This circuit was found in documentation from the 90s. The operational amplifier is NE5514, which is uncommon today. The configuration can be adapted to use modern components.
This circuit, from a 1973 documentation, can be made with BC549 transistors. The frequency depends on...
Suggested by General Electric in a 1965 documentation, this circuit can use the TIC106 as an SCR....
This circuit is from an old Popular Electronics. The circuit is based on a 555 and a 5V voltage...