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 the August 1970 issue of Radio Elecronics magazine. It uses the CA3018...
This circuit was found in a 1997 documentation. The signals are adjusted in a double 555 (556) and the...
This circuit was suggested by Sprague Electric in an 80's documentation. The circuit uses a...