This circuit found in the February 1992 issue of Radio Electronics magazine is designed to adapt an audio signal for use in a computer's ADC. It can be adapted to operate with microcontrollers.
The critical component of this circuit is the transformer. T1 and T2 can be BD135 and T3 / T4 can...
The figure shows how to implement an R-S flip-flop or bistable reset-Set with an LM3900...
This circuit was found in a 1987 documentation but uses components quite common in the current...