LED lighting, part 2

tin plated My first attempt at using the tiny Luxeon Rebel LEDs taught me several things, among them how difficult it is to hand-solder them. I also realized that I would need to experiment more with the different white LEDs that are available, or potentially using red green and blue LEDs to produce the white I’m looking for (or a combination thereof). So I designed two new PCBs… a new six-LED board with individual control over each of the LEDs, and a tiny single-Luxeon Rebel breakout board so I can mix-and-match different combinations quickly and easily. To both designs I also added very tiny dots at the corners of each LED location to help position the LEDs. These designs are two-sided, with the back side consisting of a large copper field to help transfer heat to the aluminum heat sink.

I made the boards the same way as before, except that I tin-plated the finished boards with Tinnit to protect the copper from tarnishing and to improve the solderability.

For this batch of boards I decided to finally try my hand at reflow soldering, using the skillet method described by the Sparkfun guys. I bought an inexpensive skillet at Target, an infrared thermometer at Lowes, and some no-clean solder paste. The solder paste came in a syringe package but didn’t come with any needles, so I squeezed a little paste onto a paper towel and carefully dabbed it onto the PCBs with a toothpick. Using a pair of tweezers I placed each LED into position and pressed it into the paste, which held the component fairly well.

Before trying any soldering I looked up the reflow profiles for both the solder paste and the LEDs, and experimented with the skillet to see what settings would yield the target temperatures. I may one day build an Arduino temperature control for the skillet to more precisely control the profile, but I think these reflow characteristics are pretty flexible and for now it’s working just fine.

reflow skillet reflowing

I put the PCBs into the middle of the skillet (I got wildly different temperature readings from different spots on the skillet) and turned it up to “LOW”, watching the temperature with the thermometer. As the temperature leveled off at around 165°C the boards began to smoke and I turned it up to “MED”. Within a minute or so the solder liquified and flowed nicely, in some cases shifting the LED into perfect alignment with the solder pads (apparently a result of the solder’s surface tension).

soldered assembled with headers

I made breakout boards of several different LEDs: “warm white“, “ANSI 2700K white” (an even warmer white), “amber“, and red, green and blue. After the skillet reflowing I hand-soldered on a couple of header pins for the breadboard.

breadboard

This breadboard setup allows me to swap out different combinations of LEDs to evaluate the color of the light (here I have six 2700K Rebels installed). For this mock-up I used a 24VDC desktop power supply powering a BuckPuck LED driver, switched with a momentary pushbutton. I’m getting closer to the color temperature I want, but right now these are all on (full power) or all off. The next step will be PWM control over individual LEDs or groups of two or more to start precisely dialing in the settings.

8 Responses to “LED lighting, part 2”

  1. Jeff writes:

    You might want to look into the Cypress Semiconductor “EZ Color” LED color “mixing” chips (see: http://www.cypress.com/?id=1579&source=header). They’ll drive bucks or LM3402′s and the firmware and PC interfaces make it really easy to mix R/G/B LEDs into specific colors.

  2. Matt writes:

    I really like this skillet reflow idea. I’ve been wanting to get some Rebel LED’s, but didn’t think I could handle the SMD soldering.

    I’ll definitely kick the idea around now. Any chance you can throw the PCB files up?

    Thanks!

  3. Beth. writes:

    Hi Joel, I had just seen your site via hackaday (I previously was here seeing your 55 gallon big black thing ;) ), and you are doing a fantastic work.

    Nowadays I’m finishing my home and I always had in my mind using leds for lighting. But with the little free time I have doing some research is hard.

    There is a lot of information, led suppliers, electrical and mechanical design considerations, led properties, etc, etc.

    And is nice to see your work, as hey I must to do the same before I decide something, I really don’t want a blueish or cold white on my lights.

    The low CRI of some leds, high prices of others and such things make me feel lost in these sea of leds, strips, drivers, cri, color temperatures, lumens, lumens/watt, lumens/price, china, … aarrrgg.

    Well friend, before boring you, I don’t know if you had seen that but I find it interesting, I got some useful conclusions from that videos:

    http://www.national.com/analog/nationaltv

    There are a lot of interesting videos there but for leds, “Driving High-Power LEDs Without Getting Burned – Part 1″ and “Driving High-Power LEDs Without Getting Burned – Part 2″ maybe useful for you.

    On the first one, the second man starting from the left (green led man at http://www.national.com/nationaltv/abdshow6/index.html , sorry but I don’t remember his name), give a nice explanation about light theory and why colors are not seen the same with led lighting than with sun light.

    Also they recommend for RGB applications, using an RGB led for color mixing and a white one when we need white light.

    That’s all, nice work and thanks for helping me, this 2700K Rebels seems to be warm enough for me ( I must try them ).

    Best regards from Spain, Beth.

  4. joel writes:

    Hi Beth- Wow, I think I really bit off more than I first realized here in terms of color/illumination theory, and that first video really made it clearer… thank you so much for the links! You’re right– it looks like the red-green-blue solution won’t work for illumination, unless you add in the other colors as well (per the Mona Lisa example in the video… hmm).

    Stay tuned, I hope my project here helps you find a good solution!

  5. Patrick writes:

    I’m into quality of light and I have been using filters to manage the color temperature of compact fluorescent lights in my home. I got the idea from when I used FL-D fluorescent filters on my camera to help remove the green from the lights on daylight film, and now I purchased some high temperature ones used for movies and it makes the light the correct balance. It’s basically adding cyan and magenta. I’m not sure, due to the nature of an LED, if it can produce the color of light you are looking for. Have you considered the type of semiconductor material it is made from, there may be purity differences from one batch to the next. Well, good luck and I hope to keep seeing great work from you.

  6. joel writes:

    Hi Patrick- Thanks for reading! Yeah there’s some info on batching in the Rebel datasheets, which I didn’t really understand until seeing the videos Beth linked to in the comment above. I’m going to try prototyping a fixture with some warm white Rebels, stay tuned…

  7. Adam writes:

    Joel, you are a friggin’ animal from another planet. I thought I was getting pretty geeky over here, but you’re taking this to a whole new level. We need to have some kind of “outrageous DIY projects” competition, similar to the ‘stache contest. You may have beaten me out on the facial hair front, but I refuse to be out-geeked by a pretty boy like you… Consider the gauntlet thrown, and if you were here, I’d slap you across the face with a Nintendo Power Glove.

  8. joel writes:

    Dude I’m humbled by your awesome geekitude. But if you’re suggesting a DIY-off then I accept!
    We miss you around here man. But it sounds like you’re doing awesome things in L.A. Hope it keeps going well, and your path leads you back to Boston some day…