Can I simply tell Arduino #1 that a pin is an output, set it to HIGH and then tell the second Arduino that a pin is input, connect those two pins with a wire and use that as a switch to do something on the second Arduino?
I thought this, then read that I also needed to connect the grounds of the two Arduino's. I've killed one UNO, but not sure what killed it, so I'm trouble shooting.
Yes you have to connect the grounds. Voltages always have to be referred to something and that's ground. If the two devices have different grounds the second one won't recognise the high from the first as meaning anything.
ArduinoNewSS:
Can I simply tell Arduino #1 that a pin is an output, set it to HIGH and then tell the second Arduino that a pin is input, connect those two pins with a wire and use that as a switch to do something on the second Arduino?
I thought this, then read that I also needed to connect the grounds of the two Arduino's. I've killed one UNO, but not sure what killed it, so I'm trouble shooting.
Yes, you also must join grounds. There is no 'circuit' without it.
You also must join the 5volt pins if you simply connect pins from different Arduinos together,
so both are powered up at the same time.
If you don't, then there is the danger of one Arduino powering the other one through it's input pin (phantom-powering). That (~50mA) could fry the input protection diodes of the input pin.
Another way is to use a 10k resistor between output pin and input pin.
That will reduce pin fault curent to <500uA in case one of the Arduinos is off.
Leo..