Whats the difference between LDR´s and phototransistors?

I've looked at a couple of circuits that I´m 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´m 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´s
(Light Dependant Resistors) and some use phototransistors.
I have trouble finding any LDR´s to buy and by the info I´ve seen
(not very much I´m afraid) about phototransistors they seem to be the
better choise, but can I use them as a substitute for LDR´s or do they
have fundemantal difrences?

Would I have to use them in pair with LED´s 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´s so quite a lot
of the components are outdated)

Thanks!
/David

 

[John Popelish]

david_gillberg@spray.se wrote:

I've looked at a couple of circuits that I´m 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´m 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´s
(Light Dependant Resistors) and some use phototransistors.
I have trouble finding any LDR´s to buy and by the info I´ve seen
(not very much I´m afraid) about phototransistors they seem to be the
better choise, but can I use them as a substitute for LDR´s or do they
have fundemantal difrences?

Would I have to use them in pair with LED´s 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´s so quite a lot
of the components are outdated)

Thanks!
/David

Both LDRs and photo transistors are based on the principle that in

semiconductors, photons can generate charge carriers. In the LDR, the
conductivity of the semiconductor is used , directly. In a photo
transistor, the charge carriers forward bias the base emitter junction
of a transistor, and then the current gain of the transistor amplifies
the small base current into a collector current. The response time of
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
cannot rely on ambient light, you should consider a modulated light
source and a photo transistor that has its signal AC amplified
(ignores all continuous light) and synchronously rectified (ignores
all light variations at any other frequency than the modulation).
This gives you a very sensitive and interference free signal. There
are combination emitter and detector module prepackaged that are
focused at some set distance that would make nice, compact additions
to your robot. For example (from Digikey):
http://rocky.digikey.com/WebLib/Sharp/Web%20Data/GP2S09,%2024,%2026,%2027.pdf
http://rocky.digikey.com/WebLib/Sharp/Web%20Data/gp2s28.pdf

Here is a filtered (as described, above) detector that includes the
driver for the pulsed LED:
http://www.sharpsma.com/part.php?PartID=4072