Keith's Electronics Blog

Maeve had several of these battery-operated, white LED accent lights, and let me borrow one until the next visit.

It was a good opportunity to test a few ideas about the LED puck. If a commercial product was already on the market, albeit for a different target audience, maybe it’d be good enough and I wouldn’t need to design my own. More likely, I could identify and clarify design differences between their product and my own ideas, which is what turned out to be the case.

First, the LEDs in the commercial puck are pretty directional. When placed on a nightstand pointing at the ceiling, it illuminates about a 2′ diameter area. I’d like the puck to be much more omnidirectional, casting light as evenly as possible throughout a hemispherical area.

Second, it’s not bright enough. Used like a flashlight and pointed directly at a book, it provides more than enough light to read; but pointed upward, the diffusely reflected light isn’t quite enough to read by. I specifically want to be able to read by the light of the puck, and I’d need several more LEDs and somewhat more power to be able to do that.

Third, it’s too bulky, specifically, too thick. I could never carry it in a pocket. I have been carrying it back and forth to work in my laptop satchel, but I realize I’d like the puck (or one version of it) to be small enough to carry with me in a pocket all the time. What’s the point of having cool toys if they’re never with you when you need them?

The light portion of the commercial puck tilts so it can be aimed, and the light portion is actually plenty small to carry around in a pocket. The whole enclosure is larger both to accommodate the tilting action, and because it houses three AAA cells.

The space needed to accommodate a power source concerns me. I’d like to find a flatpack LiPo battery, skinny like a cell phone battery; but I fear that power may be the driving constraint on overall puck size.

Homework Assignment

For anyone reading who’d like to build their own puck, or anyone amused enough by the idea to humor me, I have a homework assignment for you.

Find something round, (*) about the size that you envision the puck being, and carry it with you for a few days. Post a comment about what you used, its dimensions, where on your person you carried it, how comfortable/convenient it was, and whether you’d carry something that size with you all the time.

* (I envision the puck having a rounded profile like a drop of liquid sitting on a horizontal surface, rather than being an untrimmed cylinder. Rounding off the top would slightly improve the comfort of carrying it in a pocket, but cylinders that I’m able to find are good enough for a first test.)

I’ll start:

• I’ve been carrying the commercial LED light in my laptop bag for a week. It’s 3 1/2″ d and just over 1 1/2″ thick. It’s way too thick to fit into a pants pocket; it fits fine in my laptop bag, but then so would a hardback book. I would always have it nearby, although never on my person.
• My first inspiration for the physical form factor of the puck was the empty container from a roll of 3M electrical tape. It’s also 3 1/2″ d but only .9″ thick. It’s still the size that I think would look best for a puck, but it’s uncomfortably large to carry in a pocket, and I haven’t even tried. I would definitely carry it with me in my bag.
• The peanut butter jar lid is 3″ d and 1/2″ thick. I’ve been carrying it in my back jeans pocket since last night, and I hardly notice it’s there; I’d notice it even a little less if the top edge were rounded. This would be a great size for carrying with me all the time — but there’s no way to fit both battery and outward-angled 5mm LEDs into that thickness.

Maybe two puck sizes will be needed — the original puck, in the electrical tape container size, and the porta-puck, in the jar lid size.

Cost: Realistically, I see this thing climbing into the $15-30 range for materials. That’s more than I’d pay to buy a puck from ThinkGeek, but not more than I’d pay to construct one myself that I helped design. Anyone else?

Battery: Seems like we’d get a lot better power density (translating to smaller size and longer runtimes) if we used an RC-style LiPo battery, I’m thinking about the size of a matchbook or so. This’ll be more expensive; but I’m concerned that fitting in 2-3 AAA or AA cells will make the puck bulky that it’s not cool anymore. How do you feel about having to buy a special battery for it?

Charge circuit: I can find a charge-control chip and use the datasheet’s sample implementation as well as the next guy; but that’s really just a starting point, and I understand there are often subtleties involved in getting the best real-world performance. Does anyone have a lot of charging circuit design experience, or know someone who does, who’d be willing to design that portion and contribute it to an open-source hardware, community-developed project? We might want to look at LadyAda’s WaveBubble or the Chumby design for reference.

Voltage-boost circuit: Depending on what battery, microcontroller, and LEDs are used, we may need to boost the battery voltage. Same questions as on the charge circuit.

Surface-mount components: I don’t think there’s room to use through-hole technology. I’m envisioning at least the microcontroller, LED driver, and optional real-time clock chips being SMT. LEDs need to be through-hole, in my opinion, both for rated brightness and so they can be aimed slightly away from the center for better light dispersion. Will a few SMT components be enough to keep anyone from building this who would otherwise be interested?

Enclosure: My first thought was pouring this thing in resin, but I’d really like to be able to take it apart and service / enhance it. I’m now leaning toward a two-piece milled enclosure (probably lexan) that would sandwich the PCB edges in rubber for shock control and hold together with recessed screws underneath. Add an appropriate O-ring and the whole thing could probably be made watertight — which in my book counts as another feature. Thoughts?