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11 May, 2010
M3 Design Product Teardown
Optical Mouse
Why do the product teardowns?
Part of the product development process is to apply knowledge gained from prior experience during the concept
development and design phases. Some experience comes from actively designing something in the past while
other experience is gleaned from more indirect sources. It is this indirect product development experience that we
gain via product teardowns.
Teardowns are different from reverse engineering
Reverse engineering is nothing more than figuring out the design and manufacturing methods of a product,
typically for copying. Conversely, M3 Design views product teardowns as ways to gain insight into the design to
become better product developers. We focus on "why" questions.
Why did the designer make the choices they did?
Why were specific construction techniques chosen?
Why were certain features included and others left out?
Why was the particular design approach chosen?
This serves to gain more in depth understanding into the product's design rather than a superficial once-over.
How does M3 Design approach product teardowns?
Our teardown process is a rigorous approach to carefully catalog the product’s deconstruction in both pictures and
written descriptions. This procedure serves two purposes:
It forces the deconstruction team to carefully investigate the product pieces and learn as much about the
design details as possible.
2. It provides a detailed record of the process for future reference by other designers.
The end result of this meticulous process is the beneficial expansion of applicable knowledge regarding product
designs. We employ the lessons and insights garnered from these teardowns during brainstorms, design, prototype
development, and troubleshooting. This method of obtaining indirect product development experience is just one
of many important tools that sets M3 Design apart from other product development firms.
©M3 Design, Inc. 2010
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The M3 Teardown team was tasked with getting rid of a dead rodent: the ubiquitous optical mouse. Like real, living
mice, these computer accessories seem to be everywhere and tend to multiply rapidly. The teardown team seized
the opportunity to split one open and examine its guts (a computer mouse, not the real thing).
External Construction
The mouse is a wired, generic, no-brand type consisting of three external plastic shells:
• Lower shell – holds optics and glides on the desk surface
• Upper shell – encloses the mouse and provides structure
• Decorative button shell cover – flexing fingers used as the left/right buttons
The decorative button shell uses snap fits plus some additional features to attach to the upper shell. Two male
snaps in the upper shell limit the upward movement of the flexing fingers but allow downward movement for left
and right clicks. The female catches in the decorative button shell were long enough to touch the electronic
switches inside the body of the mouse.
Non-snap catches are used to connect the fronts of the upper and lower shells and a single screw at the rear
secures the pair together.
©M3 Design, Inc. 2010
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Upper and lower shells with internal components shown
Decorative button shell attaching to upper shell
©M3 Design, Inc. 2010
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The systems in a mouse are fairly simple: user input detection circuitry and the optics. All of the circuit board
electronics were through-hole type, which indicates that this is likely an older optical mouse.
User Input Detection Circuitry
The user inputs are typical: left/right buttons and a scroll wheel with an incorporated center button. Each of the
buttons presses on a standard, normally-open (NO) switch. The scroll wheel switch has a spacer underneath it to
raise it 4mm higher than the other two. The team guessed that the leads on these switches are long enough to
allow such mounting versatility.
The scroll wheel utilizes a single IR emitter/detector pair, which was confusing at first. How does the mouse know
which way the wheel is turning? Upon closer inspection we noticed that the center leg of the detector and one of
the emitter legs are tied together, possibly to V+. The other two pins of the detector connect to two pins on the
main microcontroller. Thus, by monitoring the two output pins the microcontroller can determine which direction
the scroll wheel is turning.
The scroll wheel also provides tactile feedback to the user during scrolling. The inner surface of the rotating wheel
has 12 flats rather than a continuous smooth surface. A fixed plastic flexure in contact with these flats flexes then
returns to its normal shape as each flat rotates by it. This mechanism generates gentle “snaps” as the scroll wheel
is rotated. Polypropylene is a commonly used material for long life flexures and is likely the material used here.
©M3 Design, Inc. 2010
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Button switches, and IR emitter/detector pair
Scroll wheel (background) with plastic flexure (foreground)
©M3 Design, Inc. 2010
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Since this is an optical mouse, there are no moving parts to detect the position of the mouse as it moves on a
surface. The detection system consisted of a red LED, a periscope/focusing lens, and a detection chip. The solder
mount holes and length of the LED anode and cathode do not provide precise enough positioning, so a molded
plastic shell fits over the LED to hold it in a specific place relative to the lens.
The optical surfaces on the plastic periscope/focusing lens are highly polished. It is unknown how forgiving the
detection chip is of imperfections in the lens but it seems logical that the lenses have a large impact on how well
the mouse functions.
Plastic shell to position red LED
Molded periscope/focusing lens
©M3 Design, Inc. 2010
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Summary & Conclusions
Like a CD player, optical mouse technology is largely taken for granted. The large R&D investment in developing
these products is invisible to most consumers. Optics play a large role in both CD players and optical mice and the
end results are elegant designs.
It’s unknown how old this mouse is, but the through-hole mounted electronics indicate that it is relatively outdated.
Modern mice likely use surface-mount components to reduce cost and to allow smaller packaging. Given that many
mice now use wireless technology, space needs to be made inside the housing for those electronics as well.
©M3 Design, Inc. 2010
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