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 for telecommunications was found in a 1987 documentation. Connected to the...
This circuit is from a 1966 American documentation. The circuit uses a photo-FET, an unusual...
Currents up to 1.5 A can be regulated with this circuit. The resistor R that determines the...