A flash of light on one sensor or the touch of your fingers on the other type of sensor will turn on the relay. To turn off the relay, simply repeat the flash of light or the touch on the sensor. The circuit is shown bellow and uses one flip-flop of a 4013. Any relay with coil, depending on the power-supply voltage and a current up to 50 mA, can be used. The potentiometer in the version using an LDR adjusts the sensitivity of the circuit. The other flip-flop of the 4013 can be used in other applications.
