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Author Topic: Phototransistor vs. Photodiode  (Read 2447 times)

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Offline pterrusTopic starter

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Phototransistor vs. Photodiode
« on: January 23, 2012, 12:21:05 PM »
I bought an IR emitter/detector from radioshack and built the basic circuit shown here.  However the circuit didn't work as I expected at all.  The voltage across the detector seemed to be 0.7 V no matter what I did, although I did notice a really subtle change in resistance if pointed at the emitter.

Then I read the reviews here and it seems that some of these packages are mislabeled, and I may actually have a photodiode, not a phototransistor.  That would seem to explain a lot.

My question is: what exactly is the difference?  Can a photodiode work just as well as a phototransistor for a line following robot?  If so, can someone recommend a circuit?  Or should I just return the mislabeled diodes?  Thanks for the help.  :)

Offline Billy

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Re: Phototransistor vs. Photodiode
« Reply #1 on: January 23, 2012, 02:29:28 PM »
The voltage across the detector seemed to be 0.7 V no matter what I did, although I did notice a really subtle change in resistance if pointed at the emitter.

If you did get a diode instead of a transistor, and it always reads 0.7V, try turning the diode around to reverse the bias.

Offline Soeren

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Re: Phototransistor vs. Photodiode
« Reply #2 on: January 23, 2012, 03:46:49 PM »
Hi,

I bought an IR emitter/detector from radioshack and built the basic circuit shown here.  However the circuit didn't work as I expected at all.  The voltage across the detector seemed to be 0.7 V no matter what I did, although I did notice a really subtle change in resistance if pointed at the emitter.

Did you successfully identify the IR-LED of each pair?


My question is: what exactly is the difference? 

A photo transistor is a transistor with a photo/light sensitive basis. Like a regular transistor with a photodiode on the basis


Can a photodiode work just as well as a phototransistor for a line following robot? 

Not in the "Infrared Emitter Detector Basic Circuit".

If you decide to go with an op-amp per detector, it can be coupled to work.


If so, can someone recommend a circuit?  Or should I just return the mislabeled diodes?  Thanks for the help.  :)

Your call, whether you'll let the Shack get away with it (they won't change their ways unless enough people demand the right parts).
If you decide to keep them AND use an op-amp per detector, let us know.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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