This circuit operates with frequencies from 0.5 to 400 MHz. We find it in a 90’s NASA documentation. The transistors are from that time.
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...