Relay
Controlling Relay 4CH with Arduino Uno
What is the Relay 4CH?
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
- Confirm the exact part number and breakout-board revision.
- Verify supply voltage, current demand, logic levels, and signal direction against the datasheet.
- Check controller pin assignments and bus address or interface mode against the example code.
- Power up with external loads disconnected, then test signals before full integration.
- 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.
Setting Up Relay 4CH with Arduino Uno
Wiring
| Component pin | Arduino Uno pin | Note |
|---|---|---|
| VCC | 5V | Power a 4-relay board from a dedicated 5V supply where possible, not the Uno's onboard regulator, if switching all 4 at once |
| GND | GND | |
| IN1, IN2, IN3, IN4 | D4, D5, D6, D7 | Active LOW on most modules — a LOW signal energizes that channel |
Swipe sideways to see every column →
- 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.
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