Automatic light/dark activated systems have very common utilities in day to day life. For example, street lighting at night, morning alarm, automatic emergency [[wysiwyg_imageupload::]]light, dark activated buzzers and so on. The project explained here can be used in several such systems. This circuit has a sensor component which works in response to the ambient light condition. The output of this sensor is compared with a set reference value and whenever the light condition changes from dark to light or light to dark, the output goes high. In this project, the surrounding illumination is observed by a light dependent resistor (LDR) which acts as a sensor for the circuit. The voltage drop across this LDR corresponds to a particular intensity of light. Read on more to find out how the circuit is made and how it works.