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:

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.)

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)

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

Step 1: Intro to Breadboard

Gotta cover our bases.  Skip this if you've used a breadboard before.

A breadboard provide a stable and easily adjustable way of connecting and playing around with different circuit elements.

Each row of five holes in the main section of the breadboard is connected below the surface by a piece of metal, so current can flow between the 5 holes and each of the 5 holes is at the same voltage. So if you want to connect two elements, put them in the same row of the breadboard.

In the breadboard we used, there are four rows that are connected all the way across the top. They are as follows (from the BOTTOM up): ground, negative 15 volts, positive 15 volts, and +5 volts. We will not be using the top row. 
<p>For higher voltages a voltage regulator could be added too. I am going to make a type of spectrometer with this device. (Or I will try).</p><p>Light &gt;&gt;&gt; 15 W &gt;&gt;&gt; Pinhole CD case &gt;&gt;&gt;&gt; Prism glass &gt;&gt;&gt;&gt;&gt; flask cleaned &gt;&gt;&gt;&gt;&gt; several light input diodes (Photodiodes).</p>
<p>Here is the diagram.</p>
<p>Below is a digaram that uses an dual NPN transistor and capacitors could be added between base and collector to improve output.</p>
<p>Hello,</p><p>I tried to copy the circuit you describe, but i don't have a breadboard like yours with a built in supply and ground.</p><p>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.</p><p>But the circuit doesn't work...</p><p>The output of the Op Amp stays at +11V regardless of the intensity of light hitting the photodiode...</p><p>I presume the problem is the way I am trying to set up the supply and ground, and would appreciate any advice!</p><p>Thanks.</p>
Actually, isn't the capacitor in parallel with the resistor a high-pass filter? It lets the <em>high</em> 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!
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.

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More by SarahandDillon:Use the "Force" to drive a Remote Controlled Device with a DIY 3D capacitor! A cool Op-Amp demonstration: Differential Light Meter 
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