdriver, which can pit the terminals and throw sparks. Then confirm ~0 V with a meter.

10.9 How capacitors fail

When replacing electrolytics: same capacitance, same or higher voltage, same or better temperature rating (105 °C), and low-ESR types in switching supplies.

11 — Inductors and transformers

An inductor is a coil of wire that stores energy in a magnetic field. A transformer is two or more coils sharing a magnetic core, transferring energy from one to the other. They're the reason switching power supplies are efficient, the reason AC can be sent across countries, and the reason switching off a relay can destroy a transistor.

11.1 How an inductor works

Any current through a wire creates a magnetic field around it. Coil the wire and the fields add up. Wind it around iron or ferrite and the field becomes far stronger. That field stores energy.

The key behavior: an inductor resists changes in current.

Water analogy: a heavy water wheel or turbine in the pipe. Start pushing and it takes time to spin up, so the current builds gradually. Once spinning, it has momentum. Try to stop the flow suddenly and it keeps pushing, hard, creating a pressure spike.

That last point is critical:

Interrupting the current through an inductor produces a voltage spike, often hundreds of volts from a 12 V circuit, as the collapsing magnetic field tries to keep the current flowing. This is called inductive kickback, flyback, or back-EMF.

Kickback is:

• Destructive when a transistor switches off a relay coil, solenoid, or motor. The fix is a flyback diode across the coil (chapter 13).
• Useful in boost converters, ignition coils, electric fence chargers, and flyback transformers, which deliberately generate high voltage this way.

Inductance

Measured in henries (H). Practical values run from nH (RF) through µH (power converters) to mH and H (audio crossovers, mains filters). Inductance increases with more turns (it goes as the square of turns), larger coil area, and a magnetic core.

Inductors and AC

The opposite of a capacitor: an inductor passes DC easily and increasingly blocks AC as frequency rises. Its reactance is X_L = 2π × f × L. That makes inductors natural filters for keeping high-frequency noise out of a supply line.

11.2 Types of inductor

Key specs

Reading markings

SMD power inductors often use a 3-digit code in microhenries: 100 = 10 µH, 4R7 = 4.7 µH, 221 = 220 µH. Through-hole axial inductors may use the resistor color code in µH.

11.3 What inductors are used for

• Switching regulators: the inductor stores energy during each switching cycle and releases it smoothly, converting voltages at 85–95 %+ efficiency (chapter 20).
• Filters: LC filters in power supplies and audio. Speaker crossovers send bass through an inductor to the woofer (the inductor blocks treble).
• Noise suppression: ferrite beads and chokes keep interference in or out.
• Tuned circuits: an inductor and capacitor resonate at one frequency, which is how radios select stations.
• Energy transfer: wireless charging, RFID/NFC, induction cooktops.
• Sensing: metal detectors, inductive proximity sensors, and traffic loop detectors all notice when metal changes a coil's inductance.

11.4 Transformers

How a transformer works

Two coils (the primary and the secondary) share a magnetic core. Changing current in the primary creates a changing magnetic field, which induces a voltage in the secondary. There's no electrical connection between the windings, only magnetic.

• Voltage ratio = turns ratio. A 10:1 turns ratio turns 120 V AC into 12 V AC.
• Current goes the other way. Stepping voltage down steps current up. Power in ≈ power out (real transformers are 90–99 % efficient).
• Transformers only work with changing current (AC or pulses). Put steady DC on a primary and it's just a short circuit through a low-resistance wire. The coil will overheat.
• Isolation: because there's no direct connection, a transformer separates the output from dangerous mains voltage. This is a key safety function.

Types of transformer

Reading transformer specs

• Primary voltage and frequency (e.g. 120 V 60 Hz, or 230 V 50 Hz).
• Secondary voltage(s) at rated load (it reads higher unloaded, often 10–20 % more).
• VA rating (volt-amperes, roughly watts for resistive loads).
• Wire colors: there's no universal standard. Check the datasheet or label.

11.5 Did you know?

• Michael Faraday discovered electromagnetic induction in 1831 by wrapping two coils on an iron ring. It was essentially the first transformer.
• The mains "hum" you hear from transformers is magnetostriction: the core physically flexes 100 or 120 times a second, twice per AC cycle.
• Phone chargers got tiny because switching supplies run their transformer at hundreds of kHz instead of 60 Hz. Higher frequency needs far less iron.
• Wireless phone charging is a transformer with its primary in the pad and its secondary in the phone.
• Tesla coils are resonant air-core transformers that make spectacular million-volt arcs.

11.6 Testing inductors and transformers

A basic multimeter can't measure inductance, but it catches most failures:

⚠️ Shorted turns (a few turns touching through damaged insulation) barely change the DC resistance but make the part overheat. An LCR meter or a "ring tester" finds them.

11.7 How they fail

12 — Crystals and oscillators

Every digital device needs a heartbeat, a steady clock signal that paces its operations. Watches, computers, radios, and microcontrollers get that heartbeat from a tiny sliver of quartz that vibrates at an extraordinarily precise frequency.

12.1 How a crystal works

Quartz is piezoelectric: squeeze it and it produces a voltage, and apply a voltage and it flexes. A thin, precisely cut piece of quartz, with metal electrodes on each side, has a natural mechanical resonant frequency, like a tuning fork. Placed in an amplifier circuit that keeps nudging it, it vibrates at exactly that frequency and produces an electrical signal at the same rate.

• The frequency is set by the cut and thickness of the quartz.
• Typical accuracy: ±10–50 ppm (parts per million). A 20 ppm crystal clock drifts under 2 seconds per day.
• Quartz is very stable with temperature and time, far better than any resistor–capacitor timer.

12.2 Types of timing component

Common crystal frequencies

• 32.768 kHz: "watch crystals." 32,768 = 2¹⁵, so dividing by two fifteen times gives exactly 1 Hz. They're in almost every real-time clock and wristwatch.
• 8, 12, 16, 25 MHz: common microcontroller clocks.
• 12 or 48 MHz: USB timing.
• 3.579545 MHz: from the NTSC TV color subcarrier. Cheap, so still used in hobby radio and tone generators.
• 40 MHz / 26 MHz: Wi-Fi/Bluetooth module references.

12.3 Using a crystal with a microcontroller

The standard hookup: the crystal connects between the chip's two oscillator pins (often labelled XTAL_IN/XTAL_OUT or OSC1/OSC2). Each of those pins also has a small capacitor to ground.

• Load capacitors (C1, C2): the crystal's datasheet gives its load capacitance (CL), often 12–20 pF. A good starting value for each cap is 2 × (CL − stray), where stray capacitance is ~2–5 pF. For CL = 18 pF: 2 × (18 − 3) ≈ 30 pF each (use 27–33 pF).
• Keep the crystal and caps very close to the chip, with short traces and a solid ground beneath. Crystal circuits are sensitive to stray capacitance and noise.
• Some microcontrollers need a "fuse" or configuration setting to use the external crystal.

12.4 Did you know?

• The first quartz clock was built in 1927 at Bell Labs. Quartz wristwatches arrived in 1969 (the Seiko Astron) and ended the dominance of mechanical watches.
• Your computer has several crystals: one for the processor clock (multiplied up internally to GHz), one for the real-time clock, and more for USB, Ethernet, audio, and Wi-Fi.
• Crystals used to be hand-ground. During World War II, millions of radio crystals were cut and finished for military radios.
• A tuning-fork-shaped quartz resonator is inside nearly every 32.768 kHz watch crystal.
• "Crystal radios" have nothing to do with quartz. Their "crystal" was a galena (lead sulfide) mineral acting as an early diode detector.

12.5 Testing

A multimeter can't confirm a crystal is good. It can only show that it isn't shorted (infinite Ω between pins is expected). Practical checks:

• Oscilloscope on the oscillator output pin (or an oscillator module's output): look for a clean waveform at the right frequency. A scope probe on the crystal's input pin can stop some oscillators by adding capacitance, so probe the output side or use a ×10 probe.
• Frequency counter / meter Hz mode on an oscillator module's output.
• Symptom-based: a microcontroller that runs from its internal oscillator but not the external crystal suggests a crystal, capacitor, or configuration problem.

12.6 How they fail

13 — Diodes

A diode is a one-way valve for electric current. Current flows easily in one direction and is blocked in the other. From that single behavior come rectifiers that turn AC into DC, protection against reversed batteries, voltage references, surge suppressors, and, with the right materials, LEDs, laser diodes, and solar cells.

13.1 Semiconductors in one page

Pure silicon is a poor conductor. Adding tiny amounts of other elements (doping) changes that:

• N-type silicon is doped with an element like phosphorus that adds spare electrons, which are negative charge carriers.
• P-type silicon is doped with an element like boron that leaves holes (missing electrons), which behave like positive charge carriers.

Put P-type and N-type together and you get a PN junction. Where they meet, electrons and holes cancel out, leaving a thin depletion region with no free carriers. That region is an insulating barrier.

• Forward bias (P side more positive than N side): the applied voltage pushes carriers into the barrier. Once it exceeds roughly 0.6–0.7 V for silicon, the barrier collapses and current flows freely.
• Reverse bias (N side more positive): the barrier widens and almost no current flows, until the voltage is so high that the junction breaks down.

Every diode, transistor, LED, and integrated circuit is built from junctions like this.

13.2 How a diode behaves

• Anode (A) = the P side. Cathode (K) = the N side. Conventional current flows anode → cathode, the direction the triangle in the schematic symbol points. The bar in the symbol is the cathode.
• Forward voltage (Vf): once conducting, the diode drops a nearly fixed voltage, whatever the current. About 0.6–1.0 V for silicon, 0.2–0.5 V for Schottky, 1.2–3.5 V for LEDs. It rises a little with current, and falls about 2 mV per °C as the diode warms.
• Reverse leakage: a tiny current flows backwards (nanoamps to microamps; more for Schottky and when hot).
• Reverse breakdown: above the peak reverse voltage rating, the diode conducts backwards. That destroys ordinary diodes, but it's the intended mode for Zener and TVS diodes.
• Reverse recovery time (trr): when switched from forward to reverse, an ordinary diode keeps conducting backwards for a brief moment. That matters in fast switching circuits.

Power: a diode dissipates P = Vf × I. At 2 A and 0.8 V that's 1.6 W, enough to get hot.

13.3 Types of diode

Part-number decoding

• 1N400x: the last digit is the voltage. 4001 = 50 V, 4002 = 100 V, 4003 = 200 V, 4004 = 400 V, 4005 = 600 V, 4006 = 800 V, 4007 = 1,000 V. All are 1 A.
• 1N5817 / 5818 / 5819: Schottky, 1 A, 20 / 30 / 40 V.
• Zeners are sold by voltage (e.g. "BZX79-C5V1" = 5.1 V, ±5 %) and power (0.5 W, 1 W, 5 W).

13.4 Identifying diodes

• The band marks the cathode on nearly every diode: glass, plastic, and SMD. On SMD parts it's a line or notch at one end.
• Glass diodes (orange-ish or clear, with a black band) are usually small-signal or Zener. The part number is printed tiny, or as a color code.
• Black plastic cylinders are usually rectifiers or Schottkys.
• Bridge rectifiers are marked ~ ~ (AC inputs) and + − (DC outputs). The + corner is often chamfered.

13.5 What diodes are used for

Rectification (AC to DC)

• Half-wave: one diode passes only the positive halves of the AC wave. Simple, but lumpy and wasteful.
• Full-wave center-tapped: two diodes and a center-tapped transformer use both halves.
• Full-wave bridge: four diodes use both halves with a plain transformer. This is the most common.
• A smoothing capacitor afterwards fills the gaps (chapter 10). The DC output peaks at about AC_RMS × 1.414 minus the diode drops.

Reverse-polarity protection

A diode in series with the supply blocks a backwards battery. It costs its forward drop, so Schottkys are preferred at low voltages. Alternatives are covered in chapter 7.

Flyback (freewheeling) diode

Placed across an inductive load (relay coil, solenoid, motor) with its cathode to the positive side, it does nothing in normal operation. When the driving transistor turns off, the coil's kickback voltage forward-biases the diode, and the current circulates harmlessly until it decays. Mandatory whenever a transistor switches a coil. 1N4007 for slow relays, 1N4148 for small ones, Schottky for fast PWM.

OR-ing (combining supplies)

Two sources, each through its own diode, feed one rail. Whichever is higher powers the load, and neither can backfeed the other. It's used for USB-or-battery power and redundant supplies.

Clamping and protection

Diodes from a signal line to the supply and to ground clamp the signal within the rails. Almost every IC pin has such clamp diodes built in (see §13.7).

Voltage reference (Zener)

A Zener in reverse, fed through a resistor, holds its breakdown voltage across itself: a simple shunt regulator. Choose the resistor so a few mA flows through the Zener even at maximum load. Dedicated reference ICs (e.g. TL431, chapter 28) are more precise.

Dropping voltage

Each series silicon diode drops ~0.7 V, a crude way to lower a supply slightly (e.g. to run a 3.6 V-max device from 4.2 V, or to shave a volt off a fan's supply).

Logic and signal jobs

Diode logic (simple AND/OR gates), steering signals, detecting AM radio (demodulation), and keyboard matrices (a diode per key prevents "ghost" keypresses; chapter 25).

Voltage multipliers

Chains of diodes and capacitors (the Cockcroft–Walton multiplier) build high DC voltages from AC. They're used in bug zappers, photocopiers, CRTs, and particle accelerators.

13.6 Did you know?

• The first semiconductor diode was a "cat's whisker" detector (1906): a fine wire touching a galena crystal, used in early crystal radio sets.
• Before semiconductors, diodes were vacuum tubes. The first, invented by John Ambrose Fleming in 1904, was called a "valve", a name still used in the UK.
• A diode's forward voltage makes a decent thermometer. It drops about 2 mV per °C. Many chips measure their own temperature this way.
• Solar cells are large photodiodes. Light knocks electrons loose at the PN junction.
• Schottky diodes are named after Walter Schottky, who explained the metal–semiconductor barrier in 1938.

13.7 The diodes inside every chip

Most IC pins have protection diodes from the pin to the chip's supply and to ground. They guard against static and small overvoltages, but they also cause two common problems:

• Backfeeding: putting a higher voltage on an input than the chip's own supply (e.g. 5 V into a 3.3 V chip) forward-biases the upper clamp diode. Current flows into the chip's supply rail and can partly power the chip, or damage it.
• Phantom powering: an unpowered board can "half-wake" from a signal line connected to a powered board.

Rule: never drive a pin above its supply voltage or below ground unless the datasheet says it's tolerant. Use series resistors or level shifters (chapter 31).

13.8 Testing diodes

Use the meter's diode test mode, with power off.

• Near 0 V both ways (beeps) = shorted.
• OL both ways = open (or an LED/high-voltage device beyond the meter's test voltage).
• Bridge rectifier: test each of the four internal diodes. From − to each ~, and from each ~ to +, the red-probe-forward direction reads ~0.5–0.7 V, and reversed reads OL.
• Zener voltage: a meter can't reach it. Feed the Zener from a supply higher than its rating through a resistor (e.g. 12 V through 1 kΩ) and measure the voltage across it.
• In-circuit: readings can be skewed by parallel parts. Lift a leg if in doubt.

13.9 How diodes fail

14 — LEDs and optoelectronics

Optoelectronic components convert between electricity and light. LEDs and laser diodes make light. Photodiodes, phototransistors, and solar cells respond to it. Optocouplers pair the two to pass signals across an electrical gap. This chapter covers them all, starting with the humble indicator LED.

14.1 How an LED works

An LED is a diode (chapter 13) made from compound semiconductors such as gallium arsenide, gallium phosphide, or indium gallium nitride. When current flows forward across the junction, electrons fall into lower-energy "holes" and release the difference as a photon, a particle of light.

• The size of that energy step sets the color: bigger step = bluer light.
• It also sets the forward voltage: blue and white LEDs need more voltage than red.
• No filament, nothing to burn out: an LED can last 25,000–100,000 hours, gradually dimming rather than failing suddenly.

The crucial behavior: LEDs are current-driven

Below its forward voltage, an LED is off. Above it, current rises extremely steeply with only a small increase in voltage, and the forward voltage falls as the LED warms. Connected directly to a voltage source, it draws far too much current and burns out. You must limit the current, usually with a series resistor, or with a constant-current driver for high-power LEDs. Brightness follows current.

14.2 Colors and forward voltages

Vf also rises with current and varies between batches. The datasheet gives it at a stated current (typically 20 mA).

14.3 Types of LED

Finding the polarity

• Longer lead = anode (+). Shorter lead = cathode (−).
• Flat spot on the rim = cathode side.
• Inside a clear 5 mm LED, the larger "anvil" piece is usually the cathode (a common convention, but not guaranteed).
• SMD LEDs: a dot, notch, green tint, or "T" mark, which varies by manufacturer. A meter's diode test confirms it.

14.4 Driving an LED

The resistor formula

R = (V_supply − Vf) ÷ I

Guidelines:

• Round the resistor up to the next standard value. Slightly dimmer is safer.
• Modern LEDs are bright at 2–5 mA. 20 mA is the maximum, not the target.
• Low headroom is trouble. A blue LED (3.1 V) on 3.3 V leaves only 0.2 V for the resistor, so small Vf differences cause large brightness differences. Use a higher supply voltage and a transistor switch, or a constant-current driver.
• Each parallel LED needs its own resistor. LEDs sharing one resistor "current hog": the one with the lowest Vf takes most of the current, heats up, lowers its Vf further, and fails.
• LEDs in series share current perfectly, and their Vf's add.
• Reverse voltage rating is low (~5 V). On AC, or where reverse voltage is possible, add a reverse diode across or in series.

Constant-current drivers

High-power LEDs use drivers that regulate current rather than voltage: linear driver ICs, switching LED drivers (buck or boost), or simple transistor current sinks. LED strips usually run on 12 V or 24 V with small resistors built in for each group of LEDs.

From a microcontroller

• Active-high: pin → resistor → LED → ground. The pin HIGH turns it on.
• Active-low: supply → resistor → LED → pin. The pin LOW turns it on. It's common, and the logic is inverted.
• A GPIO pin can typically supply 5–20 mA (chapter 29). More LEDs or more current needs a transistor (chapters 16–17) or a driver chip (e.g. ULN2003, TLC5940).

Dimming

• PWM (chapter 1): fast on/off switching. Brightness follows duty cycle, and color stays consistent. Use a frequency above ~200 Hz to avoid visible flicker (higher still to avoid flicker on camera).
• Current reduction: simple, but LED color can shift at very low currents.
• Human vision is roughly logarithmic. 10 % duty looks much brighter than "a tenth." Smooth fades use a gamma correction curve.

14.5 LED displays

• 7-segment display: seven bar-shaped LEDs (a–g) plus a decimal point, in common anode or common cathode form. Multi-digit displays are multiplexed: each digit is lit in quick rotation, fast enough that all appear on at once. Driver chips include the MAX7219 and TM1637.
• 14/16-segment (alphanumeric), bar-graph arrays, and dot-matrix grids (8×8 and larger) work the same way.
• LED matrix panels (e.g. HUB75 RGB panels) are large, scanned grids used for signs and video walls.

14.6 Lasers

A laser diode is an LED with polished, mirrored ends that make light bounce back and forth until it emerges as a narrow, coherent beam.

• ⚠️ Eye safety: laser classes run from 1 (safe) and 2 (visible, low power; the blink reflex protects you) through 3R and 3B (hazardous with direct viewing) to 4 (burns skin, starts fires, blinds from reflections). Never look into a beam or point one at people, animals, or aircraft.
• Laser diodes are extremely sensitive to over-current and static. They're driven by constant-current drivers with feedback from a built-in photodiode.
• Uses: pointers, barcode scanners, optical drives, time-of-flight distance sensors, fiber-optic communication, engravers.

14.7 Light detectors

Reading a phototransistor: connect its collector to the supply through a pull-up resistor (e.g. 10 kΩ), and its emitter to ground. The collector voltage falls as light increases. Read it with an analog input, or a digital input for simple detection.

Infrared remote receivers

The 3-pin modules in TVs (e.g. TSOP382xx, VS1838B) contain a photodiode, amplifier, filter, and demodulator tuned to light flashing at ~38 kHz. They ignore sunlight and room lighting, and output a clean digital signal of the remote's code.

💡 You can see infrared LEDs with a phone camera. Many camera sensors pick up 850–940 nm as a purple-white glow. It's a quick way to check whether a TV remote works. (Some rear cameras filter IR, so try the front camera.)

14.8 Optocouplers (optoisolators)

An LED and a light detector sealed facing each other in one package, with no electrical connection between the two sides. Current through the LED turns on the detector, so a signal crosses a galvanic isolation barrier that can withstand thousands of volts.

Key spec: CTR (current transfer ratio) = output current ÷ LED current, as a percentage. A PC817 with 50–600 % CTR gives 5–60 mA of output capacity per 10 mA through its LED. CTR drops as the LED ages.

Uses: isolating microcontrollers from mains-referenced circuits, breaking ground loops, reading 12/24 V industrial signals, MIDI inputs, and the voltage feedback in most phone chargers.

Photo-interrupters and reflective sensors

• Slotted (transmissive) sensors: an IR LED and phototransistor facing each other across a gap. An object in the slot blocks the beam. Used for encoder wheels, paper detection in printers, and end stops.
• Reflective sensors (TCRT5000 and similar): side by side, detecting light reflected off a nearby surface. Used for line-following robots and proximity detection.

14.9 Did you know?

• The first visible (red) LED was made in 1962 by Nick Holonyak Jr. at General Electric.
• Efficient blue LEDs came in the early 1990s (Nakamura, Akasaki, Amano). They made white LEDs possible and won the 2014 Nobel Prize in Physics.
• LEDs are now ~5–10× more efficient than incandescent bulbs.
• LEDs can work in reverse, generating a tiny current from light, so an LED can act as a crude, color-selective light sensor.
• "Blue light hazard" is why high-power blue and white LEDs shouldn't be stared at closely.

14.10 Testing optoelectronic parts

14.11 How they fail

15 — Addressable LEDs

An addressable LED ("smart pixel," "NeoPixel") combines an RGB LED with a tiny controller chip. Instead of three wires per LED, hundreds or thousands can be chained on a single data line, each set to its own color. They're behind LED strips, holiday displays, ambient lighting, signs, and countless maker projects.

15.1 How they work

Each pixel has a data in (DIN) and data out (DOUT) pin, plus power and ground. The controller sends a stream of color data down the chain:

1. The first pixel keeps the first 24 bits (8 each for red, green, blue) and forwards the rest out of its DOUT.
2. The second pixel keeps the next 24 bits, forwards the rest, and so on down the line.
3. When the data line stays idle for a short reset / latch time (tens to hundreds of µs), every pixel updates to its new color at once.

Because each pixel regenerates the signal, a long chain is really a series of short hops. Only the wire from the controller to the first pixel is critical. The downside: a failed pixel's DOUT breaks the chain for every pixel after it.

15.2 Common families

Form factors: flexible strips (30/60/144 LEDs per meter), bullet or seed strings, rings, matrices (8×8, 16×16), single pixels on breakout boards, and bare 5050 / 2020 packages.

15.3 The single-wire protocol (WS2812-style)

There's no clock line. Each bit is a pulse whose width encodes 0 or 1 (roughly: a short high then a long low = 0, a long high then a short low = 1), about 1.25 µs per bit at 800 kHz. Consequences:

• Timing must be precise to within a few hundred nanoseconds. Libraries (FastLED, Adafruit NeoPixel) generate it for you.
• On simpler microcontrollers, the library disables interrupts while sending. Other interrupt-driven code (software serial, some timers, IR receiving) can miss events during updates.
• More capable chips use hardware peripherals (DMA, SPI, I²S, RMT, PIO) so the CPU and interrupts aren't tied up.
• Refresh time grows with length: ~30 µs per pixel, so 1,000 pixels take ~30 ms (about 33 updates per second maximum).
• Color order varies: many WS2812B parts expect GRB, while WS2811 strings are often RGB. If red and green are swapped, change the color order setting. Test with pure red, green, and blue.

Clocked types (APA102) have a separate clock wire, so timing isn't critical and they can be driven much faster.

15.4 Power

Current per pixel

Each color channel draws up to ~20 mA at full brightness, so a 5 V RGB pixel draws up to ~60 mA at full white. Real parts vary (often 35–50 mA, and some "WS2811" strings far less), so measure a few if it matters.

Power budget: pixels × current per pixel at full white × expected brightness.

• 300 pixels × 60 mA = 18 A at full white, 5 V (90 W).
• Typical animations average 20–40 % of full white. But a single "all white" frame draws the full amount, so either size the supply for it, or cap current in software (libraries offer a max-milliamps limit).
• Even "off" pixels draw ~0.5–1 mA each for their controller chips.

Voltage drop and power injection

Thin strip copper and pixel wires have resistance. Every pixel's current flows through the copper before it, so voltage sags along the strip. The far end dims and shifts toward red/orange, because blue and green need more voltage (chapter 14.2). White turning peach is the telltale sign.

Power injection fixes it: run separate, heavier wires from the supply to several points along the strip, connecting both +V and ground.

• Rough guide for 5 V strips: inject every ~50–150 pixels (fewer for dense 144/m strips or full-white use).
• 12 V pixel types carry the same power at less than half the current, so they need far less injection.
• Both ends and the middle is a common pattern.

Supplies, fuses and capacitors

• Pick a supply rated ~20–25 % above your realistic maximum.
• Put a large electrolytic capacitor (470–1,000 µF, 6.3 V+) across power at the strip's input to absorb inrush and sudden brightness changes.
• Fuse each branch for its wire size (chapter 7). A big supply will happily melt thin strip wiring during a short.
• Connect power before (or together with) data. Sending data into an unpowered strip can backfeed the first pixel through its input protection diodes and damage it.

15.5 The data line

Logic levels

A 5 V pixel typically needs its data HIGH above ~0.7 × its supply, i.e. ~3.5 V. A 3.3 V microcontroller's HIGH (3.3 V) is below spec. It often works on the bench, and then fails with longer wires, temperature changes, or a different batch.

Fixes, best first:

1. A level shifter: a 74AHCT125 or 74HCT245 powered from 5 V accepts 3.3 V inputs as HIGH and outputs a full 5 V (chapter 31). Bidirectional MOSFET-type shifters are often too slow for pixel data.
2. A "sacrificial pixel": power the first pixel through a diode (~4.3 V). Its threshold drops to ~3.0 V, so 3.3 V works, and it re-drives clean data to the rest at full level.
3. Run the strip slightly below 5 V, which lowers the threshold. Sometimes enough, but marginal.

Wiring the data line

• Series resistor of 220–470 Ω in the data line at the driving end. It damps ringing and reflections and protects the first pixel's input.
• Keep the controller → first pixel wire short, ideally under ~1–2 m for 800 kHz. Run a ground wire alongside the data wire (or twist them together).
• Common ground between controller and strip is essential.
• Longer data runs: use a differential line driver (e.g. RS-485 transceivers at each end) or a slower 400 kHz mode where supported.
• Follow the arrows: strips are marked with an arrow or DIN/DOUT. Connecting to the wrong end does nothing.

15.6 Did you know?

• The WS2812 was released around 2013 by Worldsemi. Adafruit's "NeoPixel" brand name became the generic term.
• Every pixel contains a small PWM controller, typically running at ~400 Hz–2 kHz for WS2812-types (it can show flicker on camera), and much higher for APA102.
• Large LED art installations chain tens of thousands of pixels across many parallel data outputs to keep refresh rates high.
• Cutting a strip: cut only across the copper pads marked with a scissors symbol. Each cut segment is a complete strip.

15.7 Testing and diagnosis

Meter checks:

• DC volts between + and GND at several points under load: everything should be above ~4.5 V on a 5 V strip.
• DC volts on the data line: ~0 V idle. While animating, a meter shows a meaningless average (~1–2 V), which at least proves data is being sent.
• Continuity from one pixel's DOUT to the next pixel's DIN.

15.8 How they fail

A strip that fails "from pixel N onward" almost always has a single bad pixel or joint at N. Cutting that pixel out and bridging the gap usually brings the rest back.

16 — Bipolar transistors (BJTs)

The transistor is the most important invention in electronics. It lets a small signal control a larger current: as a switch (on or off) or as an amplifier (a proportional copy). The bipolar junction transistor (BJT) was the first practical type and is still used everywhere for small switching and amplifying jobs.

16.1 Structure

A BJT is a sandwich of three semiconductor layers, forming two PN junctions:

• NPN: N-type / P-type / N-type. The most common type.
• PNP: P-type / N-type / P-type. The mirror image.

The three terminals:

16.2 How it works

For an NPN transistor: when a small current flows into the base and out of the emitter (which requires the base to be ~0.6–0.7 V above the emitter), the transistor allows a much larger current to flow from collector to emitter.

Water analogy: a large valve in the main pipe (collector → emitter) whose handle is pushed open by a small side stream (the base current). More side flow opens the valve further.

The ratio is the current gain, called hFE or β (beta):

I_C ≈ hFE × I_B

With hFE = 100, a 1 mA base current allows up to 100 mA of collector current. hFE varies enormously between parts, even of the same type (often 100–400), and changes with temperature and current. Good circuits don't depend on its exact value.

Three operating regions

16.3 The BJT as a switch

The most common hobby use: letting a microcontroller pin (which can supply only a few mA) turn on a relay, buzzer, motor, lamp, or group of LEDs.

Low-side NPN switch

• The load connects from the positive supply to the collector.
• The emitter goes to ground.
• A base resistor connects the control signal (e.g. a GPIO pin) to the base.
• The load's supply can be higher than the control voltage: a 3.3 V pin can switch a 12 V load, as long as the grounds are connected.
• For a coil load (relay, solenoid, motor), add a flyback diode across the load (chapter 13).

Sizing the base resistor

To guarantee saturation, supply about one tenth of the load current as base current (a "forced beta" of 10), regardless of the transistor's actual hFE:

I_B = I_load ÷ 10 and R_B = (V_control − 0.7) ÷ I_B

If the required base current is more than the pin can supply (see the chip's limits), use a Darlington, a MOSFET (chapter 17), or a driver IC.

A pull-down resistor (e.g. 10–100 kΩ) from base to emitter keeps the transistor firmly off when the control pin is floating (during boot, for example).

High-side PNP switch

A PNP sits between the positive supply and the load, and turns on when its base is pulled low (about 0.7 V below its emitter). A PNP whose emitter is at a higher voltage than the controlling microcontroller can't be turned fully off by the pin. Add a small NPN to pull its base down, or use a P-channel MOSFET with a driver.

Heat

A saturated switch dissipates P = V_CE(sat) × I_C: small (e.g. 0.2 V × 0.5 A = 0.1 W). A transistor that isn't saturated has volts across it and heats rapidly. A hot switching transistor usually needs more base current.

16.4 The BJT as an amplifier (the basics)

In the active region, small changes in base current produce proportionally larger changes in collector current. With a collector resistor, that becomes a larger voltage swing: an amplified copy of the input.

The classic common-emitter amplifier uses:

• Bias resistors (a voltage divider on the base) to set the transistor half-way on, so the signal can swing up and down.
• Coupling capacitors to pass the AC signal in and out while blocking DC.
• An emitter resistor (and often a bypass capacitor) to stabilize gain against hFE and temperature variation.

Other configurations:

• Common-collector ("emitter follower"): voltage gain ≈ 1, but high current gain. Used as a buffer.
• Common-base: used in high-frequency circuits.

Today most amplification is done with op-amp ICs (chapter 28), but they're built from exactly these transistor stages.

16.5 Special configurations

16.6 Packages and common parts

⚠️ TO-92 pinouts are not standardized. With the flat face toward you and the legs down, the 2N3904 is E-B-C, the BC547 is C-B-E, and the 2SC1815 is E-C-B. Even one part number can vary: onsemi's PN2222A is E-B-C, but its P2N2222A is C-B-E. Check the datasheet for the exact part, or use a component tester or the diode-test method below. More pinouts are in Appendix D.

16.7 Did you know?

• The transistor was invented at Bell Labs in 1947 by John Bardeen, Walter Brattain, and William Shockley (a point-contact transistor). They shared the 1956 Nobel Prize. Shockley's junction transistor followed in 1948.
• The first commercial transistor radio (Regency TR-1) appeared in 1954. It had four transistors.
• A modern processor contains tens of billions of transistors, almost all MOSFETs (chapter 17). But BJTs still dominate in analog, RF, and some power applications.
• "Transistor" combines "transfer" and "resistor".
• Transistors replaced vacuum tubes, which were larger, hotter, fragile, and needed warm-up time.

16.8 Testing a BJT

A BJT looks like two diodes sharing the base: base–emitter and base–collector.

Diode-test mode, NPN:

PNP: the same with black on the base (the diodes point the other way).

Identifying unknown pins: find the pin that conducts to both others with the same probe on it. That's the base. If that probe is red, it's NPN; if black, PNP. The junction with the slightly higher reading is usually base–emitter.

Many meters have an hFE socket that measures gain directly. It's a rough value, but it confirms the type and pin order.

16.9 How BJTs fail

17 — MOSFETs and other FETs

The field-effect transistor (FET) controls current with voltage instead of current. Its best-known form, the MOSFET, is the most manufactured device in human history. Every microchip contains billions of them, and individual power MOSFETs switch everything from LED strips to electric cars.

17.1 How a MOSFET works

A MOSFET has three terminals:

The gate is a metal (or polysilicon) electrode separated from the silicon underneath by an extremely thin layer of insulating oxide. Putting a voltage on the gate creates an electric field that pulls charge carriers into the region beneath it, forming a conductive channel between drain and source. No voltage, no channel.

Key consequences:

• The gate draws no steady current. It's a small capacitor. Once charged, it holds the MOSFET on with zero power. (Charging and discharging that capacitance quickly does take current, which matters at high switching speeds.)
• Fully on, a MOSFET is a very small resistor, Rds(on), often just milliohms. Power lost = I² × Rds(on). At 10 A and 10 mΩ, only 1 W.
• Fully off, it's an open circuit up to its voltage rating.

17.2 Types

Enhancement vs depletion

• Enhancement mode (almost all MOSFETs you'll use): off with 0 V on the gate, turned on by gate voltage.
• Depletion mode: on at 0 V, turned off by gate voltage. Rare. Used in some startup and current-source circuits.

N-channel vs P-channel

N-channel low-side switching is the easiest and most common.

Other members of the FET family

17.3 Reading a MOSFET datasheet

Logic-level MOSFETs: the big trap

The threshold voltage is not the turn-on voltage. A MOSFET with Vgs(th) = 2–4 V may need 10 V on the gate to reach its rated Rds(on). Driven from a 3.3 V or 5 V microcontroller, it's only partly on: high resistance, heat, and possible failure.

For direct microcontroller drive, choose a "logic-level" MOSFET whose datasheet specifies Rds(on) at Vgs = 4.5 V (for 5 V logic) or 2.5 V (for 3.3 V logic).

💡 The "L" in IRL-series part numbers means "logic level." IRLZ44N works from a 5 V pin, and its near-twin IRFZ44N doesn't.

17.4 Driving a MOSFET

N-channel low-side switch (the standard circuit)

• Load from the positive supply to the drain.
• Source to ground (shared with the microcontroller).
• Gate to the control pin through a gate resistor (~100 Ω). It limits the brief current spike when charging the gate, and damps ringing.
• Gate pull-down resistor (~10–100 kΩ, gate to source). It holds the MOSFET off while the control pin floats (at power-up, during reset, if a wire comes loose). Without it, a floating gate can drift partly on, and the load may flash or twitch at startup.
• Flyback diode across inductive loads.

Gate drivers

Driving a MOSFET fast (PWM at tens of kHz or more) means charging its gate capacitance quickly, which needs amps of peak current. A microcontroller pin can't provide that, so the MOSFET switches slowly, spends time half-on, and heats up. Gate driver ICs (e.g. TC4420, UCC27517, or half-bridge drivers like the IR2110) supply the current and can also shift the gate voltage up to 10–12 V. For slow switching (on/off, low-frequency PWM), a direct connection to a logic-level part is fine.

P-channel high-side switch

• Source to the supply, drain to the load, load to ground.
• Gate pulled up to the source with a resistor (off by default).
• To turn it on, pull the gate below the source by more than its threshold, usually with a small NPN transistor or N-MOSFET pulling the gate to ground.
• If the supply is higher than the MOSFET's Vgs(max), add a Zener or a divider to protect the gate.

This circuit is used to switch power to a whole subsystem (sensors, radio modules) in battery devices.

Reverse-polarity protection with a MOSFET

A P-channel MOSFET in the positive lead (or an N-channel in the ground lead), wired so its body diode conducts in the correct direction, turns fully on when the battery is connected correctly and stays off when reversed. It's far more efficient than a series diode.

Bidirectional level shifting

A single N-channel MOSFET (typically BSS138) with pull-up resistors on both sides translates open-drain signals like I²C between 3.3 V and 5 V in both directions (chapter 31).

17.5 The body diode

Power MOSFETs contain a built-in body diode from source to drain (for N-channel). It conducts whenever the drain goes below the source, whether the gate is on or not.

• Useful: in H-bridge motor drivers, it acts as a free flyback path.
• Surprising: a single MOSFET can't block current in both directions. Battery disconnect switches often use two MOSFETs back to back (source to source) to block both ways.

17.6 Heat and heatsinking

Conduction loss is P = I² × Rds(on). Switching loss adds to it at high PWM frequencies. Estimating temperature:

Temperature rise = P × thermal resistance

Example: a TO-220 MOSFET with R_θJA ≈ 62 °C/W (no heatsink) dissipating 1 W rises ~62 °C above ambient. That's already hot. With a heatsink, total thermal resistance might fall to ~10 °C/W. Keep junction temperatures well under the 150–175 °C maximum, ideally below ~100 °C.

Rules of thumb for through-hole packages without a heatsink: TO-92 ≤ ~0.5 W, TO-220 ≤ ~1–1.5 W. In SMD packages, the PCB copper is the heatsink.

17.7 Did you know?

• The MOSFET was invented at Bell Labs in 1959 by Mohamed Atalla and Dawon Kahng.
• An estimated 10²² (tens of sextillions) MOSFETs have been manufactured, more than any other device ever made.
• The gate oxide in modern processors is only a few atoms thick. Modern processes use exotic "high-k" materials to keep leakage down.
• MOSFETs are extremely sensitive to static. A discharge you can't even feel can puncture the gate oxide. Handle them with grounded wrists, and keep them in their conductive packaging.
• The IGBT made modern variable-speed motor drives, induction cooktops, and electric trains practical.

17.8 Testing a MOSFET

N-channel enhancement, diode-test mode (power off):

1. Discharge the gate first: briefly short gate to source (touch them together, or with the probe tips).
2. Red on source, black on drain: the body diode reads ~0.4–0.7 V.
3. Red on drain, black on source: OL (off).
4. Turn it on: touch red to the gate and black to the source for a moment. The meter's test voltage charges the gate.
5. Now red on drain, black on source: it reads low (near 0 or a small value). The channel is on. (Low-threshold parts turn on fully. Higher-threshold ones may only partly turn on with a meter's ~3 V test voltage.)
6. Short gate to source again: drain–source goes back to OL.

For P-channel, reverse the probe polarities.

Bad signs:

• Drain–source reads near 0 Ω both ways even after discharging the gate: shorted (the most common failure).
• The gate shows a reading (conducts) to source or drain: gate oxide punctured.
• It won't turn on or off with gate charge: damaged.

A component tester identifies MOSFETs, their type, pinout, and threshold, which is useful for unmarked parts.

17.9 How MOSFETs fail

When a MOSFET fails shorted, check the driver and gate resistor too. A failing MOSFET often takes the gate driver or the microcontroller pin with it.

18 — Thyristors, TRIACs and solid-state relays

Thyristors are latching semiconductor switches. Once triggered, they stay on by themselves until the current through them drops to nearly zero. That makes them natural switches for AC mains power, where the current passes through zero many times per second. They're the heart of light dimmers, motor speed controls, and solid-state relays.

⚠️ Most of these parts are used at mains voltage. Read chapter 33 before building mains circuits, and prefer ready-made, certified modules when in doubt.

18.1 The SCR (silicon-controlled rectifier)

How it works

A four-layer (PNPN) device with three terminals: anode, cathode, and gate.

• With no gate signal, it blocks current in both directions.
• A short gate pulse (a few mA, gate positive relative to the cathode) turns it on, and it conducts from anode to cathode like a diode.
• It then stays on (latches) even after the gate signal is removed, until the anode current falls below its holding current.
• On AC, it switches off naturally at the end of each half-cycle. On DC, it stays on until the circuit is broken.

Uses

• Controlled rectifiers: adjustable DC power supplies, battery chargers, and DC motor controls fed from AC.
• Crowbar protection: an SCR across a power supply's output fires on over-voltage, shorting the supply and blowing a fuse, to save the load from a failed regulator.
• Latching alarms: once triggered, the alarm stays on until reset.
• Capacitor discharge: camera flashes, ignition systems.

Common parts: C106, 2N6504, TYN612, BT151.

18.2 The TRIAC

A TRIAC is effectively two SCRs back to back sharing one gate. It conducts in both directions once triggered, making it a switch for AC.

• Terminals: MT1 (main terminal 1), MT2, and Gate. The gate is triggered relative to MT1.
• It turns off each time the AC current crosses zero, so it must be re-triggered every half-cycle.
• Phase-angle control: triggering part-way through each half-cycle delivers only the remaining portion of the wave. Triggering later means less power. This is how incandescent light dimmers and universal-motor speed controls (drills, blenders) work.
• Snubbers: an RC network across the TRIAC prevents false triggering from fast voltage changes (dV/dt) when switching inductive loads like motors. Snubberless TRIACs are designed to need it less.

Common parts: BT136 (4 A), BT137, BT138 (12 A), BTA16 (16 A, insulated tab), Q4008.

💡 BTA vs BTB: in ST's naming, BTA parts have an insulated mounting tab. BTB tabs are connected to MT2, which is at mains voltage.

18.3 The DIAC

A DIAC is a two-terminal trigger device. It blocks until the voltage across it (in either direction) reaches its breakover voltage (~30 V), then suddenly conducts. Paired with an RC network, it produces sharp trigger pulses for a TRIAC. That's the classic simple light dimmer: a potentiometer + capacitor sets the delay, and the DIAC fires the TRIAC.

18.4 Optotriacs

An optotriac (e.g. MOC3021, MOC3041) is an optocoupler whose output is a small TRIAC. A microcontroller lights the internal LED (~10–15 mA), which triggers a power TRIAC safely isolated from mains.

• Random-phase types (MOC3020–3023): trigger whenever the LED is on. Needed for phase dimming.
• Zero-cross types (MOC3041–3043, MOC3061–3063): only trigger near the AC zero crossing. They produce far less electrical noise and inrush when switching resistive loads on and off. Can't be used for dimming.

18.5 Solid-state relays (SSRs)

An SSR does the job of an electromechanical relay (chapter 6) with no moving parts. A low-voltage input drives an internal LED, which controls output semiconductors across an isolation barrier.

SSR pros and cons

⚠️ Heatsink rule: an SSR carrying 10 A with a 1.5 V drop dissipates 15 W. That's far too much without a proper heatsink. Budget-module "25 A" ratings assume a large heatsink and good airflow.

18.6 Did you know?

• The SCR was developed by General Electric in 1957, launching the field of power electronics.
• High-voltage DC transmission once relied on huge stacks of thyristors, some of the largest semiconductor devices ever made (wafers up to ~15 cm across).
• Wall dimmers can make LED bulbs flicker or buzz, because they were designed for the resistive load of an incandescent filament, not an LED driver. Look for LED-compatible dimmers.
• Phase-control dimmers create electrical noise (the sharp turn-on edge), which is why some AM radios buzz near them.

18.7 Testing

⚠️ Unplug mains circuits and discharge capacitors before testing.

18.8 How they fail

19 — Batteries

A battery converts stored chemical energy into electricity. Choosing one means balancing voltage, capacity, current capability, size, weight, cost, rechargeability, and safety. Lithium cells in particular demand respect: they pack enough energy to start a fire if abused.

19.1 How a battery works

A cell has two electrodes of different materials (the anode and cathode) in an electrolyte, with a separator between them. The chemistry "wants" to move electrons from one electrode to the other but can only do so through the external circuit. That push is the cell's voltage, set by the chemistry. In a rechargeable cell, charging forces the reaction backwards.

A battery, strictly speaking, is several cells connected together, although the word is used for single cells too.

Internal resistance

A real cell behaves like an ideal voltage source with a resistor inside it. Drawing current drops some voltage internally:

V_terminal = V_open-circuit − I × R_internal

This voltage sag is why a weak battery reads fine on a meter (no load) but collapses when something draws current. Internal resistance rises as a cell ages, gets cold, or runs low.

19.2 Capacity, energy and current

Capacity isn't fixed. It shrinks at high discharge currents, in the cold, and with age. Rated capacity is usually measured at a gentle rate (e.g. 0.2C).

Runtime estimate: hours ≈ capacity (Ah) ÷ average current (A), or Wh ÷ W. Derate by 20–30 % for real-world conditions.

19.3 Chemistries

Primary (non-rechargeable)

Secondary (rechargeable)

Supercapacitors vs batteries

Supercaps (chapter 10) store far less energy but charge in seconds and last millions of cycles. They complement batteries for burst power and backup.

19.4 Sizes and naming

Cylindrical cells

Cylindrical Li-ion names encode size: 18650 = 18 mm diameter × 65.0 mm long.

Coin and button cells

• CR2032 = Chemistry (lithium-manganese), Round, 20 mm diameter, 3.2 mm thick. 3 V, ~220 mAh. CR2025 and CR2016 are thinner variants.
• LR44 / AG13 / 357: 1.5 V alkaline or silver-oxide button, 11.6 × 5.4 mm.
• SR = silver oxide, LR = alkaline, PR = zinc-air.

The 9 V battery

A rectangular battery containing six small 1.5 V cells in series (some alkaline brands use six AAAA-size cells inside). About 500 mAh alkaline. Great for smoke alarms, poor for motors.

19.5 Discharge curves

• Alkaline voltage falls steadily from ~1.6 V to ~1.0 V, so devices dim gradually.
• NiMH holds a flat ~1.2 V, then drops quickly at the end.
• Li-ion falls from 4.2 V through a long plateau around 3.6–3.8 V, then drops quickly below ~3.4 V.
• LiFePO₄ is extremely flat (~3.2–3.3 V) for most of its capacity, which makes state-of-charge hard to judge from voltage.

Li-ion resting voltage vs charge (approximate)

Measure at rest (after the load has been off for a while). Voltage under load reads lower, and it bounces back afterwards. Low-battery detection needs hysteresis so it doesn't flip on and off.

19.6 Series and parallel packs

Rules:

• Use identical cells: same model, age, and state of charge.
• Match voltages (within ~0.05 V) before paralleling Li-ion cells, or a big current rushes between them.
• Series packs drift out of balance. Over cycles, one cell ends up higher or lower than the others. A BMS (battery management system) monitors each series group through balance leads and bleeds charge from high cells.
• Never solder directly to Li-ion cells. The heat damages them. Use spot-welded nickel strip or proper cell holders.

19.7 Charging

Common Li-ion charger ICs and modules: TP4056 (single cell, 1 A, linear), MCP73831 (small, 500 mA), BQ-series (TI) chargers with power-path and fuel gauging, and multi-cell balance chargers for RC packs.

• Linear chargers (like the TP4056) burn the difference between input and battery voltage as heat: (5 V − 3.7 V) × 1 A ≈ 1.3 W, which is normal warmth.
• Charging current is set with a resistor on most charger ICs.
• Load-sharing / power-path chargers let a device run while charging without confusing charge termination.
• Charging temperature limits: Li-ion must not be charged below 0 °C (it plates lithium metal inside, which is dangerous) or above ~45 °C.

19.8 Protection and the BMS

A Li-ion protection circuit (built into "protected" cells, pack BMS boards, and many charger modules) disconnects the cell on:

• Over-charge (above ~4.25–4.3 V)
• Over-discharge (below ~2.4–3.0 V)
• Over-current / short circuit
• Over-temperature (in fuller BMS designs)

A classic small protection circuit is a DW01 controller plus a dual MOSFET (FS8205). Multi-cell BMS boards add balancing. Protection is a safety net, not a charger: it should rarely have to act.

19.9 Safety

• ⚠️ Never short-circuit a battery. Li-ion and lead-acid cells can deliver hundreds of amps, enough to melt wires, weld tools, and cause burns or fire. Tape the terminals of loose cells, and never carry them loose with coins or keys.
• ⚠️ Never charge damaged, swollen, dented, or deeply over-discharged lithium cells.
• ⚠️ Charge lithium packs on a non-flammable surface, and don't leave them charging unattended for long periods.
• ⚠️ "Thermal runaway": a damaged or abused Li-ion cell can heat itself uncontrollably, venting flammable gas and flame. It's hard to extinguish. Get clear and use lots of water for small cells.
• ⚠️ Lead-acid batteries release hydrogen when charging. Ventilate them, and avoid sparks near the terminals.
• Alkaline leakage corrodes devices. Remove batteries from equipment in storage.
• Button and coin cells are extremely dangerous if swallowed, especially by children. They can burn through tissue in hours.
• Recycle rechargeable and lithium batteries at designated drop-offs, never in household trash (they cause fires in garbage trucks and recycling plants).

19.10 Did you know?

• Alessandro Volta built the first battery (the "voltaic pile") in 1800: stacked zinc and copper discs separated by brine-soaked cloth. The volt is named after him.
• The lead-acid battery (1859) is still the most widely used rechargeable chemistry by sheer volume.
• The 2019 Nobel Prize in Chemistry went to Goodenough, Whittingham, and Yoshino for developing the lithium-ion battery.
• A car battery can deliver 500+ A for starting, which is why jump leads are so thick.
• The "Baghdad Battery," a 2,000-year-old clay jar with a copper cylinder and iron rod, is sometimes claimed to be an ancient battery. Most archaeologists are skeptical.

19.11 Testing batteries

⚠️ Never measure a battery with the meter in current (amps) mode. That's a direct short through the meter.

19.12 How batteries fail

20 — Voltage regulators

Electronic circuits need a steady, specific voltage (3.3 V, 5 V, 12 V), no matter how the supply varies or how much current the load draws. A voltage regulator provides it. There are two fundamentally different approaches: linear regulators burn off excess voltage as heat, and switching regulators convert it efficiently using an inductor or capacitors.

20.1 Linear regulators

How they work

A linear regulator is a transistor acting as an automatically adjusted resistor between input and output. An internal circuit compares the output with a precise reference voltage and continuously adjusts the transistor to hold the output exactly on target.

• Output current ≈ input current. The regulator simply drops the excess voltage.
• Wasted power = (V_in − V_out) × I. All of it becomes heat.
• Efficiency ≈ V_out ÷ V_in. 5 V → 3.3 V is 66 %. 12 V → 5 V is 42 %.

Key specs

Common parts

Heat math

A 7805 fed 12 V supplying 500 mA wastes (12 − 5) × 0.5 = 3.5 W. A bare TO-220 package (~50–65 °C/W) would rise ~200 °C, so it needs a heatsink or, better, a switching regulator. Rule of thumb without a heatsink: TO-220 ≤ ~1 W, SOT-223 ≤ ~0.5–1 W (with copper area), SOT-23 ≤ ~0.2–0.3 W.

Most linear regulators have thermal shutdown. When too hot they turn off, cool, and restart, which shows up as a device that cuts out after running a while.

When linear is the right choice

• The voltage difference and current are small (e.g. 5 V → 3.3 V at 100 mA).
• Low noise matters: audio, precision analog, RF, sensors.
• Simplicity, tiny size, and cost matter.
• As a clean "post-regulator" after a switching supply.

20.2 Switching regulators

How they work

A switching regulator ("switcher," "SMPS," "DC-DC converter") rapidly switches a transistor fully on and off, typically 100 kHz–3 MHz, and uses an inductor and capacitors to store and release energy in controlled bursts. The ratio of on-time to off-time (the duty cycle) sets the output voltage, and a feedback loop adjusts it continuously.

Because the transistor is always fully on or fully off, it wastes little power. Efficiency is typically 80–95 %, regardless of the voltage ratio.

Power in ≈ power out ÷ efficiency. So:

• A buck (step-down) converter draws less input current than it outputs. 12 V → 5 V at 2 A (10 W) at 90 % draws about 0.93 A from 12 V.
• A boost (step-up) converter draws more input current than it outputs. 3.7 V → 5 V at 1 A (5 W) at 85 % draws about 1.6 A from the battery.

Topologies

Key specs

Using switching modules well

• Set adjustable modules before connecting the load. They ship at random voltages. A boost module can output 20–30 V straight out of the bag. Connect input power, measure the output with a meter, turn the trimmer (often multi-turn: it may take 10+ turns before anything changes), then connect the load.
• Keep the input and output capacitors close, and keep high-current loops short.
• Mind the input voltage range (e.g. many buck modules need ~1.5–3 V more input than output).
• Current ratings on cheap modules are optimistic. Assume about half the headline figure for continuous use without extra cooling.
• Add an LDO after a switcher for noise-sensitive circuits, or add LC filtering.

Noise and layout

The fast switching produces electrical noise (EMI) and output ripple:

• Audible whine or buzz at light loads (burst mode, or inductor vibration).
• Interference with radios, audio, or sensitive analog measurements.
• Remedies: good layout, short loops, a solid ground plane, input and output filtering, ferrite beads, shielded inductors, spread-spectrum clocking, distance from sensitive circuits.