Glossary

A classic chip that makes timed pulses or a steady oscillation using resistors and a capacitor to set the timing.

Related: Astable. Learn more

On I2C, the receiver pulls SDA low in the ninth clock to acknowledge a byte (ACK). Leaving it high is a NACK: no device, or “no more, thanks”.

Related: I2C. Learn more

Current that keeps reversing direction, usually smoothly as a sine wave. Mains electricity is AC at 50 or 60 Hz.

Related: Direct current (DC), Frequency, RMS (root mean square). Learn more

The unit of current. One ampere is one coulomb of charge passing a point every second. Small circuits use milliamps (mA): thousandths of an amp.

Related: Current. Learn more

The part of a microcontroller that turns a voltage into a number. A 10-bit ADC gives 0 to 1023 across its reference voltage.

Related: Microcontroller, Voltage divider. Learn more

The terminal of a diode or LED that current flows into. On an LED it is the longer leg; it goes towards +.

Related: Cathode, Diode, LED (light-emitting diode). Learn more

A circuit with no stable state, so it keeps switching back and forth on its own: an oscillator. The 555 Blinky is a 555 astable.

Related: 555 timer, Frequency. Learn more

The voltage a spinning motor generates, opposing its supply. It rises with speed, so a fast, lightly loaded motor draws little current.

Related: Stall current. Learn more

The control terminal of a bipolar transistor. A small current into the base lets a larger current flow from collector to emitter.

Related: Transistor, Collector, Emitter. Learn more

A source of voltage made from one or more chemical cells. Its voltage falls a little under load because of its internal resistance.

Related: Voltage, Internal resistance. Learn more

How much charge a battery holds, in milliamp-hours (mAh): 2000 mAh can supply 20 mA for about 100 hours. It falls at high currents and in the cold.

Related: Battery. Learn more

How many symbols per second a serial line sends; for a UART, bits per second. Both ends must use the same rate, such as 9600 or 115200.

Related: Serial (UART). Learn more

A chart of a circuit’s gain (in decibels) against frequency on a logarithmic scale, showing at a glance which frequencies pass.

Related: Decibel (dB), Filter. Learn more

A board for building circuits without soldering. Holes in each short column are joined by metal strips; the long rails carry power.

Related: Supply rail. Learn more

A switching regulator that steps a voltage down by switching on and off very fast and smoothing the result. Typically 80–95% efficient, so it wastes far less than a linear regulator.

Related: Voltage regulator, Efficiency. Learn more

How much charge a capacitor stores for each volt across it. Measured in farads; real parts are usually microfarads (µF) or nanofarads (nF).

Related: Capacitor, Time constant (τ). Learn more

A part that stores charge between two plates. It blocks steady DC once charged, passes changing signals, and smooths or times circuits.

Related: Capacitance, Electrolytic capacitor, Time constant (τ). Learn more

The terminal of a diode or LED that current flows out of. It is marked by a stripe on a diode and the shorter leg (and flat edge) on an LED.

Related: Anode, Diode, LED (light-emitting diode). Learn more

An SPI line, one per chip, held high when idle. The controller pulls a chip’s CS low to talk to it.

Related: SPI. Learn more

A complete loop that current can flow around, from a source, through parts, and back. Break the loop anywhere and the current stops.

Related: Open circuit, Short circuit. Learn more

A dull, grainy solder joint that did not get hot enough or moved while setting. It may work at first and fail later.

Related: Solder. Learn more

The terminal of an NPN transistor that the load current flows into, from the positive side of the circuit.

Related: Transistor, Base, Emitter. Learn more

A circuit that compares two voltages and sets its output high or low depending on which is bigger. An op-amp with no feedback works as one.

Related: Op-amp (operational amplifier), Hysteresis. Learn more

A material that lets current flow easily, such as copper. Metals are good conductors because their outer electrons move freely.

Related: Insulator. Learn more

The rate at which electric charge flows past a point, measured in amperes. In a series loop the same current flows through every part.

Related: Ampere (amp), Voltage, Ohm’s law. Learn more

The frequency where a filter’s output has fallen to 70.7% of its input (−3 dB). For a resistor and capacitor, fc = 1 ÷ (2π × R × C).

Related: Filter, Decibel (dB). Learn more

A logarithmic way to compare two signals: 20 × log₁₀ of the voltage ratio. −3 dB is about 70.7%, −20 dB is a tenth, +20 dB is ten times.

Related: Gain, Filter. Learn more

Fine copper braid that soaks up melted solder when heated on a joint, used to remove excess solder or bridges. Also called solder wick.

Related: Solder bridge. Learn more

A one-way valve for current: it conducts from anode to cathode, once the forward voltage (about 0.7 V for silicon) is reached, and blocks the other way.

Related: LED (light-emitting diode), Forward voltage, Flyback diode. Learn more

Current that flows in one direction only, as from a battery or a USB supply.

Related: Alternating current (AC). Learn more

How far a regulator’s input must stay above its output for it to keep regulating: about 2 V for a 7805, a fraction of a volt for a low-dropout (LDO) regulator.

Related: Voltage regulator. Learn more

The fraction of each cycle that a signal is on, as a percentage. A 25% duty cycle is on a quarter of the time.

Related: PWM (pulse-width modulation), Period. Learn more

The useful power out divided by the power in, as a percentage. The rest is wasted, usually as heat.

Related: Power, Buck converter. Learn more

A capacitor type with large values in a small can. It is polarised: the − leg (striped side) must go to the more negative side, or it can fail.

Related: Capacitor. Learn more

A tiny particle with negative charge. In wires, current is the drift of electrons, which actually move from − to +.

Related: Current. Learn more

The terminal of an NPN transistor that both the base and collector currents flow out of, usually to 0 V.

Related: Transistor, Base, Collector. Learn more

A circuit that lets some frequencies through and reduces others. A low-pass filter passes slow changes; a high-pass filter passes fast ones.

Related: Cutoff frequency, Decibel (dB). Learn more

A circuit that stores one bit. A D flip-flop copies its input to its output only at the moment its clock rises.

Related: Logic gate. Learn more

A chemical that cleans oxide off metal as solder melts, so the solder flows and bonds. It is usually inside the solder wire.

Related: Solder. Learn more

A diode across a coil (motor, relay, inductor) that gives its current a safe path when switched off, preventing a damaging voltage spike.

Related: Inductor, Diode. Learn more

The voltage a diode or LED drops when conducting: about 0.7 V for a silicon diode, about 2 V for a red LED and about 3 V for blue or white.

Related: Diode, LED (light-emitting diode). Learn more

How many times a signal repeats each second, measured in hertz (Hz). It is 1 divided by the period.

Related: Period, Alternating current (AC). Learn more

How many times bigger a circuit makes its output than its input. A non-inverting op-amp amplifier has gain 1 + Rf ÷ Rg.

Related: Op-amp (operational amplifier). Learn more

The point in a circuit chosen as 0 V, from which other voltages are measured. Often the battery’s − terminal; labelled GND.

Related: Voltage. Learn more

Four switches arranged around a motor in an H shape. Closing one diagonal pair drives it forwards and the other pair drives it backwards.

Related: Motor driver, Shoot-through. Learn more

A piece of metal, often with fins, attached to a hot part to carry heat away into the air, so the part runs cooler.

Related: Voltage regulator. Learn more

Using different switch-on and switch-off points so a circuit does not flicker when its input hovers near the threshold.

Related: Comparator. Learn more

A two-wire bus (SDA for data, SCL for clock) shared by many chips, each with its own 7-bit address. Both lines need pull-up resistors.

Related: Open-drain, ACK and NACK, SPI. Learn more

A coil of wire that resists changes in current by storing energy in a magnetic field. Switching it off suddenly causes a voltage spike.

Related: Flyback diode. Learn more

A material that hardly lets current flow, such as plastic, rubber or glass. Used to keep conductors apart and people safe.

Related: Conductor. Learn more

The small resistance inside a battery or supply. Under load it drops some voltage, which is why a worn battery reads fine unloaded but sags in use.

Related: Battery. Learn more

The current flowing into any junction equals the current flowing out. Charge does not pile up or vanish.

Related: Kirchhoff’s voltage law, Parallel. Learn more

Around any closed loop, the voltage rises add up to the voltage drops. In a series loop, the part voltages add up to the supply.

Related: Kirchhoff’s current law, Series. Learn more

A diode that gives out light when current flows through it the right way. It always needs a resistor (or other current limit) in series.

Related: Diode, Forward voltage, Anode. Learn more

A resistor whose resistance falls as light increases, from about a megohm in the dark to about a kilohm in bright light.

Related: Voltage divider, Sensor. Learn more

A tool that records several digital signals at once and shows them as highs and lows over time, often decoding the bytes on a bus.

Related: Oscilloscope. Learn more

A circuit that sets a digital output (1 or 0) from its inputs by a fixed rule, such as AND, OR, NOT, NAND or XOR.

Related: Truth table, Logic level. Learn more

The voltage ranges a digital chip reads as 0 (low) or 1 (high). For a 5 V 74HC chip: below about 1.5 V is 0, above about 3.5 V is 1.

Related: Logic gate, Pull-up resistor. Learn more

A small computer on one chip, with inputs and outputs for sensors and actuators, such as the ATmega328P on an Arduino Uno.

Related: Analogue-to-digital converter (ADC), PWM (pulse-width modulation). Learn more

A transistor switched by the voltage on its gate rather than a current. Above its threshold voltage, it conducts with a low on-resistance.

Related: Transistor. Learn more

A chip or module containing an H-bridge and its protection, so a microcontroller’s logic signals can control a motor’s direction and speed.

Related: H-bridge, PWM (pulse-width modulation). Learn more

A meter that measures voltage, current and resistance. Measure voltage across a part; measure current by putting the meter in the loop.

Related: Voltage, Current. Learn more

A point in a circuit where parts connect. Everything joined by wire (or a breadboard strip) is one node, at one voltage.

Related: Kirchhoff’s current law. Learn more

The unit of resistance, written Ω. 1 kΩ is 1000 Ω and 1 MΩ is a million ohms.

Related: Resistance, Ohm’s law. Learn more

Voltage equals current times resistance: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I.

Related: Voltage, Current, Resistance. Learn more

A chip that amplifies the difference between its two inputs by a very large factor. With feedback it makes precise amplifiers; without, a comparator.

Related: Gain, Comparator, Virtual ground. Learn more

A break in a circuit, so no current flows. A meter across the break reads the full supply voltage.

Related: Short circuit, Circuit. Learn more

An output that can only pull a line low or let it go; a pull-up resistor makes it high. Used by I2C so many chips can share a wire.

Related: Pull-up resistor, I2C. Learn more

An instrument that draws voltage against time, so you can see how a signal changes: its shape, size and frequency.

Related: Frequency, Period. Learn more

Parts connected across the same two points, so they share the same voltage and the current divides between them. Two equal resistors in parallel give half the resistance.

Related: Series, Kirchhoff’s current law. Learn more

The time one cycle of a repeating signal takes. Period = 1 ÷ frequency.

Related: Frequency, Duty cycle. Learn more

How far one waveform is behind or ahead of another, as an angle: a whole cycle is 360°. A low-pass filter at its cutoff delays a sine wave by 45°.

Related: Filter, Period. Learn more

A variable resistor with a sliding contact (the wiper). Turning it changes the voltage at the wiper between the two ends.

Related: Voltage divider. Learn more

The rate at which energy is used or delivered, in watts: P = V × I. Resistors turn it into heat, so they have power ratings.

Related: Voltage, Current. Learn more

A resistor that holds a digital input at high when nothing else drives it, so it never floats. A pull-down holds it low instead.

Related: Logic level. Learn more

Switching an output fully on and off quickly, and varying the on-time, to control the average power, for example to dim an LED.

Related: Duty cycle. Learn more

Diodes arranged to turn AC into DC that flows one way only. A bridge rectifier (four diodes) uses both halves of each AC cycle.

Related: Diode, Alternating current (AC), Ripple. Learn more

How strongly something opposes current, measured in ohms (Ω). More resistance means less current for the same voltage.

Related: Ohm, Ohm’s law, Resistor. Learn more

A part with a set resistance, used to limit current or divide voltage. Its value is shown by coloured bands.

Related: Resistance, Resistor colour code, Tolerance. Learn more

Coloured bands that give a resistor’s value: two digits, then a multiplier, then the tolerance. Brown, black, red, gold is 1 kΩ ± 5%.

Related: Resistor, Tolerance. Learn more

The small, repeating dip left on a smoothed DC supply as the capacitor discharges between peaks. Roughly I ÷ (f × C).

Related: Rectifier, Capacitor. Learn more

The DC-equivalent value of an AC voltage: the steady voltage that would heat a resistor the same amount. For a sine wave, RMS = peak ÷ √2.

Related: Alternating current (AC). Learn more

When a transistor is fully on, or an op-amp output has hit its limit, so the input no longer controls the output in proportion.

Related: Transistor, Op-amp (operational amplifier). Learn more

A circuit diagram using standard symbols. It shows what connects to what, not where the parts physically sit.

Related: Breadboard. Learn more

A part whose property changes with something in the world (light, temperature, pressure), turned into a voltage a circuit can read.

Related: Light-dependent resistor (LDR), Thermistor (NTC), Voltage divider. Learn more

A simple way for chips to send data one bit at a time over a wire. An Arduino’s Serial.println() uses it to talk to a computer over USB.

Related: Microcontroller. Learn more

Parts connected one after another in a single path, so the same current flows through each and their voltages add up.

Related: Parallel, Kirchhoff’s voltage law. Learn more

A motor with gears, a position sensor and its own driver. The width of a pulse sent about every 20 ms (roughly 1–2 ms) sets its angle.

Related: PWM (pulse-width modulation). Learn more

A short circuit in an H-bridge when both switches on one side are on at once, joining the supply straight to ground.

Related: H-bridge, Short circuit. Learn more

An accidental low-resistance path, often straight from + to −. It lets a large current flow, which can overheat wires and batteries.

Related: Open circuit. Learn more

A metal alloy that melts at around 180–230 °C, used to join component legs to copper pads. Most has a core of flux.

Related: Flux, Cold joint. Learn more

Solder that joins two pads or tracks that should be separate: a short circuit. Removed with desoldering braid.

Related: Short circuit, Desoldering braid. Learn more

A fast bus with a clock (SCK), data each way (MOSI and MISO) and a chip-select line for each chip. Data moves both ways at once.

Related: Chip select (CS), I2C. Learn more

The current a motor draws when it is not turning: only its coil resistance limits it, so it is many times the running current.

Related: Back-EMF. Learn more

A motor that turns in exact steps (often 200 per revolution), driven by energising its coils in sequence through a driver.

Related: Motor driver. Learn more

A wire or strip carrying the supply voltage (+) or 0 V (−) to the rest of a circuit. On a breadboard, the long red and blue lines.

Related: Breadboard. Learn more

A resistor whose resistance falls as it warms up (negative temperature coefficient). A common one is 10 kΩ at 25 °C.

Related: Voltage divider, Sensor. Learn more

For a resistor and capacitor, τ = R × C. After one time constant a charging capacitor reaches about 63% of the supply; after five, it is nearly full.

Related: Capacitor. Learn more

How far a part’s real value may be from its printed value, as a percentage. A 1 kΩ ± 5% resistor can be 950 Ω to 1050 Ω.

Related: Resistor, Resistor colour code. Learn more

A semiconductor part that lets a small signal control a larger current: as a switch or as an amplifier. The common types are BJTs and MOSFETs.

Related: Base, MOSFET, Saturation. Learn more

A table listing a logic circuit’s output for every combination of its inputs.

Related: Logic gate. Learn more

In an inverting op-amp amplifier, the − input is held at 0 V by feedback without being connected to ground.

Related: Op-amp (operational amplifier). Learn more

The unit of voltage. A typical AA cell gives 1.5 V; USB gives 5 V.

Related: Voltage. Learn more

The electrical “push” between two points: the energy given to each unit of charge, measured in volts. It is always measured between two points.

Related: Volt, Current, Ohm’s law. Learn more

Two resistors in series across a supply; the point between them gives a fraction of the supply: Vout = Vin × R2 ÷ (R1 + R2).

Related: Light-dependent resistor (LDR), Thermistor (NTC), Potentiometer. Learn more

A chip that turns a higher, varying voltage into a steady one, such as 5 V or 3.3 V. A linear regulator turns the extra voltage into heat.

Related: Dropout voltage, Buck converter, Heatsink. Learn more