Other settings like this can be found on this website. This makes use of old operational amplifiers, as the publication where we obtained, but equivalents can be used.
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...