How would a Victorian engineer make a mouse? You can't build an optical mouse with a mechanical difference engine. So you need wheels and mechanics.
This Instructable shows how to dissect a (computer) mouse and how to mechanically connect to the encoders of a mouse. I'm sure you can think of lots of ways of making a mouse with wheels.
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Step 1: Size
It looks large but it's actually not much bigger than a standard desktop mouse. Body length 111mm; Width 68mm; Height 57mm.
It's fairly comfortable to hold and can be used for most mouse tasks although it's a little more tiring than an optical mouse. It needs a mouse mat (in the same way that a ball-mouse does).
With a painting program, it's very easy to draw exactly horizontal and vertical lines - just lift the other wheels off the mat. Diagonal lines and circles are drawn OK: both sets of wheels will turn smoothly at the same time. Of course, it's never easy to draw a circle with a mouse.
Step 2: How It Was Made
Start with a USB ball-mouse. They are not too common. Ball-mice usually have the old PS/2 connector which no longer fits most computers and USB mice are generally optical. But if you search, you can find the occasional USB ball-mouse in a charity shop or car boot sale. It shouldn't cost more than £1.
I wondered if there should be two wheels at the front or at the back so I built models out of Lego. It turns out that it's much more stable with them at the back. The "mudguards" are needed to stop your hand touching the wheels.
Take a photo of the pcb layout to trace the tracks - it's easier with a photo. Find the tracks that lead to the optical encoders and switches and make a note of the circuit.
Cut off the unnecessary parts of the pcb
Step 3: Gears and Axles
Gears from a broken clock movement are used as wheels. The back wheels are both the same size, the front wheel is larger.
The axles are made from brass tube. A coin is soldered near the middle to drive the wheel encoders. During construction, temporarily stuff the gears (and their axles) into the tubes with rolled-up paper.
Stripboard is used to support the encoder wheel, the LED and the sensor. One end of the shaft rotates in a washer held in place by brass wire. The other end is held in a slot bent out of brass wire.
A short length of rubber tube is fitted over the encoder shaft. The coin fits in a slot in the PCB and a spring pushes it against the rubber tube. One end of the shaft can slide in the brass slot and the rubber tube is held against the coin with a small spring.
Carefully bend the LED and the sensor leads so they point at each other and successfully encode the rotation.
Step 4: The Front Wheel
The arrangement for the front wheel is similar. The wheel axles rotate in bearings bent from brass wire. They are prevented from sliding by collars soldered onto the axle tubes.
You can test the sensors by temporarily wiring them into the original mouse PCB.
Each sensor consists of an infra-red LED light (with two pins) and two IR receivers in one chip (with three pins). The LED has to shine at the receiver. The receiver has two slots which the light shines through; the slots must align with the slots in the encoder wheel.
Step 5: The Mouse Wheel
The mouse wheel fits in a hinged carrier which operates a microswitch. Remove the rubber tyre and glue on a brass olive (used in plumbing). Some olives have a groove which looks very nice.
The carrier is hinged onto another piece of stripboard. The mouse wheel encoders and switches are soldered into place.
Step 6: The Pushbuttons
Pushbuttons are made from "Tactile Switches". The corners are filed off the plastic switch button and it then fits inside a 14mm brass olive.
Circular letters are printed and cut out of paper. They are covered with small circles of celluloid.
The buttons are mounted on PCBs made from yet more stripboard.
Mount what's left of the old PCB over the new stripboard. I used "lacquered" copper wire - transformer wire - to connect the sensors to the old PCB. You can buy wire with a coating that burns off when you solder it; it's much easier to work with.
Step 7: The Bottom Shell
The bottom part of the mouse shell is made from papier mache. During construction, the mechanics are protected from the glue and are padded so that the shell won't touch the coins or encoder wheels.
After smoothing, painting and varnishing the result is quite good.
Step 8: The Top Shell
The sides and top are made out of old tinplate. Cardboard templates are made first then copied in metal. Tinplate is easy to soft-solder and is nice material to work with.
The tinplate is bent, soldered, filed, filled and smoothed. Car body-filler would be good but I didn't have any so I used a mixture of plaster of paris and PVA glue. It took a couple of cycles of fill, sand, fill, sand. Then three cycles of primer, sand, primer, sand.
The final coat of paint is red aerosol car paint with a light spray of black car paint while the red is still wet. It gives a nice textured appearance.
The coach lines are hand-painted in gold arcylic. It is then given several coats of varnish.
When it's all finished, the gears and their original axles are glued into the brass axle tubes with epoxy.
The USB cable is covered in a bootlace outer to keep it in character.
A webpage describing this project is here. It has links to some of my other projects.