Krumple, I'm glad you mention your dad's old Min-X radio. It jogged my mind to a forgotten purchase from that auction site. I had picked up a Min-X six channel conversion kit that was intended to be added to a single channel Min-X receiver to make a six channel. Since their single channel receiver already had a relay, the kit came with 5 relays and a 6 channel reed bank. Just what I need to build a 5 channel receiver. These pictures show the relay bank with a hole for the 6th relay, plus the reed bank.
While very simple in concept, a reed bank is a pretty amazing work of art. The red and black wires drive the voice coil, and when the right audio frequency magnetically excites the desired finger of the steel reed, it starts to vibrate. Of course, your engine could also do that too, so placing the receiver so the reed had the right orientation with regards to the engine also helped. The contacts to the reeds only touched briefly as the reed moved, so a capacitor was also used to hold the voltage transferred by the reed. This very small current and voltage then drove a relay that in turn gave the much higher current to drive the servo motor.
So 5 different tones from the transmitter, one at a time, sent as an audio tone that modulated the transmitter's carrier. Received by a very rudimentary receiver that recovered the audio tone and drives the voice coil. The magnetic field from that voice coil resonates with one finger and that causes a capacitor to charge up, thus causing a very small sensitive relay to close with the motor driving current to get to a motor in the servo.
