This 1/2 hertz high selectivity filter was found in the December 1990 issue of Radio Electronics magazine. The source must be symmetrical. The capacitors can be changed for other frequencies.
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...