Science Buddies: "Ask an Expert"

Hello,

I am interested in the Light Energy & Frequency project (as described on the science buddies site) for my 7th grade science fair
and had a few questions.

1. The part number 889-LFL550M required for the project is no longer available. This is a RGB LED. Are these 3 different LEDs in one unit? Can the same effect be achieved by using 3 separate (Red, Green & Blue) LEDs?
2. Can you shed some light on where to source a RGB LED (preferably from a local store), or suggest alternatives?
3. What type of stores would you recommend for sourcing other items such as resistors, soldering iron, etc?

Rgds
Kirby16

I did the find similar on Mouser's page and selected RGB and found 889-SFT722N is in stock.
Radio Shack sells a 7-Color 5mm Blinking LED catalog #276-016. I didn't look at the project to know if the blinking feature would work or not.

I suggest you google RGB LEDs and select the "Shopping" results. Here's one page where you can get one: http://www.superbrightleds.com/cgi-bin/ ... oduct=LEDS

Three R, G, and B LEDs wouldn't give really the same result... they have to be close enough to each other that the eye can fool itself into thinking that they are at the same place.

Hi - I apologize for the problem you ran into with the materials list. Other Experts have given you good input for possible replacements. I've also had the project idea updated by our team of scientists. The updated part from Mouser has been identified as: 604-WP154A4-RGB

Good luck with the project!

Amy
Science Buddies

What should be the optimal distance of the LED from the drop of isopropanol?

Just so that I can validate my results, what are typical basal evaporation rates and evaporation rates using the LED?
When I did a first run, the basal rates were in the region of 17+ minutes....

Using the index card it is rather difficult to distinguish when the drop has evaporated. Is there a better option here?

Thanks in advance
Kirby16

Hi - I'll try and answer your questions. You asked about typical evaporation rates for the alcohol. I found some guidelines in the science fair project summary link at the bottom of the project guide (here's the link: http://www.usc.edu/CSSF/History/2007/Pr ... S1610.pdf.) The red LED caused the alcohol to evaporate in 1 minute 18 seconds. This would suggest to me that a base evaporation rate greater than 2 or 3 minutes is enough. One could argue that longer rates make for better data
I would adjust the LED distance until I got an evaporation rate in this range.
I don't have a better method for observing the evaporation of the alcohol, but I have posted this question in the expert forum, and will post any answers here.
In reviewing the experiment, I noticed something that I would like to mention. Please note that the load resistor for the blue LED element should be less than the other 2 segments.
Keith

Can you possibly hold forth on what the resistor values should be (for R, G & B)
as well as the 'safe' value of the forward current that should be used for the experiment?

I was also wondering if there were any further updates on the use of the index card for the
drop of alcohol.....

Thanks in advance
Kirby16

I'm still checking with other experts on ways to measure when the evaporation is finished.

I've tried to get a good idea of what the circuits should be for the 3 color LED. One of the problems I have with this experiment is that it assumes that all 3 LED colors output the same wattage / lumens / whatever. They don't. I've been reading the data sheets for a couple of these LEDs, and they confirm that the outputs of each color are very different. I'll try and attach a file that will show the data. (I finally found a format this forum will accept - I apologize if you have trouble reading the .xls file, I had very few choices.)

From this you can see that the blue LED is much less efficient. A "back of the envelope" calculation suggests that you would want to bias the red LED at ~10mA, the green LED at 3 mA and the blue LED at 22 mA to have them all output about the same number of lumens (about 300 each).

If you were using a 9V battery (as in the experiment), this would imply the red circuit would need 680 ohms in series, the greeen about 2200 ohms, and the blue about 270 ohms. (These are standard value resistors).

The appropriate circuit is shown in the project description. Let me know if this works for you.

Keith

Keith,

Thanks for all the info. I am using the LED (604-WP154A4-RGB) that Amy updated on the project description.
The datasheet was not readily available but I managed to get it from Mouser. This does not have the lumen
info that you are alluding to. The datasheets that you are referring to - would one of those happen to be
for the LED that I am using?

Thanks
Kirby16

I went to the Mouser site and cross referenced their P/N to the manufacture P/N and put that into a search engine and came up with this online data sheet http://www.us.kingbright.com/images/cat ... BGVGAW.pdf.

It lists the luminosity in mcd's at 20 mA forward current for each of the three colors with minimum and typical values. The data sheet shows the typical forward voltage drop for the different colors as being different. Later in the data sheet it shows a curve for the luminosity vs forward current and it is a straight line (linear relationship going through the origin, can't get much better in terms of simplicity than that).

From this and data and Kirkoff's Law and Ohm's Law, you can calculate the resistors needed if you know the power supply voltage. Keith did the calculation for a significantly different LED. Based on typical mcd outputs for this part, the red will need 2/3 of the current that the green needs (750 vs 500). The blue will need 10% more than the green.

If you calculate the blue for 20 ma, 9v - 3.2v (forward voltage from curve) = 5.8v, 5.8v / 0.02a = 290 ohms.
The green for 18 ma, 9v - 3.15v = 5.85v / 0.018 = 325 ohms.
You can do the red yourself.

Note these resistor values are not the standard 5% resistor values so you will have to determine what the closest standard tolerance resistor is.

Craig,

I had worked out the resistor values - that was pretty simple. However, I could not corelate the luminosity info in the datasheet with
the difference in lumens that Keith was alluding to. If you look at the spreadsheet that he had attached, if you take the lumens into
account, the resistor values change radically. I wonder if we can get some input from the folks that put up this project - in terms of
how we can ensure a level playing field for the 3 LEDs in order to be able to conduct the experiment and record accurate data.

Thanks
Kirby16

Unless you actually have some means of measuring mcd (luminosity), I already gave you an approach for the WP154A4SUREPBGVGAW part based on typical mcd output. Blue takes the most current, so calculate resistor for 20ma (the recommended maximum forward operating current). The green typically needs 90% of the current for of the blue, so calculate resistor for 18ma. The red takes 2/3 of the current for the green, so that that comes out to 12ma. If you do what I suggested, you will typically have a level playing field - no further adjustments required. As I said before, Keith's spread sheet was for a much different part with different characteristics so it does NOT apply to this part!