Found in a European magazine from the 90s, this circuit emits beeps that are several seconds apart to indicate that a device is on. The transducer is a ceramic or crystal capsule.
I found this simple circuit from an English magazine from 1984. The circuit can be easily...
Information about the high voltage transformer and the detecting circuits was not given in this...
The purpose of this circuit is to supply a direct reference voltage from the power grid. The...