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.
In fact, the original American publication exaggerated a little about the range that a good...
We found this circuit in an American documentation from the 90s. Through an LED it indicates the...
August 2017 - Mouser Electronics is announcing the availability of TDK ICS-52000 microphones with...