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Topic: OpAmp comparator (Read 612 times) previous topic - next topic


The 10K or lower impedance is what the analog input 'wants'.  According to this data sheet PDF here the DC resistance can be as high as 100M Ohm.  But, the 10K or lower is the recommended maximum DC external impedance. Hence the 10K resistor.  You could go lower if there was a need, IOW if your photocel required a lower load resistance, perhaps for linearity at a particular wavelength.

Because the external recommended resistance is 10K or higher, there is really not a reason to get too hung-up on this part of the circuit.  The biggest challenge is to breadboard the photocell, a 10K, and then do some testing with the light-source you intend on using.  The ceiling and floor values are going to be experimental ones.


DC external impedance in this case i believe translates to output impedance, right?
I think i know what you're trying to say, but i'm afraid it's not applicable in my scenario. 
I've been told i need some kind of amplification for the photodiode. But i'll still try your method just to see what happens.

All analog inputs on all analog input  [EDIT  (such as ADCs) ] semiconductor devices are designed to be high-impedance ("your results may vary"). High Impedance in general would be considered 1 Mohm => 100 M 0hm with 10 Mohm probably being more common. This is why Mr. B. said "forget about it !...."
The only thing you need to be careful about is applying any voltage which exceeds the arduino allowed range for Analog Input , which 0 V to 5V . Stay in that range and your golden.
I suspected it was something in that range yes. I came across those numbers several times in literature. Good to know. Tnx


I've been told i need some kind of amplification for the photodiode.
Yes, you'll need to convert a very small current in the pA, nA or µA range (depends on the part# of your photodiode) to a useful voltage range that can be connected to the Arduino's analog input. Also, the type of photodiode you have could infuence which op-amp would work best, or be the most practical. Rail-rail type with low input current is desireable. Example: LMC6482

An iteresting presentation: A review of common problems experienced in photodiode op-amp circuits


That is extremely helpful, thanks dlloyd.


Hey guys, me again.

I got the real circuit working well enough for my needs. I did not use the LM339 (i didn't yet buy one), but salvaged a 741, which works fairly well with a slight output voltage adjustment. The arduino is getting a clear digital signal. So i'm content.

The photodiode was amplified using again a scrounged relic LM324N op amp which obviously is not best suited for the application, but it works...almost enough.  :)

What i want to ask is, what modern op amp would you recommend for the photodiode amplification...that is cheap?
The transimpedance, low current mosfet based are insanely expensive.
I'd like something cheap but decent enough. Any recommendations?

Runaway Pancake

Dec 25, 2014, 05:49 pm Last Edit: Dec 25, 2014, 05:52 pm by Runaway Pancake
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Dec 25, 2014, 10:07 pm Last Edit: Dec 25, 2014, 10:10 pm by raschemmel
So I guess that rules out this one ?

That is a good link.
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