This circuit was found in a 90s Wels' Think Tank. The frequency depends on C2 and R3. The handset or speaker must be at least 80-ohm. Observe the low supply voltage.
This circuit was published in an English magazine from 1980. The magazine no longer exists, but...
We found this circuit in an old American documentation, from 1971, but the component used is still...
This circuit is from a 1991 publication, but it can be assembled with more modern components. We...