We found this circuit in a Popular Electronics from the 80s. This circuit eventually doesn't work with modern, higher speed controls. Experiments must be done.
This circuit takes advantage of the inductance of the phone in the resonant circuit. The circuit is...
Found in old English documentation, this circuit is still current. The circuit trips with a sound...
This 555-based circuit was found in the August 1976 issue of Radio Electronics magazine. The tone...