Ghetto Wireless Mouse Inductive Charger Mod (that didnt friggin work)
Wireless mice were created for guys who want to look at porn at a distance away from their computer so they don't mess up their keyboard. If you look at porn half as much as I do, this means that battery life will be about 2 days. The latest thing to hit the wireless desktop scene is Logitech's MX700 mouse but somehow I know I would be just too lazy to remember to stick the mouse in the recharger all the time.
So Endo and I came up with
the concept of using inductive charging to recharge the mouse constantly any
time it is just placed on top of the mouse pad. Its basically the same thing
used in those toothbrushes
that are for nonghetto people who would rather pay 50 bucks than move their
hand. This is kinda how it would look:
The circuit (the part I will explain badly and you won't understand)
Inside the wireless mouse receiver,
fix up some crap that will cause AC to flow through a coil that you have embedded
in the bottom of your mousepad or desk. The current flowing through the coil
will create an expanding and collapsing magnetic field around the surface
of the pad. Inside the mouse, you have another coil wrapped around a little
piece of ferrite. The expanding and collapsing magnetic field around the pad
will cause AC to flow through the coil inside the mouse. Then you just take
that AC, rectify it and smooth it and you will have DC suitable for charging
batteries. I would use NiMH batteries for this project, because they have
none of that memory
effect crap so you can just leave them being constantly used or trickle
charged without them getting messed up.
Here's the circuit diagram. There are like 1000 ways to build a suitable oscillator - I only showed one. On the primary side of things, you have the oscillator causing the transistor to turn off/get saturated on. This will cause current to flow, and stop flowing, from the USB power supply. A BJT or a FET will do pretty much the same thing here all you want is AC flowing through the primary coil so it shouldn't matter what you use. On the secondary side, that ferrite chunk helps maximise the magnetic field picked up by the secondary coil. A full wave bridge rectifier made from a few diodes, and a capacitor (anything above 0.1uF works) will give you a reasonably clean DC supply with which you can recharge the mouse's batteries. In fact, with small modifications, you would actually be able to power the mouse this way and not need batteries at all as long as it was near the pad. Normally you would have to have to make a constant current source to trickle charge the NiMH batteries, but I found the current flowing through the secondary to be so small that theres no way I could possibly have gone above the max charging current for the batteries. Which is good, because exploding batteries gets acid on your weiner and that hurts.
Scavenging for parts (the
I found a suitable 2N2904 transistor on an old apple printer motherboard. I measured its max current rating to be around 300mA - perfect for a USB powered device.
A dead ATX power supply provided
me with 4 diodes and a small capacitor for the rectifier. The tiny diodes
here were really good because they only had a 0.35V forward voltage drop as
opposed to the 0.8v of most larger diodes. Since I wasn't expecting to get
much of a potential difference across the secondary I wanted the smallest
voltage drop possible.
Another dead power supply or two provided a supply of little transformers, ferrite, and magnet wire to play with.
I based my oscillator out of
a 74LS02 chip
that I had found on the circuit board of an old 5.25" floppy drive. Desoldering
chips is a real pain with a soldering iron, so I did it Arnold style with
a butane torch.
First I gripped the chip with a vice, then melted all the solder points simultaneously
with the torch whilst putting tension on the chip legs. Amazingly it still
worked after this abuse. Make
sure you open a window cause burning PCBs almost smells worse than my old
roommate's farts. Almost.
Building it (the fun part if you are a loser like me)
5v taken from the USB port,
from the wireless mouse receiver.
My bridge rectifier looked like a spider mastermind!
The oscillator. I used a 1k and a 100 ohm pot in series for large and fine tuning adjustments.
Other testing and prototyping stuff was done on a breadboard so I could check it all worked before shoving it inside the mouse. During testing, rather than using my simple oscillator, I borrowed a signal generator from the university (thanks Elaine!) because it allowed me to change frequencies much easier.
Possible expansions for this project:
Inductive charge living room table, where you just put your TV remote or wavebird controllers on the table for them to charge.
Magic tricks - have an led light up when you move your hand close to a 'magic box'.