Thursday, July 16, 2009

Automatic Dark Detecting Joule Thief Nighlight

I was shopping at Wacko's for toys to fit electronics into, and the Munny was pretty much the only thing I found that was big enough, hollow enough, cheap enough, and made of vinyl (pliable enough). For about $10 it made an affordable and fashionable chassis for this project.

After decorating it with various electronic components, I set to work on the insides.

Part 1: The dark detecting circuit. I found these schematics on Watson's Blog, but since I couldn't find the mosfet required for Watson's modified circuit, I used the EvilMadScientist original circuit. I added one more LED in parallel (total 2). I also replaced the 1kohm resistor for two resistors totaling ~30kohm, which decreased the sensitivity. A note: this also seemed to dull the maximum brightness the LEDs can attain, but at 30kohm they still shine pretty bright.

Part 2: The Supercharged Joule Thief. I used my favorite schematic (Fig. 2, w/o the button cell) also from Watson's Blog for this one.

Part 3: putting it together (EDIT): I just kind of mashed the two circuits together. It was guesswork, and while it worked, I'm pretty sure it didn't work well. Mainly, I was hoping to find a design that would be very efficient so that I could leave it on all the time, and in this sense it did not work at all (the battery only lasted a few days). So I guess just have to read my physics book, do some calculations, and redesign...

I had planned to do this with the standard AA battery, but the holder wasn't going to fit in the doll very nicely. So I swapped that for an AAA battery, which works just fine as well. I improvised my own holder by cutting a piece of tube from the center of a CD holder (the kind blank CDs come on, when you buy them in gigantic packs at costco). I improved terminals at the top and bottom with aluminum foil, and then stuck the whole tube into Munny's lower torso (his head pops on and off for easy battery replacement). I'm not entirely happy with it, but it is easy to replace and if I ever find a thin single AAA battery holder, I will swap it in.

Result: in daylight (even indoors, with closed shutters) the light is off. And at night:

Thursday, July 9, 2009


I'm still enamored with the Joule thief idea, but more importantly, I have a goal: to get the most out of this bucket of old batteries (pictured below) before the batteries start to leak (or before I amass any more of them).

I'm not too fond of flashlights anymore, because I can't make them small enough to be keyring friendly, and because most people don't use flashlights that frequently. So I've switched to mini LED trees (mini tree, not mini LED), like this one I gave to my sister to liven up her dorm room desk:

But making LED trees can get boring, so enter the brushbot:

The brushbot isn't a very good roomba substitute. He's useless on rugs and he tends to just circle in one place. But he could be useful in polishing stuff, and besides, he makes a mean zen garden.

By the way, the brushbot does NOT run on wheels! He just vibrates his way across a surface, like a cell phone or rumble-equipped controller left on a table. Brushbot is made of two red LEDs, a motor with an asymmetric thingamabob attached to it, and a joule thief per this schematic (Fig. 2) from "Waston's eBlog."