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.
Using a 4046 PLL, which is uncommon today, this circuit was found in the October 1994 issue of...
This circuit was found in an 80's Electronic Design. The frequency depends on the crystal and the...
This circuit was found in a Popular Electronics of the 1990s. The sensor is a rotating mechanical...