This small low current inverter operates at 20 kHz frequency. We found it in documentation from the 1980s. The integrated circuit is 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...