Just bought this:
Normaly it's VCC GND and a lot of pins like CH1 - CH8 = Easy.
Here... Whats with the COM?
Common what?
Please help before I start messing around with external 5Vpower and burn something.
/O
Yeay, that's one with a stupid optocoupler on it. You can connect a seperate power supply for the relays. Most have a Vcc and a JCVcc or something. But this has one labled COM. I bed the COM is the relay power and Vcc for the logic. Anyway, if you just want to drive it from a simple supply connect COM and Vcc to 5V.
Stupid is the word!
And it seems to be on at LOW. Makes a mess in my codes!
For anyone else with the same question:
COM & GND to separate power supply (5V works).
VCC & GND to Arduino.
Thanks Septillion!
VCC & GND to Arduino.
You can loose the GND wire to Arduino and gain full isolation.
Dlloyd
For the record: I tried loosing the Arduino ground but the relays won't work then.
/O
Ahh, then those opto-isolators can never be utilized for isolation unless the board is hacked.
I guess so.
I think stupid is still the word to use here 
dlloyd:
You can loose the GND wire to Arduino and gain full isolation.
Of course you can not! How is something suppose to work with one wire 
The opto is only use for high side switching.
The fact they use a opto for nothing and they are active low is why a call them stupid / useless. Just stick with the opto-less relay boards 
Maybe the "COM" is opto supply, and "VCC" is relay supply.
Then "COM" and the "INs" are opto drive, and VCC/GND is relay supply.
Just like all other common relay boards with JD-VCC removed.
It takes two minutes for half a tech to follow the tracks and work this out.
Leo..
Olberg I did found the schematics for the relay board on the internet but I think it was without the opto...ill search it again and post it here so you can make more sense out of connections.
https://arduino-info.wikispaces.com/file/view/OptoRelayChannelData-800.jpg/308571794/OptoRelayChannelData-800.jpg I think this It or at least u will get the idea
Stupid is the word!
And it seems to be on at LOW. Makes a mess in my codes!
Typically , inputs are grounded for on, and outputs are open collector (that's for the opto itself).
Computer I/O boards (plug in to Motherboard type ) have typically been that way in general.
(as of the first IBM PC )
I haven't used any lately so they may have changed.
Stupid or not, it makes sense when you consider that the purpose of the optocoupler is to provide electrical isolation. By having the opto input led powered by the arduino 5V /GND and the output circuit powered by separate relay power supply Vcc/GND, the uController is protected from anything bad that might happen in the "outside world". This has been a standard practice since day 1. (1970 on =>)
The only reason you are calling it stupid is because you are not aware that this is the way it has always been done.
