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.
This circuit, obtained from a 1965 magazine, can still be assembled using some modern equivalent...
The LEDs act as diodes, so if good they will show the same characteristic curve. Arrangment shown...
We found this circuit in an ancient American documentation. It can be adapted to operate in other...