Electronics glossary

555 timer

A classic chip that makes timed pulses and blinking signals.

The 555 charges and discharges a capacitor through resistors and flips its output each time the capacitor voltage crosses 1/3 and 2/3 of the supply. The resistor and capacitor values set the timing.

AC

Alternating current — keeps reversing direction.

AC voltage swings back and forth, usually as a smooth sine wave. Wall sockets are AC (230 V or 120 V — dangerous!).

Amplifier class

How long the output transistors are on during each wave: A, B, AB, C or D.

The class of a power amplifier says how its output transistors share the work. Class A keeps one on all the time, class B and AB split the wave between two, class C only pulses at the peaks and class D switches fully on and off. It trades sound quality against wasted heat.

Class A is a car with the engine always revving; class D is a dimmer flicking a light on and off too fast to see.

Battery

A store of chemical energy that gives a steady voltage.

A battery pushes charge from its + terminal round the circuit to its − terminal. Real batteries have a small internal resistance, so their voltage sags when you draw a lot of current.

A water pump powered by a slowly emptying fuel tank: steady pressure until the fuel runs low.

Bias

The steady DC that sets where a transistor rests before any signal arrives.

Biasing sets a transistor's resting current and voltage (the Q-point) with a few resistors, so a signal can swing it up and down without running into fully off or fully on.

Parking a swing half-way up so it can move freely both ways.

BJT (bipolar transistor)

A transistor where a small base current controls a bigger collector current.

A BJT has three pins: base, collector and emitter. A small current into the base lets a current about 100 times bigger flow from collector to emitter. It is the classic transistor used for switching LEDs and relays and for amplifying signals.

A small hand on a lever that lifts a heavy gate: a little effort (base current) controls a big flow (collector current).

Buzzer / speaker

Turns an electrical signal into sound.

A speaker moves a cone with a coil and a magnet; a piezo buzzer bends a ceramic disc. Feeding them a changing signal makes them vibrate the air, and the frequency sets the pitch.

Capacitorfarad (F) · 1 µF = 0.000001 F

Stores a little charge, like a tiny rechargeable tank.

A capacitor charges up when voltage is applied and gives the charge back later. It blocks steady (DC) current but lets changes pass. Used to smooth supplies and set timing.

A water tank with a rubber membrane across it: water pushes in and stretches it, but cannot flow straight through.

Chargecoulomb (C)

The "stuff" that flows — carried by electrons in metal wires.

Electric charge is carried by tiny particles (electrons) that are already present in every wire. A battery does not supply new electrons; it pushes the ones that are already there around the loop.

Circuit

A closed loop that current can flow around.

A circuit is a complete path from one side of a power source, through some parts, and back to the other side. Break the loop anywhere and current stops everywhere in that loop.

Crystal (quartz)

A sliver of quartz that vibrates at one very precise frequency.

Quartz bends slightly when a voltage is applied and makes a voltage when it bends. Cut to size, it rings at an exact frequency, which clock circuits use to keep accurate time.

A tuning fork for electronics.

Currentampere, amp (A) · 1 mA = 0.001 A

How much electric charge flows past a point each second.

Current is the flow of charge through a wire or part. It needs a complete loop to flow. The same current that leaves the battery comes back to it — current is not used up along the way.

Like litres of water per second through a pipe. The water is not consumed by a water wheel; it just loses pressure.

Current source

A source that pushes a fixed current, whatever it takes.

A current source adjusts its voltage so a set current flows. LED drivers and battery chargers often behave like this.

Datasheet

The manufacturer’s document of what a part can do.

Every part has a datasheet listing its pins, typical behaviour and absolute maximum ratings. Going beyond those ratings can destroy the part.

DC

Direct current — flows steadily in one direction.

DC is what batteries give: a steady voltage that does not change direction.

DC motor

Turns current into spinning motion using magnets and coils.

Current through the motor’s coils makes a magnetic field that pushes against permanent magnets, turning the shaft. Reverse the current and it spins the other way. A motor is also an inductor, so it needs a flyback diode.

Derating

Using a part well below its maximum, for safety.

A resistor rated 100 mW is usually given no more than about 50 mW in a real design. Cooler parts last much longer.

Diode

A one-way valve for current.

A diode lets current flow in one direction (anode → cathode) and blocks it in the other. It needs about 0.6–0.7 V of push before it starts to conduct.

A one-way flap valve in a pipe.

Energyjoule (J) · 1 W for 1 s = 1 J

The ability to do work — what a battery actually stores.

A battery stores energy, not current. As charge flows round the circuit it carries energy from the battery to the parts, which turn it into light, heat or motion. Power is how fast that happens.

Frequencyhertz (Hz) · 1 kHz = 1000 Hz

How many times per second something repeats.

Frequency counts cycles per second. 1 kHz means a thousand repetitions every second.

Fuse

A deliberate weak link that melts to stop too much current.

A fuse is a thin wire that heats up and melts if the current is too high, opening the circuit before wires or parts catch fire. A PTC "resettable fuse" does the same job and recovers once it cools.

A safety valve that bursts before the pipe does.

GPIO pin

A microcontroller pin that a program can set HIGH or LOW.

General-purpose input/output pins can be outputs (switched to the supply voltage or to 0 V) or inputs (reading whether a voltage is high or low).

Ground (GND)

The reference point we call 0 V.

Voltages are always measured between two points. Ground is the point everyone agrees to call zero, usually the battery’s minus side. "This wire is at 5 V" means 5 V above ground.

Like sea level for heights: a mountain is 1000 m above sea level.

Heat & temperature

Every part turns some power into heat.

The more power a part dissipates, the hotter it gets. Above its rating the temperature keeps climbing until the part fails — in this app it glows, smokes and burns open.

Inductorhenry (H) · 1 µH = 0.000001 H

A coil of wire that resists changes in current.

An inductor stores energy in a magnetic field. It tries to keep current flowing at the same rate. Stop the current suddenly and it produces a big voltage spike.

A heavy water wheel: hard to start, and hard to stop once spinning.

Integrated circuit (chip)

A whole circuit squeezed into one package.

A chip (IC) contains many transistors and other parts. Its datasheet tells you what each pin does and the limits you must not exceed.

Kirchhoff’s current law

Current into a junction = current out of it.

Charge cannot pile up in a wire, so whatever current flows into a junction must flow out again. This is why current is never "used up".

Kirchhoff’s voltage law

Going round any loop, the voltages add up to zero.

The push the battery gives is exactly shared out between the parts in the loop. The voltage "drops" across each part add up to the supply voltage.

LED

A diode that gives off light when current flows through it.

An LED (light-emitting diode) lights up when current flows forwards. It needs a series resistor to limit the current, or it will burn out. Most small LEDs like 5–20 mA.

A one-way valve that glows when water rushes through it.

Light sensor (LDR)

A resistor whose resistance falls as light gets brighter.

A light-dependent resistor (LDR) is a few MΩ in the dark and only a few hundred ohms in bright light. Put it in a voltage divider and the voltage tells you how bright it is.

Load

Whatever the circuit is powering.

The load is the thing that uses the energy: a lamp, a motor, a chip. Connecting a load draws current, which can pull the source voltage down a little.

Logic gate

A tiny circuit that makes a yes/no decision from yes/no inputs.

Logic gates (NOT, AND, OR, NAND, XOR …) take HIGH or LOW inputs and give a HIGH or LOW output by a fixed rule. They are built from transistors, and every processor is made of millions of them.

Microcontroller (MCU)

A tiny computer on one chip, like the one in an Arduino.

A microcontroller runs a program and reads or drives its pins. Its pins can only supply small currents (a few mA to ~20 mA), so bigger loads need a transistor.

MOSFET

An electronic switch controlled by voltage.

A MOSFET is a transistor: a small voltage on its gate pin turns on a path between drain and source that can carry a big current. It is how microcontrollers switch motors and lights.

A tap: a light touch on the handle (gate) controls a big flow of water.

Multimeter (DMM)

Measures voltage, current and resistance.

To measure voltage, touch the probes across a part. To measure current, the meter must be in the loop, so the current flows through it. The DMM cards in this app show live readings.

Multivibrator

A circuit that flips between two states: astable, monostable or bistable.

Two switching stages wired so that each one controls the other. Depending on how they are coupled, the circuit keeps flipping by itself (astable), gives one timed pulse when triggered (monostable) or stays in whichever state it was last set to (bistable).

Astable is a metronome, monostable is a stairwell light timer, bistable is a light switch.

Node

A set of connected wires — all at the same voltage.

Wires that are directly connected form one node and share one voltage. Hover any wire in the schematic and its whole node lights up.

Ohm's law

U = R × I — the most-used equation in electronics.

Voltage equals current times resistance. Know any two, and you can work out the third: I = U / R, R = U / I. It holds for resistors and is a good first estimate almost everywhere.

Op-amp

An amplifier chip that makes a small voltage bigger.

An operational amplifier compares its two inputs and drives its output to make them equal, using the resistors around it. Those resistors set how much it amplifies.

A very strong servant that copies whatever you point at, as hard as needed, until its two inputs match.

Open circuit

A break in the loop — no current can flow.

An open circuit is a gap: an open switch, a cut wire or a burned-out part. The voltage is still there, but with no path the current is zero.

Optocoupler

An LED and a light sensor sealed together, to pass a signal across a gap.

An optocoupler sends a signal as light from an LED to a phototransistor inside one package. The two sides share no wires, so it keeps high voltages or noise on one side from reaching the other.

Signalling with a torch across a river instead of building a bridge.

Oscillator

A circuit that makes its own steady wave from DC.

An oscillator is an amplifier whose output is fed back to its own input through a network that only lets one frequency come back in step. Any tiny noise at that frequency grows each trip round the loop until the amplifier limits it.

A microphone held near its own speaker: the howl picks one note and stays loud.

Oscilloscope

Draws a graph of voltage against time.

An oscilloscope (scope) shows how a voltage changes over time, which a multimeter is too slow to show. Horizontal = time, vertical = voltage.

Parallel

Side by side, each across the same two points.

Parts in parallel all get the same voltage. The current splits between them and adds back together afterwards. Adding a parallel resistor always lowers the total resistance.

PCB

Printed circuit board — the green board that holds the parts.

A PCB has copper traces instead of wires. Switch to the SMD Footprint view to see this circuit as a real board.

PCB trace

A flat copper "wire" printed on a circuit board.

Traces are thin strips of copper on a circuit board. They have a small resistance, so a trace that is too narrow for its current gets hot.

Photodiode

A diode that makes a small current when light hits it.

Light knocks electrons loose in the junction of a photodiode, so a current flows that is proportional to the light. It reacts much faster than an LDR.

Potentiometer

An adjustable resistor — a knob or slider.

A potentiometer is a resistor with a sliding contact. Volume knobs are potentiometers.

A tap on a pipe with a side outlet: turning it chooses how much of the pressure reaches the outlet.

Powerwatt (W) · 1 mW = 0.001 W

How fast energy is being used or turned into heat.

Power is voltage × current (P = U·I). In a resistor all of that power becomes heat. Every part has a power rating — exceed it and the part overheats and fails.

Like how hard a water wheel is working: high pressure and lots of flow together give lots of power.

Power supply

A source that holds a set voltage.

A voltage source keeps a fixed voltage between its terminals and supplies whatever current the circuit asks for (up to its limit).

Pull-up resistor

A resistor that gently holds a wire HIGH when nothing drives it.

An input pin that is not connected to anything floats and reads random values. A pull-up resistor to the supply gives it a default HIGH level that a button or chip can pull LOW.

Push button

A switch that is only closed while you hold it.

A push button (momentary switch) connects while pressed. Real buttons "bounce" — they flicker on and off for a few milliseconds when pressed.

PWM

Switching on and off very fast to give an "average" level.

Pulse-width modulation dims LEDs and controls motor speed by switching fully on and off many times a second. The fraction of time it is on (the duty cycle) sets the average.

Rating

The most a part can safely take (voltage, current, power, heat).

Ratings are limits from the datasheet. Engineers usually stay at or below half the power rating so parts run cool and last longer — this is called derating.

Relay

A switch flipped by an electromagnet.

A relay has a coil and a set of switch contacts. A small current through the coil pulls the contacts over, so a weak signal can switch a big or mains-voltage load that is completely separate from it.

Ringing a doorbell that wakes someone to throw a heavy lever for you.

Resistanceohm (Ω) · 1 kΩ = 1000 Ω

How strongly something opposes current.

Resistance sets how much current flows for a given voltage (Ohm’s law: I = U / R). Higher resistance means less current. The energy lost in a resistance turns into heat.

Like a narrow section of pipe: the narrower it is, the less water gets through for the same pressure.

Resistor

Limits how much current flows; turns the excess into heat.

A resistor has a fixed resistance. It is used to limit current (for example to protect an LED) or to set voltages. It gets warm in proportion to the power it handles.

A narrow section of pipe.

Schmitt trigger

A switch with two thresholds, so slow or noisy inputs flip it cleanly once.

A Schmitt trigger switches HIGH when the input rises past an upper threshold and only switches back when it falls below a lower one. The gap between them (hysteresis) stops noise from making the output chatter.

Series

One after another, in a single path.

Parts in series share one path, so the same current flows through all of them. Their voltages add up to the supply voltage. Resistances in series add: R = R1 + R2.

Short circuit

A path with almost no resistance — huge current flows.

A short circuit connects the two sides of a source with (almost) nothing in between. Current is limited only by the wires and the battery itself, so things get very hot very fast.

Signal source

A source whose voltage changes over time (wave or pulses).

A signal source produces a changing voltage — a smooth sine wave (AC) or on/off pulses. Use the oscilloscope to see its shape.

SMD

Surface-mount device — tiny parts soldered flat onto a board.

Most modern parts are surface-mount. Size codes like 0402 or 0805 give the length and width (0805 = 0.08 × 0.05 inches). Smaller parts can handle less power.

Switch

Opens or closes the loop. Click it in the schematic!

A closed switch is a wire; an open switch is a gap. Opening it stops current everywhere in that loop at once.

Thermistor / PTC fuse

A resistor whose resistance changes with temperature.

Some thermistors rise sharply in resistance when they get hot. That makes them a self-resetting fuse: too much current heats them up and they choke the current down.

Time constant (τ)

How long a capacitor or inductor takes to mostly charge.

For a resistor and capacitor, τ = R × C. After one τ a capacitor is about 63 % charged; after five τ it is basically full.

Transformer

Two coils on one core that change AC voltage up or down.

A transformer passes AC power from one coil to another through a shared magnetic core. The ratio of turns sets the ratio of voltages. It only works with AC (changing) current, and it isolates the two sides from each other.

Two gears of different sizes: speed (voltage) goes one way, force (current) the other.

Transistor

A part that lets a small signal control a big one.

Transistors are electronic switches and amplifiers. Every chip contains millions or billions of them. The MOSFET is the most common kind.

TVS diode

A surge protector that clamps short voltage spikes.

A TVS (transient voltage suppressor) diode does nothing in normal use. When a spike, such as static electricity, arrives it conducts hard and dumps the energy safely to ground.

A lightning conductor: idle almost always, but it takes the hit when a spike arrives.

Voltagevolt (V)

The electrical "push" between two points.

Voltage is the difference in electrical pressure between two points. It is what pushes current through a circuit. A voltage is always measured between two places — usually from a point to ground. In formulas its letter is U (US books and datasheets often use V for both the letter and the unit).

Like water pressure: a taller water tank pushes harder. No pressure difference, no flow.

Voltage divider

Two resistors in series that make a smaller voltage.

Two resistors in series split the supply voltage in proportion to their sizes. The point between them is a lower voltage. It is the standard way to scale a voltage down for a sensor input.

Voltage regulator

Turns a higher, messy voltage into a steady lower one.

A regulator keeps its output at a fixed voltage (for example 3.3 V) even when the input or the load changes. Linear regulators burn the extra as heat; switching (buck) regulators are much more efficient.

A pressure-reducing valve on a water main: high, varying pressure in, steady lower pressure out.

Zener diode

A diode built to conduct backwards at a set voltage.

A Zener diode blocks reverse voltage up to its rated value, then conducts. That clamps the voltage, so it is used as a simple voltage limiter.

A pressure-relief valve that opens at an exact pressure and holds it there.