This circuit was found in a 90s Popular Electronics. The sensors can be common speakers that reach from 15k to 25 kHz, such as piezoelectric tweeters.
The UAA180 integrated circuit is no longer manufactured, but if the reader finds one in a scrap...
This circuit is from an old National Semiconductor component manual. The circuit can be...
This circuit makes use of a diac to stop the activation of a sensitive reed relay 1 mA. The trip...