Photodiode

[ahmedragia21]

Hi , i got a photodiode somewhere and i dont have its datasheet i connected it as a common emitter , used a 10k resistor , illuminated it with red led . i want to use it as a line follower sensor , to detect white and green surfaces , i used an LM324 for this purpose , i found that the distance between the reflected surface and the photo diode while using the red led is so small less than a cm , dont know what to change ? reduce the 10k ? also the swing voltage from a full room light and with my hands conver the photodiode is so small like 33mV .. is this a good swing ?

 

[dknguyen]

Try using a green diode to detect a green line. Green won't reflect a lot of red light back for the photo diode. I don't know about the swing, but 33mV sounds like you can easily amplify if you need to.

Are you trying to detect a green line on a white surface? Please explain more about the white and green surfaces.

 

[ahmedragia21]

Thanks for reply
im trying to detect white line (3cm width ) on a green surface.
why the distance between the surface and the pair (Tx,Rx ) is so small ? also the thershold in the LM324 is so small even , which means the difference between green and white is so small in voltage right ?

 

[dknguyen]

Well the easiest thing to detect is black and white. No matter what colour LED you shine at it, the white will reflect all the light back and the black will absorb all the light.

Now with a green and white, it's not as clear since the green will reflect back green light and the white will reflect back ALL light. If you are shining a red light onto it, then the green will only reflect back the small amount of green light that the LED is shining in the first place while the white will reflect back all the light. So it probably would still work, but you can probably imagine the difference between the two is much smaller than with black and white.

(NOw that I've typed this out, it seems that a green LED might not necessarily improve things since the green line will become stronger since it will reflect back the green light, but the white would do the same. It just depends on which one increases more.) Also, if you used a green LED you would have to make sure the photo-diode responds to green (which I think it does if it's cadmium sulphide). You'd have to look at the datasheet to know what wavelengths of light it responds to.

I don't think you would decrease the 10K resistor to increase the amplifcation of the CE circuit. WOuldn't you increase the resistor? I read the photodiode work as a current source. So V=IR means larger resistor means larger V. I have no idea what kinds of current the photodiode will output and since you don't have the datasheet maybe just mess around with the resistor value?

It could also be the LED is a bit weak so your photodiode output is also low (and requires a close distance). You could increase LED power but it's probably easier just to increase the CE amp resistor value.

 

[ahmedragia21]

Its the first time i use a photodiode , i were using LDRs and they were working very nice with green- white surfaces .. just connected them to the LM324 as a compartor ... but im still didnt know how to increase the distance between the tx,rx to the surface ? its still too low , less than a cm !! how to increase it ? because if that's the distance , the sensor might touch the floor then ...

 

[dknguyen]

How much less than a cm? Don't line followers hug the floor realy closely anyways? It's not like they are travelling over dirt full of mounds and rocks. It's usually a hard flat floor.

Maybe you need to place your LED and LDR closer together. Or maybe it realy is the colour of the lines. Does it work better with black and white?

Here are some line follower websites with diagrams about the distance between each sensor and the ground so you have something to compare to:

Sensors 5mm apart from each other, 4.5mm from the ground


Sensors 2.5mm apart, 1.5cm from the ground
**broken link removed**

And remember you can always amplify it with another op-amp before feeding it into a comparator. THis would let you increase the distance from the floor.

 

[ahmedragia21]

Amplifiying it would increase the output voltage of the photodiode or what ? would you please post a simple schematic for the op-amp ampilifier idea ? i know it but dont want to do mistakes ..etc

 

[dknguyen]

Amplifiying it would increase the output voltage of the photodiode or what ? would you please post a simple schematic for the op-amp ampilifier idea ? i know it but dont want to do mistakes ..etc

What you need is a non-inverting amplifier (or an inverting amplifier). It doesn't realy matter since it will just reverse the output of your comparator. BUt change the mess around with the resistor value in CE amplifier first. Try and use that amplifier if you can.

 

[I²R]

A small, short-focus converging lens with the photodiode placed at its focal point will also improve your detection range by increasing its light-gathering power.