I can't describe the circuit, being an electronic dumbo. A friend who
is a ham made up a circuit board inserted into a Radio Shack case about
the size of a cigarette pack. A 9v battery drives it. It is Velcroed to
the top of my C150's glare shield. One cable plugs into the headset
jack and there is one large LED on the case that flickers when there is
traffic on the frequency. Cost less than $10 in parts to put together.