This circuit was found in the April 1971 Radio Electronics magazine. It uses common transistors and can be adapted to use an LED or audible indicator instead of a light bulb.
Found in English documentation from the 1980s, this circuit can use any germanium diode. The...
This configuration, which requires a symmetrical power supply for the 741, can be implemented with other...
This 3-digit code switch was obtained from an old American documentation. The capacitor is 470 uF...