This circuit works on analog I/O ports and was suggested by an Electronic Design from the 90s. It can be used to work as a shield in projects with microcontrollers.
This National Semiconductor circuit appears elsewhere on this site. The circuit must be powered by...
In this circuit the frequency is determined by the coil and the variable capacitor. Transistors like the...
This circuit is from old American documentation, but the transistor used is still very common...