This adjustable frequency circuit was found in December 1986, Radio Electronics magazine. The base frequency is given by C1 and the power supply must be 5 V.
The circuit shown consists of a power stage for relay excitation. 2N3053 can be a BD135 and 2N406...
This circuit, from a 1968 Electronics World, is fed directly by the rectified voltage from the...
Using a 4046 PLL this circuit was obtained from the October 1994 issue of Electronics Now...