"Can't we cross this here and get a couple more Ohms or something?"
January 12, 2010 11:45 AM   Subscribe

I'm trying to build a LED-lit jacket, but need some help with the wiring!

I've got a jacket with 88 10mm diffused LEDs popping out through out of the surface. I've run it before with a whole bunch of watch batteries on each light (like a throwie), but it's got a couple of problems. While I love the redundancy, it's a pain to turn off and it makes a lot of waste.

How can I internally wire the LEDs such that I can have one or two power sources? I've used this calculator but I'm not sure I'm doing it correctly, and I'm not sure about the power requirements of the LEDs. I would rather avoid setting someone on fire. :)

I can solder a circuit board pretty well, but if you have an recommendations on how to specifically wire the LEDs together, I'm all ears.

Thank you so much for your help.
posted by cr_joe to Clothing, Beauty, & Fashion (11 answers total) 6 users marked this as a favorite
This answer is likely of no use for your current effort, but perhaps using these

LED Ribbon strips

in your next project would be more efficient.
posted by de void at 11:58 AM on January 12, 2010

I know next to nothing about this, but it reminded me of a turn signal biking jacket someone built that I had seen awhile back. I poked about and found this tutorial, which might help you out.
posted by urbanlenny at 12:21 PM on January 12, 2010

I'm fuzzy on the power requirements, but these sites might be a good place to tstart if you want to try working with electroconductive thread and wearable technologies.

posted by edbles at 12:37 PM on January 12, 2010

Best answer: You should read up on parallel and series circuits. That's what you're looking for.

Let's assume that the voltage drop across one LED is 1.2V at rated current. This might not be a good estimate, so check your datasheet! It can go past 3V for blue, white, or high-power devices. Let's say you also have a 12V pack of batteries, maybe 8 1.5V AAs in a holder. From there... strings of 10 LEDs in series would match the voltage. So you want to make 8 strands of 10 LEDs connected end-to-end. For the last 8 LEDs, you can run as-is and they'll be a little bright, or you can read up on resistors to get the extra 2.4V voltage drop.

Good luck! Sounds like a fun project. I'm partial to blinky things.
posted by lalas at 1:02 PM on January 12, 2010

Not sure what look you are going for but if lines would work instead of dots you could try using EL Wire. Here is a page with a link to a tutorial on working them into clothing.

I made an EL wire jacket that had a single power source hidden in one of the sleeves once. And it really wasn't that expensive to do, like 10 bucks for the power supply and buck a foot for the wire if you google up some cheap stuff. Best part: no wiring knowledge required!
posted by cirrostratus at 1:14 PM on January 12, 2010

Best answer: for 88 leds, it's probably impractical to put them all in series or all in parallel, like your calculator site shows. You are going to have to do some fiddling to figure out what combination works depending on what batteries you want to use.

Your topology should definitely be a series-parallel. In other words, join together groups of a few leds in series to suit the voltage of the battery you have and then join together the groups in parallel. If you want to use multiple batteries, make a couple of parallel groups.

what kind of leds do you have? Let's say for the sake of argument that you get good light out of them at about 20 mA and they drop around 2 V. These values may be close (or WILDLY different) from yours. First calculation: 0.02 A * 2 V = 0.04 W per led, or about 3.5W for the jacket. Depending on your battery voltage, your current limiting resistor(s) may eat half of that, so I would design figuring for 7 W when this thing is turned on. This is relevant for battery sizing. Example: If you wanted to run the whole thing off a 9 V, 200 mAh battery (which you could, but I wouldn't advise), you would drain the battery in 15 minutes (7 W @ 9V is 777 mA, and 200 mAh / 777 mA = 0.25 h)

For those types of LEDs, you could try series groups of 4 with a 9V battery, or series groups of 5 with a 12V battery. At 12 V, you would want to drop 2 V over your current limiting resistor at 20 mA, so you would want a (2 / 0.02) = 100 ohm resistor.
posted by aquafiend at 1:25 PM on January 12, 2010

Best answer: I'd spec it the other way.

Pick a battery first, then sort out the arrangements LEDs after.
I'd go for one of the sparkun Lithium packs - probably this one 1100mAh or this one 2000mAh. You'll probably need the matching charger too - USB/DC charger. These batteries are awesome - a LOT of power for very light weight, and rechargeable.

Assuming not using blue or white LEDs. You could then have 44 strings of two LEDs in series with an ~18ohm resistor.

I think you'd get about 1.5 hours out of the 1100mAh battery, or 3 from the 2000mAh one.

Please post pictures!
posted by trialex at 2:54 PM on January 12, 2010

Best answer: The Lilypad system from Sparkfun includes lots of fun bits for wiring up your clothes, including conductive thread.
posted by contraption at 5:52 PM on January 12, 2010

Response by poster: I don't have the datasheets on my LEDS - bulk supplier from HK. I've had good luck with watch batteries, though, so I've assumed that kind of power consumption.

How does conductive thread work?
posted by cr_joe at 6:37 PM on January 12, 2010

Lots of advice here on parallel and series LEDs, if you can stand the 90's internet-style webpage. Even if you don't read it, the important thing to remember is that generally you should not put lots of LEDs in series, even if the calculators say you can. The problem is that if one LED has a slightly different forward voltage from the rest, then it can fail, and if it fails as a short circuit, then the other LEDs will insta-fail as well. However, you can put them in series if you personally measure the forward voltage, and/or get them all from the same supplier in the same batch.
posted by RobotNinja at 2:27 PM on January 15, 2010

Damn the man! What I meant is that you generally shouldn't wire them in parallel.
posted by RobotNinja at 2:29 PM on January 15, 2010

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