This might be a stupid question, but I'm getting stuck in my thinking here. I'm designing a device that will be activated by a typical RC receiver that is putting out the standard PWM. My device will be on a separate power supply (battery) than the receiver, and I want to keep it that way. I'm guessing I can't just have the signal from the receiver feed into my device without a common ground of some sort. How do I make sure my device reads the signal reliably while keeping the power supplies as separate as possible?