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4-Channel 5V Relay Module

4-channel relay board for controlling multiple appliances — ideal for home automation projects.

Wiring & code for

Arduino UnoEspressif ESP32Raspberry PiSTM32 Blue Pill

Relay 4CH practical reference

A 4-relay board sharing the same opto-isolated design as the single-channel version, ideal for home-automation projects controlling several devices.

Key specifications

  • Control side: 5 V DC per channel, opto-isolated, ~15-20 mA each when energized
  • Switching side: Up to 250V AC / 10A or 30V DC / 10A per channel on the common SRD-05VDC-SL-C relays used on most of these boards
  • Trigger logic: Active LOW on most modules — a LOW signal closes that channel's relay
  • Peak current: All 4 relays switching on together draw a brief combined inrush — size the 5V supply for that, don't rely on the MCU board's onboard regulator alone

Pinout and connection notes

Same mains-voltage isolation rules as the single-channel version, times four — keep the COM/NO/NC screw terminals physically separated from the 5V logic side of the board.

  • VCC: Power, 5V DC, control side
  • GND: Ground, control side
  • IN1-IN4: Signal inputs, one per channel, from the MCU — active LOW on most modules
  • COM 1-4: Common terminal, one per channel, switching side
  • NO 1-4: Normally open, one per channel — connects to that channel's COM when energized
  • NC 1-4: Normally closed, one per channel — connects to that channel's COM when de-energized

Integration checklist

  1. Confirm the exact part number and breakout-board revision.
  2. Verify supply voltage, current demand, logic levels, and signal direction against the datasheet.
  3. Check controller pin assignments and bus address or interface mode against the example code.
  4. Power up with external loads disconnected, then test signals before full integration.
  5. If readings or communication fail, check power and ground first, then pin mapping, interface settings, and library initialization.

Troubleshooting and reproducibility

Record the board used, wiring, library/version, supply arrangement, and measured behavior. Use the selected board-specific wiring example as the source of truth. Never assume visually similar modules share identical voltage tolerance or pin order.

Board Integration

Wiring

Component pinArduino Uno pinNote
VCC5VPower a 4-relay board from a dedicated 5V supply where possible, not the Uno's onboard regulator, if switching all 4 at once
GNDGND
IN1, IN2, IN3, IN4D4, D5, D6, D7Active LOW on most modules — a LOW signal energizes that channel

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C++
  • Set all relay pins HIGH in setup(), before anything else — many Arduino boards briefly drive GPIOs LOW during reset/upload, which would otherwise flicker every relay ON for a moment on power-up.
  • If you're switching mains voltage (110/220V AC) on the COM/NO/NC side, that side is genuinely dangerous — keep it fully isolated from the low-voltage control side and don't touch it while powered, and if you're not confident with mains wiring, have someone qualified check your setup.
  • The relay's mechanical click and small LED per channel are useful sanity checks that your code is actually toggling the right channel before you wire up the real load.
View full Arduino Uno guide

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