INDEX

PHOTO TRANSISTOR
I haven't used photo transistor. But a photo transistor is one in which base is like the receiver of light. When light falls there will be a short circuit between collector and emitter. This can be used in optical communications. I heard that you can make a photo transistor by cutting the upper portion
of transistor BC107 and leaving the base. You can use either IR or laser (cheap one available). But in case of transmission we require line of sight propagation. Here is a circuit for detection of IR using photo transistor.
http://www.kmitl.ac.th/~kswichit/LFrobot/LFrobot.htm

Here when light is not there then the resistance of transistor will be high, so the V-(pin2)>V+(pin3) making output of comparator LOW. That is when no reflection from ground or any obstacle on the IR. When light is there then the resistance will be very less and V+>V-. So output of comparator is HIGH. Suppose if you are using it for line detection, then there is reflection of IR from the white surface, but IR radiations are absorbed by black surface, so no or less reflection from the surface in black strip. Remember to check the voltage across IR to see whether IR LED is working or not and it should be greater than 2V. When black strip comes, output of comparator become 0V and the LED glows(visible light LED).