The PTT line is connected to pin 2. It reads 5 volts(Pullup in the radio). The radio GND is connected to the Arduino GND. To enable PTT, I have to pull pin 2 low. The problem is the Arduino defaults to it being low when off.

Pin2 on the RPI (3?) header board is always 5V. It's connected to the power bus. When power is OFF of course it will go low. You can not control this pin via software.

You will be best served by one of the GPIO pins like Pin40. With the right software (like gpio from WiringPi) you can control the state of the pin (H or L, respectively).

• gpio -g mode 21 up

• gpio -g mode 21 down

Note though that the GPIO pins are 3.3V only! Don't mix it with 5V or you'll blow oyur RPI.

Are you sure your radio wants a 5V?