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.
Both the field effect transistor and the integrated circuit of this 1970s circuit are no longer very...
This timer, like the previous circuits, uses a currently uncommon integrated circuit. The time is...
The transistors in this circuit, found in an American publication in 1975, can be replaced by...