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My goal is to read luminance signals with a certain level of sensitivity (able to tell when an OLED screen is on or off) with at least a 1 kHz sampling rate. I am following this tutorial (https://outsidescience.wordpress.com/2012/11/03/diy-science-measuring-light-with-a-photodiode-ii/) on reading luminance with a photodiode using an Arduino.

I used components that I previously bought to follow the tutorial to create a simple transimpedance amplifier (first pic), but it's not working on my setup (second pic). There are only two differences from the tutorial. First, the op-amp used in the tutorial (LTC 1050) is different than the one I had on hand (TL972IP), so I had to adjust based off of the difference in pin assignments. I also did not have a disc capacitor. These are the two differences: the photodiode and resistor (10k) were the same as in the tutorial. I double and triple-checked the wiring, then I took it apart then re-created the circuit. Same result (4.97v, readout is not luminance sensitive, see third pic). I think the wiring is not the problem. I have no clue though what specifically it could be. Maybe the op-amp? Maybe the lack of a capacitor?

Diagram of transimpedance amplifier circuit that I tried to replicate

OP-AMP TL972IP (datasheet: https://www.ti.com/lit/ds/symlink/tl972.pdf?ts=1737634150609&ref_url=https%253A%252F%252Fwww.mouser.es%252F)

My wiring (op-amp: TL972IP)

CODE:

Code output

Note that the code outputs 4.97v regardless of the light level on the photodiode. I verified that the photodiode orientation (long = +) is correct.

Other things that I've tried: I used a 470 ohm, 10k ohm (shown here), and 1M ohm resistor. These did not change the output. I also used another photodiode. I verified that the photodiode that I used here is light-sensitive. Simply wiring the negative to ground and the positive to the output gives dark output of 0.22v and flashlight output of 0.40v. But these are not good enough for my purposes.

I have no idea what the problem could be.

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  • \$\begingroup\$ The datasheet suggests that TLP doesn't really work with inputs at the negative supply. \$\endgroup\$ Commented Jan 23 at 17:15

2 Answers 2

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Please read the words on the diagram below and note that photo-current needs to flow into the top terminal of the photodiode: -

enter image description here

Next is your op-amp choice (TL972) <-- basically it doesn't have an input voltage range that covers ground so, it won't work in that respect either. Try using the ubiquitous LM324.

I haven't check your code.

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One more issue is, that you use the inputs of OpAmp 1 (pin 2 and 3), but the output of OpAmp 2 (pin 7).

Don't connect IN+ and the diode directly to GND. Create a voltage of around 1 V with, e.g. a voltage divider, and connect them there. This produces valid input voltages for the OpAmp.

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  • \$\begingroup\$ Please don't misquote me. Think again what I said. \$\endgroup\$ Commented Jan 24 at 9:23
  • \$\begingroup\$ @Andyaka Sorry, my bad. \$\endgroup\$ Commented Jan 24 at 14:12

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