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KNACRO 4-Channel DC 5V Relay Module 30A DC 30V/AC 250V connected to a Raspberry PI 3 B+. 3 channels will work but 1 won't.

I'm assuming is a 3.3 vs 5v issue.

It doesn't matter which 3 I use they all work.

But if I connect all 4 it doesn't work.

I'm wondering if the 'Com' I'm seeing on the GPIO side needs to be connected to 5v as well. Most relays don't have a 'Com' on the GPIO side just the side that connects to devices.enter image description here

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    You have given no information about the wiring between the Pi and the relay module. Without that information no useful answer can be provided. – joan Jan 10 at 17:21
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    It has been a similar discussion in the Arduino Stack exchange group, and to supply a 5V device with 3,3V is as asking for trouble. – MatsK Jan 10 at 21:41

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