This circuit was found in the July 1986 issue of Radio Electronics magazine. The circuit has a loudspeaker warning system. The sensor is the ceramic capacitor C1 which acts as a transmission line.
Old multimeters used a 22.5 V battery for high resistance measurements. If you have one, you...
This circuit has an opposite action to the previous one, triggering when light falls on the...
This circuit was found in a Popular Electronics from 1991. The transistor could be the BC548. The...