Electronics glossary
Short, accurate definitions of the words you meet when you start building circuits and programming microcontrollers, with links to guides that use them.
Voltage
The difference in electric potential between two points, measured in volts (V). It is what pushes current round a circuit, and it is always measured between two points: when one is not named, it is ground.
A 9 V battery holds its positive terminal about 9 V above its negative one. An Arduino Uno’s pins switch between 0 V and 5 V; an ESP32’s between 0 V and 3.3 V.
Current
The rate at which electric charge flows, measured in amperes (A). One ampere is one coulomb of charge per second. Small circuits work in milliamps: an indicator LED typically takes 5 to 20 mA.
By convention current flows from positive to negative (conventional current), although the electrons in a wire move the other way. Current is the same at every point in a series loop.
Resistance
How strongly a material or part opposes current, measured in ohms (Ω). A copper wire has very little; a resistor has a chosen, stable amount; an insulator has so much that almost no current flows.
Ohm’s law
For a resistor, voltage equals current times resistance: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I. If 3 V is across a 330 Ω resistor, the current is 3 ÷ 330 = 9.1 mA.
It holds for resistors and wires, not for LEDs, diodes or transistors, whose current is not proportional to their voltage.
Power
The rate at which energy is converted, measured in watts (W). For any part, P = V × I; for a resistor this can also be written P = I² × R or P = V² ÷ R.
Every part has a power rating. A common through-hole resistor is rated 0.25 W: a resistor dropping 7 V at 15 mA dissipates 105 mW, inside that, while one dropping 12 V at 50 mA (0.6 W) would overheat.
Ground
The reference point that other voltages in a circuit are measured from, defined as 0 V, usually the negative terminal of the supply. Symbols labelled GND are all connected together.
Separate boards or modules that exchange signals must share a ground, or their voltages have no common reference and signals are read wrongly.
Short circuit
An unintended low-resistance path, most often straight across a supply. With almost no resistance, Ohm’s law allows a very large current, which can overheat wires, drain or damage batteries, and destroy parts.
On a breadboard a short is often two legs in the same row, or a stray wire between the power rails. A fuse or a current-limited bench supply protects against it.
Voltage divider
Two resistors in series across a supply, with the output taken from the point between them. The output is Vout = Vin × R2 ÷ (R1 + R2), where R2 is the resistor to ground. With 9 V, R1 = 1 kΩ and R2 = 2 kΩ, the output is 6 V.
Dividers scale a voltage down for an analogue input, or turn a varying resistance (a photoresistor or thermistor) into a voltage. They are not power supplies: drawing current from the output changes its voltage.
Resistor
A component with a fixed resistance, used to limit current, divide voltage, or pull an input high or low. Through-hole resistors show their value in coloured bands: yellow, violet, brown is 47 × 10 = 470 Ω.
A resistor’s value has a tolerance (commonly ±5% or ±1%) and it has a power rating that must not be exceeded.
Capacitor
A component that stores energy in an electric field between two conductors, measured in farads (F); practical values run from picofarads to millifarads. The voltage across a capacitor cannot change instantly: current is i = C × dv/dt.
Capacitors smooth supplies, decouple chips (a 100 nF capacitor beside each chip’s supply pins is standard), block DC while passing AC signals, and set timing with a resistor: an RC circuit has a time constant τ = R × C. Electrolytic capacitors are polarised and must be fitted the right way round.
Inductor
A coil that stores energy in a magnetic field, measured in henries (H). The current through an inductor cannot change instantly: the voltage across it is v = L × di/dt.
Inductors filter and store energy in switching power supplies. Coils in relays and motors are inductive too, which is why switching them off produces a voltage spike that needs a flyback diode.
Diode
A semiconductor part that lets current flow from its anode to its cathode and blocks it the other way. When conducting, a silicon diode drops about 0.6 to 0.7 V, and a Schottky diode less.
Diodes protect against reversed supplies, rectify AC to DC, and absorb the spike from relay and motor coils. The cathode is marked with a band.
LED
A light-emitting diode: a diode that gives off light when current flows from anode to cathode. Its forward voltage depends on colour, roughly 1.8 to 2.2 V for red and 3.0 to 3.3 V for blue and white.
An LED must have its current limited, normally by a series resistor: R = (supply voltage − forward voltage) ÷ current. The longer leg is the anode; the cathode is by the flat on the rim.
Transistor
A semiconductor part that uses a small signal to control a larger current, as a switch or an amplifier. A bipolar junction transistor (BJT) has a base, collector and emitter: a small base current allows a collector current many times larger (the current gain, often 100 or more).
A common use is letting an Arduino pin switch a load that needs more current than the pin can give, such as a relay coil or a small motor.
MOSFET
A field-effect transistor controlled by the voltage on its gate rather than a current. An N-channel MOSFET conducts from drain to source when its gate is above the source by more than its threshold voltage, and draws almost no gate current in steady state.
For switching from a microcontroller, choose a logic-level MOSFET whose on-resistance is specified at the gate voltage you have (5 V, or 3.3 V for an ESP32). Add a resistor from gate to source so it stays off while the pin is not driven.
Relay
A switch operated by an electromagnet: current in the coil pulls a contact over, so a low-voltage circuit can switch a separate, often higher-power one with no electrical connection between them.
A microcontroller pin cannot drive a relay coil directly. Use a transistor or a relay module with a driver, and a flyback diode across the coil.
Potentiometer
A resistor with a third terminal, the wiper, that slides along the track. With the ends on 5 V and ground, the wiper gives any voltage in between, which makes it the usual knob for an analogue input.
Used with only the wiper and one end, it is a variable resistor (a rheostat).
Breadboard
A reusable board for building circuits without soldering. Each numbered row is two groups of five connected holes (a to e and f to j), split by a centre channel that chips straddle, and the long rails along the edges carry power.
Two legs in the same group of five are connected; legs in different rows are not.
Servo motor
A motor with built-in gears, a position sensor and a control circuit, which turns its output shaft to an angle set by a pulse. A hobby servo expects a pulse every 20 ms whose width, nominally 1 to 2 ms (many servos accept about 0.5 to 2.5 ms), sets the angle across its range, typically about 180 degrees.
On an Arduino the Servo library generates the pulses: servo.write(90) moves to the middle. Power servos from a supply that can deliver their stall current, not from a pin.
Stepper motor
A motor that turns in fixed steps as its coils are energised in sequence, commonly 200 steps per revolution (1.8 degrees per step). It holds its position without a sensor, as long as it is not asked for more torque than it has.
Steppers need a driver (such as an A4988 or a ULN2003 for small unipolar motors); the microcontroller sends step and direction signals to the driver.
Microcontroller
A small computer on one chip: a processor, program memory (flash), working memory (RAM) and input/output peripherals such as GPIO, timers, an ADC and serial ports. It runs one program, starting when it powers up.
The Arduino Uno uses an ATmega328P (8-bit, 16 MHz, 32 KB flash, 2 KB RAM); the ESP32 is a much faster 32-bit chip with Wi-Fi and Bluetooth.
GPIO
General-purpose input/output: a microcontroller pin that software can set as an input, to read HIGH or LOW, or as an output, to drive HIGH or LOW. In Arduino code this is pinMode, digitalRead and digitalWrite.
Every GPIO has limits: a maximum voltage (5 V on the Uno, 3.3 V on the ESP32) and a maximum current, about 20 mA recommended per pin on the Uno. Some pins have extra jobs, such as serial, I2C or boot configuration.
Pull-up resistor
A resistor from an input to the supply voltage, so the input reads HIGH when nothing else drives it. A button from the input to ground then pulls it LOW when pressed. Without a pull-up or pull-down, an unconnected input floats and reads at random.
Most microcontrollers have pull-ups built in: in Arduino code, pinMode(pin, INPUT_PULLUP). I2C lines need pull-ups too, often 4.7 kΩ.
Pull-down resistor
A resistor from an input to ground, so the input reads LOW when nothing else drives it. A button from the input to the supply then makes it read HIGH when pressed. 10 kΩ is a common value.
The Arduino Uno has no internal pull-downs, so a pull-down must be an external resistor; some chips, including the ESP32 on most pins, have both.
PWM
Pulse-width modulation: switching a pin fully on and off quickly and varying the fraction of time it is on (the duty cycle). Because the eye averages an LED’s flicker and a motor’s inertia smooths its pulses, a 25% duty cycle acts like a quarter of the power.
On the Arduino Uno, analogWrite(pin, value) sets a duty cycle from 0 to 255 on pins 3, 5, 6, 9, 10 and 11, at about 490 Hz (980 Hz on pins 5 and 6). It is still a digital signal: to get a true analogue voltage it must be filtered.
ADC
An analogue-to-digital converter: it measures a voltage and returns a number. A 10-bit ADC, as on the Arduino Uno, gives 0 to 1023 across 0 V to its 5 V reference, about 4.9 mV per step; the ESP32’s 12-bit ADC gives 0 to 4095.
In Arduino code this is analogRead. The input must stay within 0 V and the reference voltage.
I2C
Inter-Integrated Circuit: a two-wire bus, with a data line (SDA) and a clock line (SCL), that lets one controller talk to many devices, each with its own 7-bit address. Both lines need pull-up resistors. Standard speeds are 100 kHz and 400 kHz.
Many sensors, displays and real-time clocks use I2C. On the Uno, SDA is A4 and SCL is A5; on the ESP32 the defaults are GPIO 21 and 22. Arduino’s Wire library drives it.
SPI
Serial Peripheral Interface: a fast synchronous bus with a clock (SCK), data out (MOSI, also called COPI), data in (MISO, also CIPO) and a chip-select line for each device. It is full duplex and runs at several megahertz.
SD cards, TFT displays and many radios use SPI. On the Uno, MOSI is D11, MISO D12 and SCK D13.
UART
Universal asynchronous receiver-transmitter: serial communication over two lines, TX (transmit) and RX (receive), with no clock. Both ends must agree on the speed (the baud rate, such as 9600 or 115200) and format; TX on one device connects to RX on the other.
Arduino’s Serial is a UART, and on the Uno it is also how the board talks to the computer over USB, on pins D0 and D1.
PCB
A printed circuit board: an insulating board, usually FR-4 fibreglass, with copper tracks that connect the parts in place of wires. A two-layer board has copper on both sides, joined by plated holes called vias.
Designing one means placing footprints, routing tracks, checking them against the maker’s design rules and exporting manufacturing files.
Gerber file
The standard file format for sending a PCB to be made. Each layer (top copper, bottom copper, solder mask, silkscreen, board outline) is a separate Gerber file; drill holes usually go in a separate Excellon file. Gerber X2 adds attributes that describe each layer’s purpose.
Board makers ask for the Gerber and drill files together, normally in one zip.
SPICE
Simulation Program with Integrated Circuit Emphasis: the circuit simulator first released at the University of California, Berkeley, in the early 1970s, and the family of simulators descended from it, such as ngspice and LTspice. It solves a circuit described as a netlist using nodal analysis and Newton iteration, and offers DC, AC (frequency response) and transient analyses.
Manufacturers publish SPICE models of their parts, which is what makes SPICE the standard tool for analogue design.