We found this CMOS circuit for excitation of a relay in an old documentation. The transistor is a general-purpose high voltage as used in the power supply.
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...