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.
Bis This two-transistor circuit was found in the January 1994 issue of Electronics Now magazine. The...
This receiver for the police band, which at the time was 45 MHz, was found in a publication by...
This circuit is the digital indicator for the previous circuit and can be used with similar...