2013-03-22 LED Vs LED In Grease Spot Experiment

I thought about doing this since I wrote a blog on it back in the mid 2000s.  Use the Grease Spot Photometer Method to compare the outputs of LEDs.  This experiment is used in Physics classes to demonstrate the relationship between candles and the distance to the light source.

In my case I decided that I should connect two white LEDs fresh out of the bag in series, so that the current through them is identical.  I put a 100 ohm resistor in series so I could measure the voltage across it and calculate the current.  For every volt, I would get 10 milliamps current.

I soldered the two LEDs onto the ends of half meter lengths of thin flexible stranded wire.  At first I was going to use some old speaker wire I had, but I changed my mind and used some twisted pair from a cat5 patch cable.  Some short lengths of shrink tubing helped to prevent things from shorting out.

I dropped a drop of shredder oil in the middle of a quarter sheet of paper.  I could have used grease, butter, lard, salad oil or similar.  I spread the drop out until it was about 30 or so millimeters diameter, then I blotted the excess oil away with a sheet of paper towel.  The oil tends to spread out over a few days, so I may have to make up a new sheet periodically.  I used a spring clamp to hold the paper up while it was resting on the floor.  I put a yard/meter stick on the floor next to it, with the paper at the fifty centimeter point.

I connected up the LED assembly to a power supply and adjusted the voltage to give 2 volts across the 100 ohm, which was 20 milliamps LED current.  Wow, nice bright beams from the new LEDs.  The power supply voltage was 8.58 volts or about 3.3 volts per LED.

I held one LED left of the paper at the 30 CM point, which was 20 cm from the paper.  I then held the other LED right of the paper at the point where the grease spot disappeared when the beams were pointed at it.  I looked at the right LED’s distance and it was at 71.5 cm, or 21.5 cm from the paper.

What this small difference means is that the right LED has as slightly brighter light output for the same current as the left LED.  But the main point is that it shows that this grease spot method does a reasonably accurate job of comparing two light sources, and the cost is almost nothing.

What I need to do now is add a switch so that I can switch one LED over to a Joule Thief and be able to compare its light output with the the other LED’s current, which will be adjustable by varying the power supply voltage.  I also need to make sure that the grease spot disappears as the current to the two LEDs is varied over a range, say 5 to 25 mA.

I plan on mounting the paper and LEDs inside of a box to keep out the ambient light.  I will have the LEDs fixed so that the grease spot disappears when their current is the same.  Then I can switch one LED over to the Joule Thief and compare  the light output from the JT with the LED connected to the adjustable  power supply.  When they are the same, I can then measure the voltage across the resistor, and get the actual current through  the LED.  Because the light output from both LEDs is the same, I can assume that the JT is putting the same average current through the LED.

Another method

I bought a Luxmeter a few years ago and it also works for this comparison.  I thought about another method.  Instead of comparing two LEDs, I could use the same LED.  I would need to connect it to a DPDT switch or relay to allow me to switch the LED between the power supply and the JT.  The currents would be equal when the LED does not change brightness when the switch is changed.  If I don’t trust my eyes to make that judgment, I can aim the LED at a CdS photocell and measure the resistance of the power supply and JT, and if the two resistances are the same, the two currents through LED should be the same.