Hi,
I got a relays card which has got 8 relays on it and an already opto-coupled trigger circuit on the circuit board. See the picture attached.
Now I do not know if I can connect one of the eight IN terminals to ground of Arduino. That can be seen as the blue line in my picture. That would cause that every of the eight input terminals would be connected to Arduino GND. Does that work ?
Thanks.
You should connect COMM to 5V pin on Arduino, and IN1..8 to 8 Arduino pins.
Do not connect the VCC and GND of the relay board to the Arduino: that would defeat the purpose of the opto-isolator.
Note that a LOW on the arduino output pin will turn the relay ON, and a HIGH will turn it OFF.