The light sensor brick is based on a photo resistor. It can be used for detecting environment illumination intensity. Connect the + and – pins to power supply and you can read the data from S pin. BY means of a simple switch on the board you can change the output to Analogue or Digital. When you choose Analogue, the S pin will output the analogue value. When you choose Digital, the S pin will output high or low depending on how you set the adjusting potentiometer.