Gas station without pumps

2014 October 5

Summer project

Filed under: Uncategorized — gasstationwithoutpumps @ 19:06
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I’m finally ready to reveal the project I’ve been working on all summer: a “kit” for making dimmable LED lamps.

The idea of the project is to have a flexible set of electronics modules that can be put together in various ways to get quite different lighting fixtures.  I ended up designing around a 9V power supply, and making two custom modules:

  • a dimmer board that reads a potentiometer and converts it (non-linearly) to a pulse-width-modulated 9V output signal.
  • LED boards that hook up to the two wires of the PWM input signal, and that can be run in parallel.

The key thing here is that the LED boards are designed to have roughly constant brightness despite variations in the LEDs or in the IR voltage drop of the wiring.  Each LED board has a constant current circuit and a little heatsink to keep the LEDs cool (they last longer that way, so I went overboard on the heatsink, keeping the LEDs well below their maximum temperature). The boards are designed to put out about 85 lumens of warm-white light at 130mA (dissipating about 1.17W on the board—with a 90% efficient 9V supply, the overall efficiency is about 65 lumens/W).

So far I’ve wired up two of the dimmer boards and tested them at currents of up to 4.8A—the transistors on the board don’t get hot (not even up to body temperature).  I don’t have a dummy load for testing at higher currents, but the board was designed to be able to handle at least 6A.  I paid extra to have the dimmer boards fabricated with 2oz copper (through ITEADstudio), both to reduce resistance on the board and to provide better heat sinking.  I want to be able to use the board in enclosed cases, and I don’t think that there is will be any trouble with that.

I’ve also tested 36 of the 100 LED boards I had made. Because the LEDs are surface-mount devices with big pads, I paid Elecrow to assemble the boards for me.  Their prices came out a little lower than Smart Prototyping for this particular board, but the difference in pricing schemes could make either one be cheaper for a given design. So far 34/36 work (~95%), which is a somewhat lower yield than I had expected for such a simple design with large-pitch components. I’ve not reworked the bad boards yet, but I did a little testing, and the problem seems to be a damaged transistor rather than an obvious soldering problem such as a short or open. I wonder if they were careless about their anti-static protocols, or whether the transistors were damaged before they installed them.

My original goal was to make an LED lighting fixture to replace the old ceiling fan in my breakfast room (using 10–20 of the LED boards, to get 850–1700 lumens), but I’ve not gotten that one done yet, because I got sidetracked into two other related projects:

  • Making a custom desk lamp for my son.
  • Making a prototype table lamp for my sister.

Originally, my son was going to design his own desk lamp to sit on a shelf above his desk, based on the desks we had seen in the dorms at orientation.  But when he moved into the dorm last weekend, he found that the desks in this dorm had no book shelf over the desk. But he really needs a desk lamp, because he lofted his bed over the desk to make more space, so the desk is quite dark.  I decided that I would make him a desk lamp as quickly as I could, designing it on the train home, and sharing sketches with him by e-mail when I got home.  This weekend I threw the project together as quickly as possible, so that I could ship it to him on Monday.

At the back of his desk is a 1″ thick wooden brace for the lofted bed, so I designed the lamp to hook over that brace, with the control box over the desk and LED lamps about 50cm above the desk on 10-gauge copper wire. The LEDs are light enough that the wire alone is enough to support them, though a thicker wire would be a little less wobbly.

Here is a side view of the desk lamp showing the hook for sliding over the bed rail.  I've only populated 4 of the 5 positions for LED boards.

Here is a side view of the desk lamp showing the hook for sliding over the bed rail. I’ve only populated 4 of the 5 positions for LED boards.

Front view of the desk lamp, as it would appear at the back of his desk.  The knob controls the dimmer, and a 9V wall wart provide power on the right side.

Front view of the desk lamp, as it would appear at the back of his desk. The knob controls the dimmer, and a 9V wall wart provide power on the right side.

Because I had to throw this thing together in a hurry, it has a very “homemade” look to it. The box is a cheap wooden craft box from the art supply store, the hook is a piece of masonite glued to a couple of  pieces of  scrap wood I had in the living room. I did not take the time to trim everything to perfect fit, nor to do more than cursory sanding.  I finished the box with Danish oil, but I only had clear oil, and one with a stain included would have looked better.

But the lamp works well. The vertical wires are attached to screws with knurled thumbnuts, and the LED boards to the wires the same way. These give the lamp a certain “steampunk” charm, though there should really be a big knurled brass knob instead of a plastic one to enhance that effect.

Next weekend I plan to try to finish a table lamp for my sister—it needs to look a bit nicer, since she is considering making a series of table lamps using stiffened-silk shades (she makes stiffened-silk bowls, and her customers have been telling her that she needs to make lamps).  I want her to see the artistic possibilities, and she isn’t into the rustic “homemade” look.  I’ll also have to provide her with instructions on how to put together the electronics for a lamp with artist-level instructions. I’ll have put the lamp together, but she’ll need to be able to figure out how to put the same electronics into a different base with different support for a shade. That means not just building the lamp, but explaining how it was built and why certain choices were made, so that she can do her own designs.

I’ll probably detail some of that build in a blog post here, so that I have a record as well as her.

3 Comments »

  1. […] simple go/no-go testing for a bunch of the LED boards that I designed for lighting fixtures (see Summer project), but I thought it would be interesting to characterize at least one of the boards more thoroughly. […]

    Pingback by LED board I-vs-V curve | Gas station without pumps — 2014 October 15 @ 21:27 | Reply

  2. […] Summer Project, I introduced the project I’ve been working on all summer: a “kit” for making […]

    Pingback by Summer project 2 | Gas station without pumps — 2014 October 19 @ 20:50 | Reply

  3. […] Summer Project and Summer Project 2, I introduced the project I’ve been working on all summer: a […]

    Pingback by Desk lamp | Gas station without pumps — 2014 December 28 @ 18:47 | Reply


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