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.
The circuit presented is based on the National Semiconductor's LM3524, a component not common...
This circuit was obtained in an old American publication from 1974. The supply must be made with 5 V...
This configuration can be made based on other operational amplifiers. For other frequencies, the...