This switched regulator configuration was found in Motorola's 1969 documentation. The integrated circuit is from that time and is difficult to obtain today.
This circuit was found in a 90s Hands-on Electronics. The transistor can be a BC548. What it does...
This circuit separates the bass from the treble in an audio signal. The circuit is from a Hands On...
The frequency of the signal produced by this circuit depends on the temperature of the diode used...