Tuesday, March 25, 2008

Boring Diet Update

I'll have something much more exciting in a few days, I think. Well, exciting to a NERD. It involves motors and integers and wires.

A month ago, I mentioned my upcoming dietversary. Today is the last day of Year One. My Official Tally of Weight Lost is 38 lbs. That's an average of 2/3-3/4 of a lb per week.

I was half-heartedly shooting for 40 during the last few weeks, knowing I wouldn't make it. But I came pretty close and in the final weeks I doubled my average rate. Also, the Official Tally is based on a moving average, so my last actual weigh-ins have been at or close to exactly 40 lbs. All in all, not at all disappointing.

My original goal was 60 lbs, with a caveat that I may want to go 10 beyond that when I get there (I'm not sure I believe the BMI chart). So I've still got 6 to 9 months ahead of me at this rate. Perhaps by Christmas? Then again, I'm actually chowing on Easter candy as I type this...

Friday, March 14, 2008

Parabola Update

I didn't realize until just now how much as happened since the last update.

The final picture in that post shows the problem. Imperfect focus. My theory was that the parallel parabola issue preventing full closing of the two halves of the form. When I tried it, the halves definitely did fit better, but the resulting mirror acted the same. The next possible culprit was Shaky Hands Syndrome. You can't freehand a perfect cut on a bandsaw, even following a line.

So I came up with an elaborate procedure that would let us use a router.

I'm not going to describe every little nuance of this thing, because seriously, it is much more complicated than it looks. I'm just going to point out the main idea.

If you recall, when I hold the mirror in place with nails it works great. So my theory was that constraining a springy object was making it closely approximate a parabola. So I took another springy object (the tan stick thing on top of the paper) and bent it to fit a parabola printed on paper. Then we used that as a guide for a router.

When I clamped the mirror into the form, it looked really good. There was not a single gap anywhere. But the result is indistinguishable from the freehand versions.

To my eye, the star pattern of the imperfect version looks like it is coming from short sections that aren't agreeing with each other. Like a faceted mirror. That would be easy to explain from a freehand form, but this latest version destroys that theory, at least as far as the form is concerned.

Yesterday I realized that uneven bending could be the result of uneven heating. Maybe some areas of the mirror aren't quite getting up to temperature. I was going to heat the entire thing up by another 10 degrees, but I accidentally broke the form. (Uh oh...it could also be shrinkage or other heat-deformation. I might be doomed.)

Also, I just realized that bending acrylic must be a technique used in industry (and art?). I should google around to see what I find. (I did that before I started, but more from the point of view of parabolas and less from the point of view of general acrylic forming.) (Update: Heh. Sounds like this guy had the exact same problem I'm having, 40 years ago. (picture caption, 2nd paragraph))

It's also possible I'm being a perfectionist. I'm going to have a 1/2" copper pipe running where the focus is. As long as all the energy hits the pipe, it isn't vital that it be pinpoint focus. Still, it's annoying that a thrown-together-in-5-minutes nail version performs so much better. It seems like I must be missing some really simple thing.

I actually do have an entirely different approach in the back of my mind. No oven needed at all...

Thursday, March 13, 2008

The Heated Mouse

Not only do I work in the basement of my building and not only are there lots of computers (and thus lots of AC) but one of the AC vents is right above my desk. Oh and of course I sit still all day. So I get cold hands. More specifically, I get a cold mouse hand.

The obvious1 solution to this problem is a heated mouse. Google can tell you this isn't a new idea but google can't tell you where to actually buy one. The only place that sold them is now dead and in any case seems to have been a scam. People didn't get their shipments or they did but the contents either failed to work as heaters or failed to work as mice.

But this can't be complicated. USB is a powered connection, meaning that a mouse can get power from the computer. If it can draw enough, it can run a tiny heater. As a matter of fact, the USB spec says that a device can draw up to 500mA at 5V, which is 2.5 watts. That's not a lot of heat, but if you were holding it literally in the palm of your hand I thought it might be enough to ward off pneumonia.

USB optical mouse and resistance wire from hair dryer.

Entire inside of optical mouse, USB cable unplugged from board. Red/black are power, don't mess with the other wires (data).

(I really should have photographed this entire build, but I was feeling my way through and didn't want to waste time with a bunch of dead ends.)

The first thing was to get a USB optical mouse. USB for the power, optical to have plenty of space inside. I had one lying around. Then I needed a way to turn electricity to heat. Resistance wire out of a trashed hair dryer turned out to be perfect. Around 3" of wire (coiled length), coated with liquid insulation, provides ~20 ohms of resistance. That draws about 250mA, providing around 125mW of heat. (I stayed well below the spec just in case, plus the mouse itself needs 100mA, according to the label.)

The rest was just a bunch of boring details. How to keep the heater wire inside the mouse. How to attach the power to the coil. How to get the mouse back together. Thinking about safety. Like, am I going to burn down the building? I don't think so...

Does it work? I've got it plugged in right now and it's working great. Works fine as a mouse (didn't even have to reboot, even though I switched from PS/2 to USB) and as a heater (I switched because my hand was cold and now I'm comfy).

I'd say the only problem is that the heat is applied more towards the palm when it needs to be more towards the fingers. It works as-is, but maybe I could uncoil the resistance wire and spread it out a little more around the inside of the mouse to hit all the areas.

1To a dork.