This circuit was found in an Elektor from the 90s. It is a circuit that generates different types of sound effects. The components are still common today.
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...