This two-output circuit was found in General Electric documentation from the 1990s. The circuit uses an NTC as a sensor and the control element is an SCR.
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...