Power over USB vs from Raspberry

Question

If I connect a relay to Raspberry PI (Ground to Ground, VCC to 5v, and switch pin to one GPIO), when I write 1 on that pin, the relay switches on and off.

But, if I connect the relay Ground and VCC to USB power wires (5v), I can't control it from Raspberry GPIOs. An external switch, also connected to the USB wires, triggers switching in the relay.

Does anybody have an explanation for this behaviour ?

TLDR; power and control signals are working only if they have the same source (Raspberry or USB powered devices)

Answer 1

When you power the relay from the same source (either the rPi or a USB power source) the relay and the controlling GPIO pin share the same ground. Since voltages are relative to ground, this means that the 3.3V GPIO voltage is sure to be high enough to trip the relay.

When you connect the rPi to one power source, and the relay to another, and you DO NOT connect the grounds together, the 3.3V GPIO output from the rPI is relative to the rPi's ground, and that ground level can be totally different than the ground of the relay - in effect turning that 3.3V into an "undefined" voltage level, likely not high enough for the relay to work.

Once you connect a wire between the GND pins on your rPi and the GND pins on your relay, the same ground voltage holds for both devices and the relay should see the correct 3.3V GPIO voltage again, and work correctly.