This 500 MHz oscillator has an output power of 1 W when powered by a 20 V source. The circuit was suggested by RCA in a 70's documentation.
This circuit was published in an English magazine from 1972, but it can be easily implemented...
This circuit from a Popular Electronics from the 90s allows you to use an old PC as a digital...
With the values of the components of this circuit, tone recognition can be done in the range of...