Amjad Yaghi

Aerodesign Payload ·

Mastering IR detection (part 1)

Studying opamps, documenting phase-reversal, and setting up for the best IR detection this world has ever seen.

What I’m trying to achieve

Today I want to build a simple circuit to detect IR using a photodiode.

Building the circuit

We start by characterizing the opamp I will be using.

The datasheet I found for the TL082 is a bit hard to understand, so I want to test the input ranges myself to see what analog outputs my photodiode is allowed to give.

I built this circuit:

TL082 wired as a unity-gain follower, driven and measured by the Analog Discovery 2 — W1 into the +input, scopes on input and output

Running it with a sine wave input gave me this result:

I don’t quite understand what’s happening… particularly what is going on with that spike to 1.6V, then surprising saturation at the output pin once my input pin is sufficiently low enough. I must explore more.

The TL082IP

This is the opamp I have right now. According to the datasheet:

PARAMETERMINTYPMAXUNIT
V_CMCommon-mode voltage range±11-12 to 15V

The common-mode (with assumed rails at -15 V and 15 V) is -12 V to 15 V… This means, roughly, we should expect “normal” behaviour around 3 V to 5 V on our input rail. We’re seeing something like a phase-reversal, which is apparently a common failure mode for these opamps.

In any case, now that we have a profile of this opamp operating at this supply range, we can carry on.

The circuit we’re aiming for

Today I will be building this circuit:

Photodiode → op-amp buffer → Arduino: reverse-biased photodiode feeding a unity-gain buffer into Arduino A0

But… because of what we just learned about phase-reversal, we need to bias our OUT to operate within ~1.6 V to 4.5 V. So, I’ll be building this:

The biased version: R's bottom tied to +1.8 V so OUT rests at 1.8 V with no light, keeping it clear of the phase-reversal floor

And in order to do this… I will need to use the second opamp in the TL082IP IC. I need to make a voltage divider and take the floppy output, run it through a unity-gain opamp and use this as the reference floor to my photodiode circuit.

(This will make more sense tomorrow when I have images for you.)

Driving questions

  • Is there any interference from one opamp that will impact the other on the TL082IP opamp?
  • How should I build this circuit to reduce wire-clutter?

Next

  • Move to the project lab and build the stiff supply!
  • Create the test outside test-plan to get good baselines on the sun.