Most Accurate IR measurement using Photodiode

hi dear readers and thanks a lot for spending time on this topic.

i'm trying to use external analog reference voltage via lm4040-2.5 to
measuring infrared intensity with photodiode (like 5mm LED's,covered black).
i did some thing working with lack of precision. here is my circuit :

my aim is detecting small Analog changes in passing IR from the Fingers to the photodiode (heart beat tracing)
i connected photodiode to reference voltage to keep maximum voltage to A1 below Aref voltage, i know i did the wrong. please guide me correcting this circuit, i need measuring an steady (as much as possible) voltage from Photodiode. thank you

Post the code, using code tags, and a sample of the results, as quoted text.

jremington:
Post the code, using code tags, and a sample of the results, as quoted text.

code is just a simple call to analogRead with no filtering yet
i just need to correct circuit. i guess connecting photodiode to Lm4040 affects Aref voltage when photodiode receives IR

As I recall, a photo diode creates a voltage based on illumination, not a change in resistance.

Paul

Paul_KD7HB:
As I recall, a photo diode creates a voltage based on illumination, not a change in resistance.

Paul

wow, if its how photodiode works i was in deep idiotism!
unfortunately i cant find datasheet for this popular photodiodes
so it has a maximum voltage output, is this maximum equals input voltage?

Search for "photodiode circuit" and you will see how it needs an amplifier.

Paul

code is just a simple call to analogRead with no filtering yet

Then you may have damaged the Arduino by not correctly setting AREF.

In their normal mode of usage, photodiodes generate a current proportional to the illumination. Here is a good reference for how to interface them: http://edge.rit.edu/edge/P09051/public/photodiodeamplifers.pdf

Paul_KD7HB:
Search for "photodiode circuit" and you will see how it needs an amplifier.

Paul

im mixed up, for an example in the above module i see no op-amp,
Also i founf many types, lm358,lm393, ... hard to choice
How op-amp affects precision in noticing IR changes? Thankkkk you :slight_smile:

jremington:
In their normal mode of usage, photodiodes generate a current proportional to the illumination. Here is a good reference for how to interface them: http://edge.rit.edu/edge/P09051/public/photodiodeamplifers.pdf

Great reference , but hard to understand, i have to read it ten more times, good informations about precision was included, thanks

Im almost crazy with it
Is it necessary to using an op-amp to improving precision? I does it helps ADC to detect small changes inside the blood?

Forgive me if i am asking about op-amps before reading about them, im going to read but asking you is a double check

Is it necessary to using an op-amp to improving precision?

Using an op amp improves the range of light levels that can be detected and the linearity of the response. It may not be necessary to use an op amp in your application.

jremington:
Using an op amp improves the range of light levels that can be detected and the linearity of the response. It may not be necessary to use an op amp in your application.

Precision will depend all together on your diode. Accuracy will depend all together on your calibration.

Paul

Precision will depend all together on your diode.

?

jremington:
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The OP stated his goal : measuring infrared intensity with photodiode ".

If the diode is unable to reflect the unstated differences in IR intensity the OP desires, then there is nothing that can be added that will change that.

Paul

my aim is detecting small Analog changes in passing IR from the Fingers to the photodiode (heart beat tracing)

Using IR photodiodes is a very common approach for this application and will work very well, if you reduce background IR as much as possible and employ a proper amplifier.

Do an image search for "IR photodiode amplifier" to see some working examples.

the most horrible thing in electronics is op amps, hateful damned old op amps which waste money with no use!
for example i bough some op amps containing a LF356 which requires +/-15v, so from which hell i can provide this kind of supply’s? it is killing me over this days of reading and reading… poooh

from this picture (transimpedance amplifier), is it the ground connected to photodiode? i mean is it a virtual ground or normal ground.

Have a look at the pulse reading gadgets you see on eBay. No idea if they work , but from £1.40 worth a go.

hammy:
Have a look at the pulse reading gadgets you see on eBay. No idea if they work , but from £1.40 worth a go.

I owned one of them by about 20$ for reverse engineering.

Payam_sbr:

from this picture (transimpedance amplifier), is it the ground connected to photodiode? i mean is it a virtual ground or normal ground.

Thats a virtual ground called mid rail , a voltage between vcc+ and vcc- of opamp. I hope im talking the true