The project about light sensor circuit shows the application of photodiode to detect the presence of light. This light sensor can be used as an intermediate circuit [[wysiwyg_imageupload::]]in various applications to detect the presence or absence of light. The sensitivity of the sensor can be adjusted using the preset. This circuit is based around a LM339 comparator along with variable capacitors, photodiode, LED and a solo resistor. A photodiode, used as a photo detector, generates current in the circuit when light incidents on it. The circuit of this project uses the photodiode in reverse bias mode with resistor R1. The LED at the output will glow when light falls on the diode. In such a condition, the inverting input drops to a lower value than the reference set at the non-inverting input and so the output goes providing the necessary forwards bias to the LED.