Relay
Controlling Relay 4CH with STM32 Blue Pill
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 STM32 Blue Pill
Wiring
| Component pin | STM32 Blue Pill pin | Note |
|---|---|---|
| VCC | 5V | Use a separate 5V supply if switching all 4 channels together |
| GND | GND | |
| IN1, IN2, IN3, IN4 | PB12, PB13, PB14, PB15 |
Swipe sideways to see every column →
- Same setup()-first HIGH-init reasoning as the other boards.
- PB12-PB15 are a convenient free block on most Blue Pill boards with nothing else wired there by default — swap if your project already uses them.
- Same mains-safety caveat as the other boards if switching AC on COM/NO/NC.
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