ComponentsMaker Modules

Relay Module

Module

A pre-assembled PCB with one or more relays plus drive transistors, flyback diodes, and optocouplers. Lets a 3.3V or 5V microcontroller safely switch 10A mains or 24V DC loads with a single logic pin.

Schematic symbol

Relay

How relay module appears in a circuit diagram.

Types & variants

1-channel relay module (most basic)2 / 4 / 8 / 16-channel modules (multi-load control)Active-low modules (common on Arduino kits — LOW energises the relay)Active-high modules (more intuitive — HIGH energises)Opto-isolated modules (galvanic isolation between MCU and relay)Solid-state relay module (no moving parts, silent, AC only)5V coil / 12V coil / 24V coil versions

Key specs

Switching capacity

A

Typically 10A at 250VAC or 30VDC. Bigger modules handle 16A or 30A.

Coil voltage

V

5VDC (Arduino), 12VDC, 24VDC. Module VCC pin must match.

Trigger current

mA

5–15mA per channel via optocoupler. MCU pin can drive directly.

Trigger level

—

Active-low (common) or active-high. Check module docs carefully.

Isolation

—

Optocoupler isolation rated 5000Vrms. JD-VCC jumper separates logic and coil supplies.

Markings

Channel count printed on PCB (1/2/4/8). Relay markings: e.g. SRD-05VDC-SL-C (5V coil, 10A). Terminal blocks labeled NO (normally open), COM (common), NC (normally closed). Headers: VCC, GND, IN1, IN2…

Standard values

Common modules: 1-ch (5V active-low), 4-ch (5V active-low), 8-ch (5V active-low). Relay: SRD series (10A) or SLA series (16A). JD-VCC jumper connects logic VCC to relay VCC by default.

How to choose

1) Match coil voltage to your supply (5V for Arduino, 3.3V for Pi — use 5V relay with transistor or level shifter). 2) Active-low is most common but check. 3) Pick channel count for number of loads. 4) Opto-isolate if switching mains. 5) For frequent switching, consider SSR module instead.

Pinout & package

module boardVCC1GND2IN3

1-channel relay module — control header

Check first: Most cheap modules are active-LOW: the relay pulls in when IN is driven to 0 V. The switched side is the screw terminal block — NO, COM and NC — which must stay isolated from the logic wiring.

PCB with 3-pin control header per channel (VCC, GND, IN). Power header (JD-VCC, VCC, GND) for relay supply. Output: 3-way screw terminal per channel (NO, COM, NC). Status LEDs show power and per-channel activation.

Example circuits

  • Arduino controls a table lamp via 1-channel relay module
  • ESP8266 WiFi smart switch with 4-channel relay module
  • Raspberry Pi garden irrigation (4 solenoid valves on 4-channel module)
  • Home automation mains distribution (8-channel module for whole-room control)
  • Solid-state relay module for PID-controlled heater (frequent switching)

Common failures

Relay contacts weld shut from inductive loads (motors, solenoids) switched without flyback/snubber. Coil transistor burns out from overcurrent. Optocoupler fails from ESD. Mains wiring mistakes cause fires — always insulate and double-check before powering.

How to test

Power VCC + GND. Apply trigger (HIGH or LOW per module spec) to IN pin. You should hear a click and see the status LED light. Multimeter continuity: COM-NO closes when triggered, COM-NC opens. For mains, never test with power on until wiring is verified.

Substitutes

Bare relay + transistor + diode circuit (DIY equivalent). MOSFET module for DC-only low-voltage loads. Solid-state relay for silent AC switching. Contactor for industrial currents.

Where to buy

DigiKeyMouserLCSCRobuAmazon