Using operational amplifiers of the time, we found this circuit in documentation from the 90s. The configuration can be adapted to use modern operational amplifiers.
This Rytron transmitter was published in a 1953 book. The circuit can be implemented with a BC558...
Foto-Darlington for applications such as light sensor. In normal applications only the collector and...
This circuit was found in a 90s Popular Electronics. It generates tones given by the oscillators and...