This circuit was found in a 1964 Electronic Design. The transistor may be a current equivalent of general use. The frequency ranges from 100 to 150 Hz.
This is the receiver for the previous circuit, also found in 90s Radio Electronics. The circuit...
This circuit from a 1978 American publication is used on vintage vehicles that still have contact...
This circuit was suggested by Popular Elecronics from the 90's. The circuit has high input...