Automatic letter box is a very simple, interesting and a DIY Working Science Model for a Science Activity Workshop or even for submission as a Physics Project for Class 11th and 12th. Designed at Projects for School, this simple and thorough kit will help you in Building breadboard projects for circuit diagrams. The circuit diagram we have chosen is an automatic letter box. As you build the project, you will realize that the key concepts behind this project include photodiodes, change in resistance and potential difference and a logical output. Effectively when an object is placed in front of the IR pair, the light bounces off the object, thus falling on the photo diode. As a result, the resistance is reduced, and so does the potential which reflects in the potentiometer and affects the IC. Thus the potentiometer reading leads to the generation of the output of logic 1 causing the LED to glow and the automatic letter box to function in principle.