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PICList Thread
'photodiodes etc.'
1999\02\13@172042 by Reginald Neale

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Fr. Tom said:

 (excellent summary of design considerations skipped)

>The circuit does not produce any "reverse bias" at all
>on the photodiode. The photodiode is forced to run with
>zero volts across itself. This is a very desireable thing,
>since the photodiode will produce a more linear current output
>when run in this mode.

 Also desirable from the standpoint that the device has a
 fairly large capacitance; so keeping the voltage across it small
 improves the frequency response.

 (more skipped)

>As a little aside, I would like to note that light emitting
>diodes very often make good photo-detectors. I often use
>one IR diode as the emitter and another (exactly the same type)
>as a detector. As usual, Your Mileage May Vary.
>

 I've heard of this but never tried to do it. Just curious - in
 your experience, does the spectral sensitivity curve as a detector
 match its output as an emitter? Do you know anything about
 whether tracking spectral characteristics for the two different
 modes would be a general rule for LEDs?

 Regards,
 Reg Neale

1999\02\13@173726 by dave vanhorn

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>  Also desirable from the standpoint that the device has a
>  fairly large capacitance; so keeping the voltage across it small
>  improves the frequency response.


Huh?? Diode junction C goes down with reverse bias voltage.
Operating it into a vitrtual short should be like a low or zero voltage.

1999\02\13@182358 by Mike Keitz

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On Sat, 13 Feb 1999 17:34:40 -0500 dave vanhorn <spam_OUTdvanhornTakeThisOuTspamCEDAR.NET>
writes:
>>  Also desirable from the standpoint that the device has a
>>  fairly large capacitance; so keeping the voltage across it small
>>  improves the frequency response.
>
>
>Huh?? Diode junction C goes down with reverse bias voltage.
>Operating it into a vitrtual short should be like a low or zero
>voltage.

Dave's right, it's the AC voltage across the diode that interacts with
the capacitance.  If you had an amplifier with zero input impedance it
would be ideal.  But a real amplifier has some non-zero input impedance,
so it is usually an advantage to reduce the diode's capacitance by
applying a DC reverse voltage.  With bias, DC and noise currents will
leak through the diode and be amplified, so measurement of low light
levels suffers.  Whether to use bias or not depends on the application.


>

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1999\02\13@225050 by Thomas McGahee

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Reg,
I have not done any kind of exhaustive study on the matter,
but I can tell you that A few years ago I had a project that
involved detecting walls using modulated IR beams.
Using identical IR LEDs as detectors did almost as good a
job as the "real" IR detectors that I had initially
purchased. I tested about 40 different kinds of detectors.
The best were the relatively expensive IR detectors that
included a built-in lens and filter. Next came the
identical IR LED used as a detector. Last of all came the
visible LEDs and generic photodetectors.

And yes, I did find that for any given IR LED, the best
low cost detector was most often a matched IR LED used as a
detector. Again, not a rigorous survey, but enough
correlation within the samples to convince me that it makes
sense to at least experiment and compare results before just
blindly going out and buying the expensive units.

Perhaps others might like to perform the experiments for
themselves and report back to the list on what they find.

Fr. Tom McGahee


----------
> From: Reginald Neale <.....nealeKILLspamspam@spam@SERVTECH.COM>
> To: PICLISTspamKILLspamMITVMA.MIT.EDU
> Subject: photodiodes etc.
> Date: Saturday, February 13, 1999 5:20 PM
>
> Fr. Tom said:

SNIP!

{Quote hidden}

1999\02\13@232908 by electme

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thanks for your input thomas ,
i have a personal  project in the near future dealing with ir.
after reading specs on a few different types & combinations & price i too
decided that a matched pair would be best.
the $ for the expensive IR detectors that
included a built-in lens and filter arn't worth it especially in the proto
stage.
thanks for varifying my decision.
glen

Thomas McGahee wrote:

{Quote hidden}

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