This two-tone decoder has a bandwidth of 24%. The circuit is from a 1994 Electronics Now. The components without the value determine the recognized frequency.
When the sensor is activated the relay turns on powering na external load such as a buzzer siren...
Found in a book on transistors from the 50's. This receiver is very sensitive and features...
Found in 1965s General Electric documentation, this circuit is intended for the control of servos...