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 battery circuit provides a charging current of 500 mA depending on the solar panel. The...
Found in Elektor Electronics from the 90s, this circuit produces the tone of 1 750 Hz for calling VHF...
Suggested by General Electric in a 1967 publication, this circuit makes use of an SCR of the time....