I am using an 8 Relay module, bought from dx.com.
It has no instruction manual…
This board has 8 inputs (IN1,…,IN8) to control each relay, 1 VCC pin that requires 5V and 1 GND pin.
I connected the VCC to the 5V pin of the raspberry and the GND to the ground of the raspberry. After that I tested each relay by connecting them to a GPIO-pin as I saw other people doing it: video
But only 2 of the 8 relays seem to work fine. The indication lights of the other 6 do change, but the relays don't 'click'.
Is somebody experiencing the same problem, or does anybody know how to fix this?
Thanks in advance!
Best Answer
You're using a board that was designed for Arduino, which has 5V GPIO signals, on a RaspberryPi, which has 3.3V GPIO signals.
Looking at the relay board, I see driver transistors for the relays, but they do not have base resistors, which means that they're probably MOSFETs. There's a good chance that these MOSFETs have a threshold voltage that works fine for 5V signals, but does not provide adequate drive to the relays when given 3.3V signals.
Try connecting the control inputs of the relay board one at a time directly to the 5V supply pin and see if they all operate solidly that way. If so, then you'll need to find a way to translate from the lower voltage to the higher voltage.
BTW, I would normally expect to see resistors even with MOSFET drivers — pulldown resistors on the gates that keep them from "floating" and providing some resilience against ESD. Treat those inputs as being very static-sensitive!