When a pulse of light reaches the sensor [a CDs cell or LDR] the flip-flop changes its state. Sensitivity is adjusted by the 1 M ohm potentiometer. Pressing the RST switch for a moment resets the circuit. Other NAND gates can be used in the same circuit as the one found in the 4093. Remember that the LDR is not a fast sensor for light pulses and a minimum duration is needed to trigger the circuit.
Light Triggered Flip Flop
