Introduction: USB Heated Mouse
I've never understood usb powered coasters that are supposed to keep your cuppa hot.
If your cuppa goes cold in the time it takes you to drink it, drink it faster. Coffee is medicinal, and you can never have enough tea.
However, with this inspiration, I can take a stab at another problem.
Why do fingers freeze when in contact with computer mice?
It seems I'm a bit late to this party, with commercial units for about $25, but it's so easy to do yourself with the seemingly endless supply of discarded mice.
With this in mind, I present my USB heated optical mouse.
Step 1: Power
So you want to make things hot. Power is the most important thing.
If you place a hot cup of tea on a set of scales and allow it to cool, it looses about 25 watts of power by evaporation alone.
(measurable by mass of water evaporated per unit time * latent heat of vaporisation)
My soldering iron would have trouble keeping up with that.
USB specifications allow you to have 2.5 Watts total. (5V * 500mA) Feeble. However, this can get comfortably warm.
To maximise power draw without your computer making a fuss, you can place 10 ohms accross the power rails in your USB device.
Two 22 Ohm resistors in parallel should do while leaving a bit of room for the original circuit.
In this case, I need a nice distributed heat so I'm going for two 10W wire-wound resistors simply for the size. Distributing the heat means you don't melt the plastic.
Step 2: Select a Victim and Tear It to Pieces.
I have no idea how I acquired this particular mouse... You know how the bottom of the cupboard is.
But as far as generic goes, this is it; version 1.0A from Microsoft.
There shouldn't be anything stopping you from doing this to a ball mouse, or even a track ball if that's what you have.
If you have options, go for one that sounds hollow. You probably won't be able to tell how much real-estate you have by the mass, mine has a chunk of iron in it to add heft.
Start by voiding whatever warranty there might be. Most of the screws will be hidden under stickers.
My mouse only had one screw but it had these crazy sliding clips either side of the scroll wheel, it took me a while to get it open without destroying it.
You'll want to work out placement of parts with the mouse mostly intact but you need to get access to the circuit board to work on it.
Step 3: Add Parts
Check what kinds of resistor will fit, there are plenty of different sizes.
10W are big and heat slowly but distribute the heat well.
If you're particularly enthusiastic, you can use 1/4W resistors but you'll need the power evenly distributed between at least ten of them.
(I'd expect nichrome wire to melt pretty much any plastic housing but feel free to try)
In my case, I could fit 10W resistors down both sides and reach the solder side from almost anywhere on the board.
I couldn't easily reach the USB header but the circuit board was single sided and had a very obvious ground plane that went all the way around the board.
I was able to solder to the button joins which were connected to ground.
The positive connection was more difficult, I had to solder a breakout wire directly to the USB header.
You may like to add a switch to the resistors, I didn't and I'll probably regret that in summer.
Step 4: Whip Out the Soldering Iron
First, the breakout. (only do this if you have to)
My favourite wire to use is from CAT5 solid core cable.
When you solder to the existing solder connection, you should have as little copper exposed as possible, 1mm exposed is probably too much.
Don't bother stripping it, just very carefully tin the very end of the copper this melts back the insulator and tins the copper neater than I could do any other way. It doesn't make much of a smell, it simply distorts the plastic.
Solder that end to the power rail wherever you like on the circuit board, just make sure it traces straight back to the red or black wire. I chose the header itself.
Bend and trim the leads of your resistors to suit the mouse, then solder them in place.
At this point, if you have resistors across the power rails of the usb cable, you can test your Frankenstein exposed circuit board. (5V can't kill you unless you happen to be a poorly constructed robot, in which case you probably shouldn't need a USB heated mouse)
If it works, pack it back in the case and pretty it up.
(you'll need the crazy little lens thing to get the cursor moving but just the light coming on means you've done it)
If it doesn't work, try removing one of the power rail connections to your resistors. If the mouse is still alive, revise your choice of resistors. You're drawing too much power.
Step 5: Put It Back in The... Case...
Yeah, honestly, mine didn't go that smoothly.
I had to trim some of the interior plastic to go over the new wires.
Because of the crazy clever design (cost cutting nonsense and only having one screw) it took me half an hour to put it back together the way it was intended.
(astounding and slightly disturbing that they do these by hand as an industrial process)
By the way, since you've put so much effort into this, why should it carry someone else's logo?
Sand off the old logo and have at it!
Make your design in pencil then go over it, CD markers have permanent ink and a fine point.
Any sort of use tends to smudge the marker. You can fix that with any spray-can varnish but be careful, the varnish has solvents that make the marker run. Do three coats extremely lightly and let them dry in between. Then you can start laying it on normally.
It takes at least 5 minutes for it to warm up but because the resistors are so big and that chunk of iron is still there, it stays warm. Yes, it does keep the blood going in my hand, but it is feeble.
Next time I'll try the ESATA power connection.