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gates2231
03-27-2008, 10:30 AM
Hello frends i am new to the field of robotics and read all the forum posts and decided to use
IR emitter+IR photodiode as everyone suggests it is a good beginner for line following

I followed this ckt
http://www.triindia.co.in/resources/?p=52

Everything was fine but the LED i kept at the output terminal was always glowing what ever may be the conditions.

So felt that the photo diode resistance is always greater than the reference voltage. So comparator is always giving high.
So i tried to measure the photo diode resistance.

I had a lot of peculiar experiences.

1)The value of the resistance remains constant under tube light as there is no IR. In 2000k range i am getting abt 1353k
It is always constant.

2)But wen i couple it IR LED i am reading different readings in different
multimeter readings
238k-2000k range
66.5-200k range
17.31-20k range
This is wen the surface is White surface

112k-200k range
466k-2000k range
This when the surface is black surface.

Why i am getting values lik this and plz suggest me a solution
plzzzzzzzzzzzzz

shreek1123581321
03-27-2008, 05:18 PM
tube light has ir light so the resistance should change when photodiode is exposed to the light and when it is not,...

next, if you couple it with ir emitter the resistance is varying....good enough, now be careful that the photodiode is reverse biased in the circuit. next now we know the resistance is changing so potential drop should also change. hence in the circuit at the anode of the photodiode voltage should vary when it is white and when it is black,. now you should find out these two voltages, very important.

now we know how a comparator works, so the reference voltage will be midpoint of these two voltages... done! it should work...

gates2231
03-27-2008, 08:08 PM
now i have some questions here

1) i want to know y my multimeter is showing diffrent resistance in diffrent resistance ranges

2) second i am using a 20k fr refernce voltage and at a specific voltage there will be sudden amplification
it is not happening

yes i kept the photodiode in the reverse bias

elecrom
03-29-2008, 09:54 AM
1) i want to know y my multimeter is showing diffrent resistance in diffrent resistance ranges

I think this is because of the different values of resistors used in the bridges inside DMM for different ranges, this in turn apply different voltages to Diode under test and hence shows different values of resistances. I too have observed this thing.


2) second i am using a 20k fr refernce voltage and at a specific voltage there will be sudden amplification
it is not happening


>> ??

This can be helpful to you :

How to make simple infrared sensor modules (http://elecrom.wordpress.com/2008/02/19/how-to-make-simple-infrared-sensor-modules/)
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allbits
03-29-2008, 12:11 PM
Why do you want to measure the resistance??

It is the voltage you should be measuring, once the circuit is powered up.

And why, why are you measuring the resistance of a photo diode?

If you want to get the resistance right, no current should be passing through the resistance under test.