This circuit was found in the July 1992 issue of Radio Electronics magazine. The circuit is based on a 741 that activates a buzzer with oscillator. R5 adjusts the sensitivity.
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...