LM 311 Light activated Switch

September 18, 2009

18 September 2009:


Well, Here is my first sensor that was rolled out for my Line following Robot ‘Cyborg’. It is a Light Detective Resistor (LDR) based circuit where the resistance of the LDR drops when light is incident on it. This results in an LED glowing at the output of the LM311 voltage comparator.

I got the circuit for this comparator from Circuits Today. At present, I am facing some problems with BC107 transistor which acts a switch at the out put of the comparator. I will either replace it or remove it from the picture at the earliest. To know how a voltage comparator works, click here.

I have plans of debugging the worn out transistor, adding up one more sensor for my Cyborg as well as trying this comparator for IR transmitter pair. Watch out for more updates!

19 September 2009:

I removed the transistor and I was able to learn that the transistor while acting as a switch, inverts the output of the comparator i.e., When the LDR resistance drops, the LED goes off and vice versa which is quite opposite to its earlier operation.

I have demonstrated the application of obstacle detection using a simple video. Here, the obstacle reflects the light from the white LED on the surface of  the LDR causing a drop in resistance. This makes the output of the comparator turn high. Eventually the LED switches off!