We found this circuit in a Popular Electronics of the 80s. The circuit can be made with almost any operational and does not require symmetrical power source. For a 12 V power supply a 3.3 V zener is suggested.

We found this circuit in a Popular Electronics of the 80s. The circuit can be made with almost any operational and does not require symmetrical power source. For a 12 V power supply a 3.3 V zener is suggested.