This circuit was found in the July 1992 Radio Electronics magazine. Sensitivity adjustment is made at R6. Supply voltage is matched to that of the relay.
This circuit was obtained from a 2005 Linear Technology application manual. The circuit is...
The tone produced by this circuit depends on the capacitor and the adjustment of the...
We found this circuit in 1969 American documentation. FETs admit equivalents, as do transistors. The...