Connecting an independently powered an RPi and Arduino

Suppose I power my RPi (Raspberry Pi) and Arduino separately. Then I want to use a GPIO pin on my Pi as output, and a pin on my Arduino as input. I can set it high or low to send a signal from the Pi to the Arduino. So there is only one wire connecting the Pi to the Arduino.

  • The question is: will this work?
  • Will I fry anything?

so there is only one wire connecting the Pi to the Arduino.
The question is: will this work?

No it will not. The grounds must be connected.

Will I fry anything?

Maybe. If the supply GPIO voltages of both the RPI and the Arduino are the same it will be OK. If not, then you will need to use extra components to make the GPIO voltages the same or make sure that a 3.3V input never sees 5V.

groundFungus:
No it will not. The grounds must be connected.
Maybe. If the supply voltages of both the RPI and the Arduino are the same it will be OK. If not, then you will need to use extra components to make the GPIO voltages the same or make sure that a 3.3V input never sees 5V.

Yep this is right, you need a common ground in pretty much any case that you have electronic signals between two electronic devices. In your case this is just needs to be a jumper wire between Arduino GND and RPi GND.

The Uno logic level is 0V to 5V and the Raspberry Pi logic is 0V to 3.3V, despite it having a 5V rail available. I only ever use 3.3V logic with the Raspberry Pi so I would recommend using a logic level converter like this: https://www.sparkfun.com/products/12009 (pretty sure they are easy to DIY as well). Lots of posts online seem to think that 5V on Raspberry GPIO is a bad idea