This circuit was found in a Motorola documentation from the 70s. The integrated circuit is from the time, an operational power amplifier according to the controlled motor.
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...