I asked for some help a bit ago, but haven't really done much since then because of everything that's been going on these days. I didn't want to reopen that old post so I'm just going to create a new one.
I'm working on a project that's basically a matrix of IR sensors and LEDs that are controlled with a microprocessor and some LED drivers.
Each IR sensor has an associated LED. The idea is that when an IR sensor sees something above, it will turn on it's associated LED, and the closer that something is to the sensor, the brighter the LED will become.
This project works, but the problem im having is that some of the sensors are way more sensitive than others, so they see the object way before the others. When I was seeking help before I was told that I was forward biasing the photo diodes, which is true, and that they needed to be reversed biased. I was linked to this document:
https://www.analog.com/en/technical-articles/optimizing-precision-photodiode-sensor-circuit-design.htmlI read through the whole document. I set up a small little test circuit to test this out. The photo diodes do actually work reversed biased. I used an oscilloscope and a DMM to observe what happens. I kept this test simple, just the transmitter, the receiver, and a resistor; no opamps.
What I noticed is that even though I was reverse biasing the photo diodes, the change was significantly different from one photo diode to the next. This is what I was experiencing when I had them forward biased. I really thought that the change would be more consistent once I had them reverse biased, but that just isn't so. Am I missing something here?
Some in the old post mentioned that I had to add some compensation to my code to basically calibrate all the diodes so they behave the same. Is that really the way it needs to be done? If some company was using these photo diodes for a product their manufacturing in the thousands or millions, would they have to do some unique compensation for each product so that they all operate exactly the same?
This is my small test with my DMM connected:
I swapped numerous photo diodes and got many different current measurements. Some of the measurements were negative and some were positive??? Some of the readings I got using the test setup were 13nA, 14nA, -28nA, -24nA, -17nA, -34nA, -31nA, etc.
I don't understand the positive and negative readings. Im thinking the negative readings are correct seeing that the photo diode is reversed biased?