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.
Found in a 1966 documentation, this “coin finder” uses components from the era operating at 2.1...
This circuit is from a documentation on the MC1539 from Motorola and may, however, be implemented...
The 747 is an old operational amplifier, but the configuration applies to modern operatives. In...