Whats the difference between LDR´s and phototransistors?

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to find just the thing on the web and my electronics books are from the 70=B4s so quite a lot of the components are outdated)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

Reply to
david_gillberg
Loading thread data ...

y

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):

formatting link
=2Epdf
formatting link

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

formatting link

Reply to
John Popelish

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.