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ok i think i have a phototransitor... over 500 of them or diode

right i have recently found out that i have about 500 photodiodes or phototransistors. 
however i have never used these before and i dont really know what they do or how they work, could someone please explain what i can do with them and what use they are to me.






New line sensor

I had some problems with the 36 KHz detector when trying to read it from the picaxe, so I have switched to using 4 infrared photodiodes in a feedback setup with two LM358 dual opamp packages as shown in the following image (for one photodiode only):


This gives an analog reading and I will use this for linear interpolation between the sensors to determine where the line is relative to the robot center.