This sensitive circuit was found in the June 1995 Electronics Now magazine. R1 can be from 10k to 1M depending on the desired sensitivity. Power is supplied by the relay.
We found this circuit in documentation from the 1970s; The circuit uses Japanese transistors of...
This circuit goes into operation producing a sound on a small speaker when the supply voltage...
Suggested by Harris in documentation from the 80s, this circuit uses components from that time. It...