This circuit can be used as an answering password system in a store, office, or other location. The circuit is from Intersil using components of the time. The circuit is from the 80's.
This circuit is from 1968 General Electric documentation. It can certainly be adapted for any...
This tester from a 1960 magazine is only for capacitors with insulation voltages above 200 V (110...
Found in a 1991 publication this circuit can be assembled with more modern components. We can use...