Whats the difference between LDR´s and phototransistors?

Jul 11, 2005 1 Replies
D

I've looked at a couple of circuits that I=B4m considering to use in my SUMO-bot. The purpose of the circuit is to detect a white line at the edge of the SUMO-ring and use this detection to steer away from it.



My bot will have two powered wheels, 1 motor per wheel. I=B4m thinking of using one detetctor at each front corner (my bot will only drive forward and turn so rear detetction will be unnesesary). I figure that if I hook it up in such a way that it will cut the power to the engine on the opposite side it will do the trick.



But when I look at possible solutions I find that some use LDR=B4s (Light Dependant Resistors) and some use phototransistors. I have trouble finding any LDR=B4s to buy and by the info I=B4ve seen (not very much I=B4m afraid) about phototransistors they seem to be the better choise, but can I use them as a substitute for LDR=B4s or do they have fundemantal difrences?



Would I have to use them in pair with LED=B4s to get enough light? (the detectors will be mounted under the robot, so its possible that not much light will get in)



Can anyone think of a circuit that might help me do the thing I want? (a www-reference would be lovely, I can't seem to find just the thing on the web and my electronics books are from the 70=B4s so quite a lot of the components are outdated)



Thanks! /David



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J

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Both LDRs and photo transistors are based on the principle that in=20 semiconductors, photons can generate charge carriers. In the LDR, the=20 conductivity of the semiconductor is used , directly. In a photo=20 transistor, the charge carriers forward bias the base emitter junction=20 of a transistor, and then the current gain of the transistor amplifies=20 the small base current into a collector current. The response time of=20 a photo transistor is generally much faster than that of an LDR.

Either type might be usable for your purposes, but I think, since you=20 cannot rely on ambient light, you should consider a modulated light=20 source and a photo transistor that has its signal AC amplified=20 (ignores all continuous light) and synchronously rectified (ignores=20 all light variations at any other frequency than the modulation).=20 This gives you a very sensitive and interference free signal. There=20 are combination emitter and detector module prepackaged that are=20 focused at some set distance that would make nice, compact additions=20 to your robot. For example (from Digikey):

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Here is a filtered (as described, above) detector that includes the=20 driver for the pulsed LED:

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