Mouse Dissection 101

By Gareth Branwyn

This document was put together for the 2007 Bay Area Maker Faire(May 19, 20). If you’re coming to the Faire to take my Building Mousey the Junkbot Workshop, you can save some build time (giving you more time to enjoy the Faire) by bringing an already-prepared, recycled computer mouse. Here are the instructions you need to do that prep. We will have more dead mice at the Faire and we’ll have two parts bundles for sale: one to build “My Mousey the Car,” a non-robotic, much simpler, version of Mousey, and one to build the full Mousey the Junkbot, as seen in my book, the Absolute Beginner’s Guide to Building Robots, MAKE Vol. 2, and as a “special guest” on The Colbert Report!

Tools Needed:
Screw Driver(s) (likely small phillips head)
Dremel Tool (w/ cut-off wheel)
Shop Goggles or Safety Glasses (a must!)
Dust Mask (ditto)
Soldering Iron (full Mousey only)
De-solder Wick or De-soldering Pump (full Mousey only)

To create My Mousey the Car or Mousey the Junkbot, you first need an empty computer mouse case to house your cute little Frankenmouse creation. For the Car version (which includes little more than two DC motors, a power switch, and a 9V battery), you can use any mouse, mechanical or optical. For the full Mousey the Junkbot, with its LM386 brain, IR sensors, bump switch, and reverser circuit, you need an older, analog/mechanical mouse (you know, the kind with that always-grody rubber ball underneath). You likely have one of these in a junk drawer someplace.

In selecting your mouse, you want one that’s as symmetrical as possible and fairly roomy inside. You can use “lopsided” mice, such as the popular IntelliMouse from Microsoft, with the curved body. It’s just a trickier to build and to get the motors aligned and the weight balanced, so if you have a choice, choose symmetry.

You also want to do a little planning ahead to see how everything is going to fit. For the Car, this is no big deal. Any mouse should have plenty of space. For the robot version, this is a factor. Getting everything inside will be a challenge with a mouse of any size and design. Below is a rough layout for a typical arrangement for Mousey the Junkbot components. You’ll need to fit all of these inside your case (tho they don’t have to go in the same places). Unscrew the mouse halves. Many mice have screws under the little rubber glide pads, so check there if your attachment screws aren’t obvious.

With your proper mouse selected, you’re ready to remove everything that’s not needed inside. First, remove the printed circuit board (PCB). This is usually attached only by the scroll-wheel mount (if your mouse had a wheel), so snapping the wheel usually frees the board. Removed the cable, scroll-wheel assembly, and PCB. Save the PCB. You’ll need parts from it (if you’re building the full Mousey).

Now you should have nothing in the mouse but the encoder wheels and all of the plastic mounts for the various components and for attaching the two body halves. Get out your Dremel tool and a cut-off wheel. You should absolutely wear eye protection for this, and a paper filter mask. The plastic dust is nasty business.

If you’ve spec’d out where the parts will go, you’ll have some idea of what you can get away with leaving in. You’ll need a place in the top half for the toggle switch tail. If the screw post is in that vicinity, you’ll have to remove it. No biggie. You can hold Mousey together with a piece of tape when he’s finished. We recommend taking out everything that’s inside, just to give yourself as much room to work as possible.

When you’re done, a clean as a whistle bottom body half should look something like this:

Now you need to cut notches for your motors. If you’re doing this dissection in preparation for my workshop, you won’t have the two Solarbotics RM1A motors to use as your guide. You need to cut two tabs out of the sides of the mouse bottom. Make them at least 3/4” wide and all the way to the floor of the mouse bottom half. That should give you plenty of mounting room. Just so you know, the Solarbotics motors are approx 5/8” wide by just shy of ½” tall. We’ve found that placing the motors forward of the halfway mark (forward-to-back) is a good spot in terms of overall parts placement and robot performance. Here’s where we tend to mount ours:

Once you cut the motor notches, if you’re just building the Car, you’re done with the bottom half. The only other thing you have to do is drill a hole in the top half for the toggle switch. The photo below shows you the basic placement. You want to drill a hole ~1/4” wide.

If you’re building the full Mousey, the other thing that needs to be cut in the bottom half is the notch for the bump switch package. See the Motor Placement image above for the basic location. You’ll be using one of the button switches from the mouse as a bump sensor that’ll engage Mousey’s reverser circuit. You want the opening to be about 5/8” wide by about 3/8” high. The package itself is (likely) 1/2” wide by 1/4” tall. Don’t worry too much about these cuts. You can adjust them as you test-fit the motors and the switches.

If you’re building the full Mousey the Junkbot, you’ll also need to remove one button switch and the two infrared EMITTERS from the mouse’s printed circuit board. See the illustration below for the likely location of these components. Your PCB may vary slightly.

In this drawing, you are looking at a top-down view of the PCB, with the front end of the mouse at the top of the image. The encoder wheels aren’t used in this project, they’re just pointed out for orientation purposes. Once you’ve located your parts, get out your soldering iron and de-soldering tools. De-solder the pins on these components and bring them with you to the Faire, along with your prepped mouse halves.

BTW: You want to save (and bring) the two plastic pieces that make up the top surface of the two mouse buttons. They likely came apart as you eviscerated your mouse case. You can follow your own creativity as you build, but when I designed Mousey, one of the things I wanted to do was make it look as much like an un-altered mouse as possible. I’d seen some other robots in mouse cases, but the top of the mouse usually didn’t fit or it had been so altered it didn’t look mouse-like anymore. I wanted Mousey to look like your garden variety computer mouse that had somehow transformed itself into a robot, chewed off its own cord, and made a grand escape from its computer servitude. So, when the build is complete, you can glue the button parts back on your case to minimize the “damage” to the mouse formfactor.

BTW 2.0: You’ll also need to drill holes in the mouse top to snake the eyestalk sensor eyes through, but you should save that until you’ve installed everything else inside the case and know where the stalks should go.

* Mousey the Junkbot’s Webpage
* Free Mousey Project PDF from MAKE Vol. 2
* Mousebot Revisited, Additional Mousey Building Tips and Tricks on Instructables.
* My Soldering Tutorial
Information about the 2007 Bay Area Maker Faire

Images are from the Absolute Beginner’s Guide to Building Robots, Gareth Branwyn (Que). Illustrations by Mark Frauenfelder, Photos by Jay Townsend.