How a mild-mannered computer mouse becomes a fast, freewheeling photon-hog.

Analog (non-optical) mice pick up movements of the ball with two axles that turn gear-like wheels. The teeth rotate between IR emitters and recep¬tors that capture the flickering shadows to read horizontal and vertical directions and speeds. Reverse-biasing the diode emitters turnsthem into Mousey’s “eyes.”

Mousey’s bumper (from one of its buttons) empties a capacitor-full of current across a relay, temporarily crossing the motors’volt¬ages and throwing Mousey into reverse.

Randy Sargent’s Herbie (below) was the first LM386-based bot. It finished last in the 1996 Robothon’s line-following race, but went on to spawn many descendent designs.

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First, you’ll need an analog (non-optical) mouse to can nibalize for its case and several parts inside. If you don’t have an old mouse or two gathering dust, ask friends and colleagues. Otherwise, you can buy a new. super-cheap model such as the Kensington ValueMouse. which costs $10 and has enough space to fit all of your components inside. The big¬ger and more symmetrical the mouse, the easier the build will be. “Handed» mice with asymmetrical, curved bodies present problems.

The other components can be scavenged, or purchased from an electronics retailer. For the motors and other specialty parts, we recommend Dave Hrynkiw’s Solarbotics (solarbotics.corrO as an excellent source. Where available, we’ve listed Solarbotics parts numbers for components, and they now offer a complete mousey kit for about $20 (without the mouse).

MOUSEY’S circuitry is freeformed

This means that we’ll solder the parts to each other without a circuit board, building everything up right inside the mouse case. But before we do this, we’ll need to prep the case and install the motors, and then breadboard the circuitry separately to make sure everything works.

Before unholstering your Dremel tool, you’ll need to determine if the mouse has enough space inside. Unscrew the mouse case and eyeball it to make sure that it will hold the two DC motors and a 9-volt battery. Screws may be hiding under little nylon feet or tape strips on the bottom of the mouse. Save these bits so you can put them back at the end of the build; they’ll help reduce friction.


Now we’re ready to figure out the arrange¬ment of the bigger components and cut openings for the motors. Mouse shapes vary, so you’ll use some judgment here, but the two motors should be oriented perpendicular to the centerline of the body, so the bot travels in a straight line. Also be sure to leave enough space behind the motors for the battery.

Once you’ve placed the motors and battery, you’re ready to cut openings for the axles and wheels, which are simply the drive shafts and gears of the motors.

You’ll want to angle the shafts coming out of the mouse body so they support the bot and set a proper speed. The steeper the angle, the less rubber will meet the road, which slows the bot down — but this is good, since many builders have complained that Mousey moves too fast. If you’re using the lively Solarbotics RM1 motors. 60 degrees is about right, as shown.


Your mousebot will have a giant “whisker»

— a bump switch (courtesy of one of the mouse’s button switches) that triggers Mousey’s scuttle- away behavior. Look on the mouse PCB (see photo in Step 7) for a tiny plastic box that clicks when

Finished motor and bump switch installation. Shown with battery test fit.

you press it down; then desolder it. Once you have the switch removed, attach the base with putty to one side of your mouse’s front end. Tape the strip of hard plastic in place, so that it covers the tiny switch button and runs along the front of the mouse like a wide bumper. The idea is to have the switch triggered by a bump anywhere along the length of the “whisker” so when you press in the plastic, you should hear an itty-bitty click. Tweak this arrangement until it looks good. Once you have your placement, drill a small opening in the mouse case bottom for the switch to stick out. Also cut the plastic strip down to size, about VA» x 2VI».

The last mechanical modification needed for the bottom half is adding tires. Find a rubber band with the same width as the sprockets on the drive shafts, and then cut it to length, wrap it around, and glue it on. You can make the wheels thicker by continuing to wrap the band around itself. Rubber or plastic tubing also makes good tires, as does corrugated tubing from a Lego Mindstorms robot kit or the rub¬ber cylinders from Dremel drum sander bits.


The LM386 op-amp. the main component of Mousey’s control circuit, “listens» to two in put signals. If one signal is lower than the other, the chip boosts that signal to equalize the one output. In our case, the inputs are light values rather than audio.

If we hook this output to two DC motors, we have a little brain that reads input from two light sensors, compares them, and boosts the power on the dim¬mer side. This creates a robot that follows a light source, auto-correcting itself as it moves.

Use this diagram as a reference as you build your mousebot.

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