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 circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...