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 is from a publication from the 70s, but it can be assembled with any diac and a Triac...
Found in American documentation from the 1990s, this circuit generates signals in the 350 to 500...
This circuit was found in a Radio Electronics from October 1952. The circuit equalizes the sounds...