TL;DR I need to do is successfully transmit a wave across a single conductor between two battery powered circuits with no common ground. I already know how to create the wave and detect the wave when there is a common ground, but I don't know how to design a circuit that can detect the wave without the ground. My scope grounded to the receiver can detect the wave.
My friends and I are are building an open source fencing box to score matches (The kind of fencing with swords). Competitive fencing is scored electronically by using a sword rigged with a button and jackets made of conductive material. We want to improve on the existing designs available today which require expensive mechanical reels to maintain tension on the cables.
To make a long story short, we need to be able to communicate over one "wire" (the sword) between two Arduinos. Our plan was to use the Arduino PWM generators to generate tones that can then be detect by FFT of the analog inputs on the other side. We're using 7 total tones between the frequencies of 9khz and 18khz. We got all that working great. The problem is what happens when we go to batteries and no common ground. The PWM signal goes from 5V peak to peak to 1.2V peak to peak. I don't understand why this is happening. Any enlightenment here would be appreciated.
Can anybody recommend a circuit design for transmitting a wave over 1 conductor between two systems with no common ground? (And amplifying it on the other end if required) I'm thinking it's going to have to be similar to radio technology. Alternatively I can try to take what I've got and feed it into some sort of amplifier and see if I can run with that, but I hate doing that without understanding the forces at play, and so far none of the circuits I've tried have reacted to the signal.
Some additional Information:
We have a button on the sword. We can always detect that button press and use it to turn off/on the transmitter, so that if we cannot tell the difference between touch and not touch via attenuation of the wave we can just make it not transmit when there is no touch.