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Electronic Components β€” Complete Guide & Cheat Sheet

Last reviewed: 2026-09-17

[!TIP] The summary table at the end is designed as a printable one-page cheat sheet.

Overview

Every electronic circuit β€” from an ESP32 sensor node to a 10 kW solar inverter β€” is built from a small set of fundamental components. This guide explains the most common ones: what they do, how to identify them, how to read their markings, and how to test them with a multimeter.

Components fall into two broad families:

  • Passive components β€” cannot amplify or switch; they only consume, store, or transfer energy (resistors, capacitors, inductors, transformers, fuses).
  • Active components β€” can amplify or switch signals using an external power supply (diodes, transistors, MOSFETs, op-amps, regulators, ICs).

Passive Components

Resistor (R)

Limits current flow and sets voltage levels. The most common component in any circuit.

  • Unit: Ohm (Ξ©). Common ranges: 1 Ξ© to 10 MΞ©.
  • Markings: axial resistors use 4- or 5-band color code; SMD resistors use 3- or 4-digit codes (e.g. 472 = 47 Γ— 10Β² = 4.7 kΞ©).
  • Power rating: 1/8 W, 1/4 W, 1/2 W, 1 W… determines physical size. Power dissipated: P = IΒ² Γ— R.
  • Variants: potentiometer (adjustable, 3 legs), trimpot, thermistor (temperature-sensitive), LDR (light-sensitive), varistor/MOV (voltage spike protection).

[!TIP] 4-band resistor color code Black 0, Brown 1, Red 2, Orange 3, Yellow 4, Green 5, Blue 6, Violet 7, Grey 8, White 9 β€” then multiplier band (same colors = Γ—10ⁿ), then tolerance (gold = Β±5%, silver = Β±10%). Example: Red–Red–Orange–Gold = 22 Γ— 10Β³ = 22 kΞ© Β±5%.

Capacitor (C)

Stores energy in an electric field. Used for filtering, decoupling, timing, and smoothing.

  • Unit: Farad (F) β€” practically Β΅F, nF, pF.
  • Polarized types: electrolytic (aluminium) and tantalum β€” must be wired the right way round (longer leg = +). Reversed polarity = explosion risk.
  • Non-polarized types: ceramic, film, mica β€” safe either way.
  • Markings: electrolytic β†’ value + voltage (e.g. 100Β΅F 25V); ceramic β†’ 3-digit code (e.g. 104 = 100 nF); film β†’ direct value.
  • Key behavior: blocks DC, passes AC. Voltage rating must exceed circuit voltage.

Inductor (L)

Stores energy in a magnetic field. Used in power supplies (buck/boost converters), filters, and RF circuits.

  • Unit: Henry (H) β€” practically Β΅H, mH.
  • Behavior: opposes changes in current. Never interrupt current through an inductor abruptly β€” the voltage spike can destroy components (hence flyback diodes across relay/motor coils).
  • Markings: often color bands (like resistors) or printed code.

Transformer (T)

Two or more coupled inductors that transfer AC energy between circuits via magnetic coupling.

  • Purpose: step up/down AC voltage, isolate circuits galvanically.
  • Only works with AC β€” a DC voltage on the primary does nothing (and can burn it).
  • Ratio: Vβ‚‚/V₁ = Nβ‚‚/N₁ (turns ratio).

Semiconductors

Diode (D)

Allows current in one direction only (from anode β†’ cathode).

  • Forward voltage (Vf): ~0.6-0.7 V silicon, ~0.3 V Schottky, ~2-3.5 V LED.
  • Uses: rectification (ACβ†’DC), reverse-polarity protection, flyback protection, voltage clamping.
  • Marking: a line on the body marks the cathode (βˆ’).
  • Variants: Schottky (fast, low Vf), Zener (reverse-breakdown voltage regulation), TVS (transient protection).

LED (D, light-emitting diode)

Diode that emits light.

  • Always needs a series resistor (unless in a designed module).
  • Resistor formula: R = (V_supply βˆ’ V_LED) / I_LED β€” e.g. 5 V supply, red LED (2 V, 20 mA): R = (5βˆ’2)/0.02 = 150 Ξ©.
  • Long leg = anode (+).

Zener Diode

Deliberately operated in reverse breakdown to hold a stable voltage.

  • Example: a 5.1 V Zener keeps a node at ~5.1 V regardless of small supply variations β€” cheap voltage reference / shunt regulator.
  • Cathode marking like a normal diode; used in reverse.

Bipolar Junction Transistor β€” BJT (Q)

Current-controlled switch or amplifier. Three legs: Base, Collector, Emitter.

  • NPN (most common): small current into the Base switches a larger Collectorβ†’Emitter current. Emitter is the arrow point on the symbol.
  • PNP: opposite polarity β€” current flows out of the base; emitter arrow points in.
  • Key rule (NPN): V_BE β‰ˆ 0.7 V to turn on; base current I_B β‰ˆ I_C / h_FE.
  • Uses: switching loads (relays, LEDs, motors), simple amplifiers, level shifting.
  • Package markings: 2N2222, BC547, S8050, etc.

MOSFET (Q)

Voltage-controlled switch β€” the workhorse of power electronics (inverters, motor drivers, solar charge controllers).

  • Three legs: Gate, Drain, Source. Voltage on Gate controls Drain↔Source conduction β€” almost no gate current.
  • N-channel vs P-channel; enhancement (normally off) is the common type.
  • Key specs: V_GS(th) (threshold), R_DS(on) (on-resistance β€” low = less heat), V_DS max, I_D max.
  • Static-sensitive β€” handle with care (ESD can punch the gate oxide).

Thyristor / SCR & Triac

  • SCR: one-way latching switch β€” once triggered by the gate, it stays on until current drops to zero (used in AC power control, crowbar protection).
  • Triac: bidirectional SCR (controls AC in both halves of the cycle β€” dimmers, motor speed).

Optocoupler / Optoisolator

LED + phototransistor in one package β€” transfers a signal across an isolation barrier (no electrical connection).

  • Uses: isolating MCU from high-voltage/inductive circuits, level shifting, protecting logic inputs.

Integrated Circuits (IC)

Operational Amplifier β€” Op-Amp (U)

High-gain differential amplifier: amplifies the difference between its two inputs.

  • Two inputs: inverting (βˆ’) and non-inverting (+), one output.
  • Common configurations:
  • Voltage follower (buffer): output = input, high input impedance, low output impedance.
  • Non-inverting amplifier: gain = 1 + Rβ‚‚/R₁.
  • Inverting amplifier: gain = βˆ’Rβ‚‚/R₁.
  • Comparator: output swings HIGH/LOW depending on which input is higher.
  • Examples: LM358, LM741, TL072, LM393 (comparator).

555 Timer (U)

The classic timing IC β€” monostable (one-shot) and astable (oscillator) modes.

  • Astable formula: f = 1.44 / ((R₁ + 2Rβ‚‚) Γ— C) β€” square-wave generator, LED flashers, buzzers, PWM.
  • Also used for debouncing, timers, tone generation.

Voltage Regulator (U)

Maintains a stable output voltage regardless of input variations and load.

  • Fixed linear: 7805 (+5 V), 7812 (+12 V), 7905 (βˆ’5 V) β€” simple but wasteful (drops the difference as heat).
  • Adjustable: LM317 β€” V_out = 1.25 Γ— (1 + Rβ‚‚/R₁).
  • LDO: low-dropout, efficient for battery applications.
  • Switching regulators (buck/boost): highly efficient (90%+), used in most modern power supplies.

Microcontroller (U)

A programmable computer on a chip (Arduino/ATmega, ESP32, STM32, Raspberry Pi Pico).

  • Runs firmware from flash, reads sensors (ADC), drives outputs (GPIO, PWM), communicates (UART, IΒ²C, SPI).
  • Needs supporting components: decoupling capacitors, crystal/resonator (or internal RC), pull-up resistors, voltage regulation.

Electromechanical Components

Relay (K)

Electromagnetic switch: a small current energizes a coil that closes/opens contacts carrying a much larger current.

  • Coil specs: 5 V / 12 V / 24 V versions.
  • Always add a flyback diode across the coil (cathode toward +) to absorb the inductive spike when switching off β€” otherwise it can kill the driving transistor/MCU pin.

Switch (S)

  • SPST / SPDT / DPDT β€” number of poles (circuits) and throws (positions).
  • Momentary (push-button, returns when released) vs latching (stays).
  • Tactile, toggle, rocker, slide, micro-switch (limit), reed (magnetic), DIP (configuration banks).

Fuse & Circuit Breaker (F)

Overcurrent protection β€” the last line of defense.

  • Fuse: one-time, rated in A and voltage (e.g. 5Γ—20 mm glass, blade/ANL for automotive, mini-fuse). Never replace with a higher rating.
  • PTC resettable fuse: self-resetting β€” heats up and opens, cools and reconnects.
  • Circuit breaker: manual reset, reusable.
  • Sizing rule: fuse β‰ˆ 1.25 Γ— maximum continuous current.

Motors

  • DC motor: spins when powered β€” speed ∝ voltage, current ∝ load. Needs a driver (MOSFET H-bridge) for direction/speed.
  • Servo: position-controlled (PWM pulse every ~20 ms; 1 ms β‰ˆ 0Β°, 1.5 ms β‰ˆ 90Β°, 2 ms β‰ˆ 180Β°).
  • Stepper: moves in precise steps β€” for 3D printers, CNC; needs a driver (A4988, DRV8825).

Buzzer

  • Active: generates its own tone when powered (just apply DC).
  • Passive: needs an external square wave (PWM) β€” can play melodies.

Sensors & Other Common Parts

Part What it senses Behavior Typical use
LDR (photoresistor) Light Resistance drops in bright light (MΞ© β†’ kΞ©) Night lights, light meters
Thermistor NTC/PTC Temperature NTC: resistance drops when hot; PTC: rises Motor protection, temp sensing
Hall sensor Magnetic field Digital or analog output RPM sensing, door/window sensors
PIR sensor Infrared motion HIGH when motion detected Security, presence
Potentiometer β€” Adjustable voltage divider Volume knobs, calibration
Crystal / Resonator β€” Precise frequency reference (e.g. 16 MHz) MCU clocking, RTC
Capacitor (decoupling) β€” Local energy reservoir near ICs Noise suppression, stability

Cheat Sheet β€” All Components at a Glance

Component Ref. Category What it does Key property / unit How to test (multimeter)
Resistor R Passive Limits current, sets voltage Resistance, Ξ© Ohmmeter: read value vs color code (Β±tolerance)
Potentiometer RV Passive Adjustable resistor / voltage divider Ξ© (max) Ohmmeter between wiper and ends while turning
Capacitor C Passive Stores charge, filters, blocks DC Capacitance, F (Β΅F/nF/pF) Ohmmeter: brief continuity then open (charging); ESR meter for precision
Electrolytic capacitor C Passive (polarized) Bulk energy storage, smoothing Β΅F + voltage rating Check no short, correct polarity; bulged top = dead
Inductor L Passive Stores magnetic energy, filters Inductance, H (Β΅H/mH) Ohmmeter: near 0 Ξ© (low DCR); no continuity = open
Transformer T Passive AC voltage step up/down, isolation Turns ratio Ohmmeter: continuity on each winding; check isolation between windings
Fuse F Protection Opens on overcurrent Amps, A Ohmmeter: ~0 Ξ© = good; OL = blown
Diode D Active One-way current valve Vf ~0.6 V (Si) Diode mode: ~0.6 V one way, OL the other
LED D Active Emits light Vf 2-3.5 V, I ~20 mA Diode mode: glows faintly; OL in reverse
Zener diode D Active Voltage reference/regulation Breakdown Vz Diode mode forward; reverse ~Vz with bench supply
BJT transistor Q Active Current-controlled switch/amp h_FE, V_BE ~0.7 V Diode mode: B–E and B–C junctions ~0.6-0.7 V
MOSFET Q Active Voltage-controlled switch V_GS(th), R_DS(on) Diode mode: body diode D–S one way; gate open
SCR / Triac Q Active Latching AC power switch Hold current, gate trigger Diode mode between anode/cathode; gate pulses
Optocoupler U Active Signal isolation CTR (transfer ratio) Power input LED, check output transistor switches
Op-Amp U IC Amplify/compare signals Gain, GBW Supply it, test follower/comparator behavior
555 timer U IC Timing, oscillation Frequency formula Wire astable, measure square wave with scope
Voltage regulator U IC Stable output voltage V_out, dropout Measure in/out; 78xx out = rated V Β±5%
Microcontroller U IC Programmable logic Clock, flash, GPIO Program it, check power pins, blink LED
Relay K Electromechanical Big switch, small control Coil V, contact A Ohmmeter: coil 50-200 Ξ©; click on coil power
Switch / button S Electromechanical Open/close circuit Poles, throws Ohmmeter: continuity in closed state
DC motor M Electromechanical Rotation V, current, RPM Ohmmeter: low Ξ© between terminals; spins with supply
Servo M Electromechanical Position control PWM pulse width PWM signal β†’ shaft angle follows
Stepper M Electromechanical Precise steps Steps/rev, coil Ξ© Ohmmeter: coil pairs have continuity; driver needed
Buzzer BZ Electromechanical Sound Active/passive Active: beeps on DC; passive: needs PWM
LDR R Sensor Light β†’ resistance kΩ–MΞ© Ohmmeter: resistance drops under light
Thermistor R Sensor Temp β†’ resistance NTC/PTC, 10 kΞ© @25 Β°C Ohmmeter: resistance changes with heat/cold
Hall sensor U Sensor Magnetic field Digital/analog out Magnet near β†’ output toggles
Crystal Y Passive Frequency reference MHz Oscilloscope (in circuit); can't test with ohmmeter

Quick Formulas

Formula Purpose
V = I Γ— R (Ohm's law) Voltage, current, resistance
P = V Γ— I = IΒ² Γ— R Power dissipation
V_divider = V_in Γ— Rβ‚‚/(R₁+Rβ‚‚) Voltage divider
R_LED = (V_supply βˆ’ V_LED) / I_LED LED series resistor
Ο„ = R Γ— C RC time constant (~63% charge in 1Ο„; 5Ο„ = settled)
f = 1.44 / ((R₁ + 2Rβ‚‚) Γ— C) 555 astable frequency
V_out = 1.25 Γ— (1 + Rβ‚‚/R₁) LM317 adjustable regulator
P = IΒ² Γ— R (fuse sizing) Fuse β‰ˆ 1.25 Γ— max continuous current

Practical Tips

  • Polarity first: electrolytic capacitors, LEDs, diodes, and most ICs are polarized β€” check before powering. A reversed electrolytic can pop.
  • Decouple every IC: place a 100 nF ceramic capacitor as close as possible to each IC's power pins.
  • Flyback diode on every inductive load (relay coil, motor, solenoid) β€” cathode toward the positive supply.
  • MOSFETs and CMOS ICs are ESD-sensitive β€” use an anti-static wrist strap or at least touch ground before handling.
  • Check polarity with a multimeter in diode mode before connecting power β€” seconds of checking saves components.
  • Derate: choose capacitors rated β‰₯1.5Γ— the circuit voltage and resistors rated β‰₯2Γ— the expected power.

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