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 with germanium transistors was found in documentation from the 50's. The transistors are...
The circuit presented was found in a documentation from the 1950s, generating a sinusoidal signal...
This circuit was originally indicated as a signal reinforcement step for the previous receiver...