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.
This circuit is old, causing LEDs to be lit sequentially. Several types of games can be programmed using...
This detector with ultra-low deviation was found in an American documentation from the 90s....
With the presented circuit it is possible to detect signals from transmitters in a frequency range...