Opto-Coupled RS Flip-Flop

RS flip-flop, or reset-set flip flop, is a kind of stable multivibrator with two input, namely reset and set. The output will be at one of two stable states, set (active) or unset (inactive). A pulse strobe on set input will activate the output, and a strobe on reset input will deactivate the output. This two stable stage are provided by the RS circuit that is built from 4N26 couplers. Giving +2V signal at reset, the output will change from high to low. The output will change to high when the set input receives +2-V signal. The optical coupling  of the input will be useful in an application where interference can damage disturb the function of the circuit, such as in electrically-noisy environment. Here is the schematic diagram of this circuit: