This old circuit from a documentation from the 70s disconnects a TV from a signal obtained on the TV itself, detecting when the tuned channel goes out of the air.
This configuration, which produces a rectangular signal compatible with CMOS and TTL logic, was...
We found this circuit in a 1977 Radio Shack brochure. Transistors can be the most modern BC548 for NPN...
The basic idea of this circuit is to convert the indication of a magnetic sensor to an audio tone...