This circuit was found in Radio Electronics from the 90s. The CMOS integrated circuit is a PLL not easy to obtain today. The output stage can be exchanged for another one with greater power.
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...