This is the receiver for the previous transmitter. The microphone is of the high impedance ultrasonic type. The circuit activates a relay. The operational amplifier supports equivalent.
This different astable configuration was found in one in a 1974 Ferranti transistor manual....
We found this circuit in an American 80's documentation. The resistors must be of high precision...
This switch or band selector for receivers was found in a Popular Electronics from the 1990s. The...