This circuit was found in old documentation, but the setting applies to any operational. Resistors and capacitors determine the frequency. For the indicated values the frequency is 10 kHz.
I found this circuit on a December 1976 Radio Elettronica. The Italian magazine no longer exists, but...
This circuit for the power grid was found in the June 1969 Radio Electronics magazine. The load is...
This circuit with common CMOS components was found in the November 1984 issue of Radio Electronics...