Wednesday, January 7, 2009

The UV LED microcontroller bracelet

I have been thinking about soft circuits for a while now. Its interesting to think about electronics as they relate to clothing, as well as building circuits with no solder, just knots. At about the same time i had come across "magic beads", which are white beads that change colors when exposed to UV light. Apparently they are one of the least expensive ways to reliably detect UV, and have some scientific use. but mainly they are sold by science education stores to teach kids about sun protection. The beads retain their dark color for a bit after the UV light source is removed, so the idea came up to make a slow refreshing solid state display out of beads and UV leds. This got mashed up with the soft circuit idea, and the ultimate raver cancer device was born!

I found some UV led's from a china importer for really cheap, and decided to use the arduino minipro from sparkfun as the microcontroller. it has such a small form factor, and is completely integrated so no external parts are needed besides power. Theres also a CR2032 battery holder from sparkfun, and a magnetic conductive snap (also from sparkfun. can you tell i love this place?). I took each bead and made a cut through it, and fit the LED through two of them.

Using conductive thread (sparkfun), and a piece of leather as a board I experimented with how close you could get the traces together in a sewing machine. The leather made a surprisingly good circuit board. the critical part here is to use the conductive thread on the *bottom* bobbin, not the main bobbin. you get a perfectly flat "wire" with a nice little thin wrap around it. This stuff frays like *crazy*, and was a giant pain in the ass to work with. Id almost recommend hairspraying over it lightly or something once you finally get it with no shorts.



So after many hours of carefully..... okay, im lying. I totally had my sewing machine master roommate sarah sew this for me. I laid it out with a ruler and had her sew over the traces. It was unusually fun designing the circuit traces, this is why i am a nerd. Its sort of hard to tell, but the microcontroller will go on the right, and there are 10 UV led's that run down the center of the bracelet.

the LED's were attached by wrapping the wire very tightly around each lead, and sealing it with a drop of glue. I had high hopes to make the entire thing with no adhesives of any sort, but the fraying was really frustrating me.

Here you can see the first few leads of the microcontroller attached. they were attached just by stitching through the hole in the board a few times around the leather. it looks fantastic, and was very solid. You can also see the detachable mini USB programming header.

Here it is! It is very very difficult to take videos and photos of this thing. all of my cameras react very badly to the UV light, even with playing with UV filters. There is no flickering to the eye, that is an artifact of the camera. It has ten individually addressable LED's, with the full power of the arduino behind them. This program reads from a stored sine wave table to do fakie PWM on all 10 pins. Sadly, the beads do not hold their color for very long since the UV is not that strong coming out of the led's. its bright, its just not enough of the ideal wavelength. The effect is pretty damn cool anyways though. i cant see actually wearing this, but it was a cool way to explore these technologies. Its pretty amazing how advanced of things you can make these days at home with fifty bucks and a few hours time.