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.
This circuit with JFET was obtained in a book on FETs of 1989. The circuit is still current since...
This circuit was obtained from National Semiconductor documentation from the 1990s. The circuit has a...
Found in 90s documentation from Analog Devices, this circuit employs operational amplifiers of the time....