Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: Kirk on August 07, 2008, 09:43:35 PM

Title: Questions about Phototransitor sensors
Post by: Kirk on August 07, 2008, 09:43:35 PM

Greetings,
         I am working on a new line follower for an event Sept 6.
I have some Fairchild QRB1114 sensors that I got from sparkfun.
http://www.sparkfun.com/datasheets/Sensors/QRB1114.pdf


I was hoping to wire them in voltage dividers and feed the outputs to an A2D.
The concern is that the max current is VERY LOW 0.60mA.  So I need a
BIG resistor (1M to 10M) to get a significant voltage swing.
My questions:
         With so little current flowing will the signal get swamped by noise?
         Should I use a smaller resistor (perhaps 10k or 100k) to get
a bit more current to overcome the noise.
         Do I need op amps?
         How about a transistor arranged as a darlington with the
photo transistor?  Or do I need feedback?
         Will some small value caps help if I have a noise problem?
         Any other ideas?
         Am I just overly concerned?
I now plan to move the controller (ATmega8) very close to the line
sensors with short wires.

Thanks
Kirk
PS this is helpful
http://www.societyofrobots.com/schematics_photoresistor.shtml

Title: Re: Questions about Phototransitor sensors
Post by: dsheller on August 07, 2008, 10:16:12 PM

I used these before for some small tests, they don't have that great of range unless you of course use a large resistor... but even with a 100kOhm resistor they seemed to have a decent signal for me... granted I was using them for detecting whether someone's finger passed in front of a beam. =/

Title: Re: Questions about Phototransitor sensors
Post by: Admin on August 10, 2008, 07:49:41 PM

You don't need voltage dividers or transisters.

Use the schematic here:
http://www.societyofrobots.com/schematics_infraredemitdet.shtml