Each frequency between the ranges of,
420Hz - 840Hz - 1260Hz - 1680Hz - Max
activates one of four LEDs depending on the input frequency, could be built simpler but I wanted to drive relays which I wasn't able to do using my simpler method.
Useful for a 4 channel RF remote control or a two wire system (signal and ground) where you need to control 4 devices from 4 buttons. Unfortunately this circuit does not allow multiple outputs to be on at the same time although I am working out how to add that.
