# A cool Op-Amp demonstration: Differential Light Meter

What is a differential light meter?

A differential light meter is a nifty little tool that will tell you how much brighter the surrounding light is on one side of the meter than the other.  With a little imagination and a little engineering, you could connect a whole bunch of these together in different directions, and make a little box that will tell you how bright the light in the room is from all directions.  You might call that box a 3D camera.

This Instructable is more of a demonstration of concept, and an introduction to how operational amplifiers work, but it's cool to think about how this can be generalized!

What you will need:

Tools:
1) A breadboard that can source +15 V and -15V OR a regular breadboard with a +/- 15 volt power supply
2) Jumper wires
3) An oscilloscope + oscilloscope probe (If you do not know how to use an oscilloscope, there are plenty of good resources online!)
4) A multimeter (not strictly required, but useful.)

Components:
1) An Operational Amplifier (We used a TL082 - http://tinyurl.com/d49o3yy)
2) Two photodiodes
3) A capacitor and resistor such that 1/RC is about 2*pi* 1 kHz and R is large (so you can see the tiny little current from the photodiode.)
We used a 1.5 nF capacitor and a 100 kOhm resistor.
(where R is the resistance of the resistor, and C is the capacitance of the capacitor)

Safety:
Be VERY careful to connect the photodiode in the right orientation!! If you connect it the wrong way it will very quickly burn out and possibly shoot out of the board.  And then, http://www.youtube.com/watch?v=YleZvTSDC6s
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cuica11 months ago

Hello,

I tried to copy the circuit you describe, but i don't have a breadboard like yours with a built in supply and ground.

So I used a 24V supply and a voltage divider using two 4.7K resistors to give me -12V (the negative pole of the 24V supply), +15V (the positive pole) and ground (the middle of the voltage divider where the two resistors meet.

But the circuit doesn't work...

The output of the Op Amp stays at +11V regardless of the intensity of light hitting the photodiode...

I presume the problem is the way I am trying to set up the supply and ground, and would appreciate any advice!

Thanks.

CBMalloch1 year ago
Actually, isn't the capacitor in parallel with the resistor a high-pass filter? It lets the high frequencies through, so the impedance of the combination is small at high frequencies, and that means the gain is also smaller at high frequencies. Otherwise, great instructable. Your explanations are clear and you have chosen a good project that will help electronics hobbyists get a grasp of the op amp. Thanks for posting!
SarahandDillon (author)  CBMalloch1 year ago
Thanks! I think when the parallel capacitor - resistor pair is in the feedback loop of an op amp though, it really is a low pass filter. Just look at the oscilloscope screen - it's the high frequencies that we have filtered out.