Inside the Mouse Systems Optical Mouse

(Model M4 pictured)

Mouse Systems Model B4 with homemade optical mousepad

Introduction

Current versions of optical mice use advanced electronics that are able to detect motion across a wide array of surfaces. The electronics give you a mouse that is far more accurate than a mechanical system can provide. Mechanical mice using a roller ball were so common for so long that people are often unaware that there was a previous generation of optical mice using technology from the early 1980s. The Mouse Systems optical mouse pictured above is from that generation.

How it works

These early optical mice were also highly accurate but they required the use of a special reflective mouse pad. In the system used by Mouse Systems Corporation two LEDs at different wavelengths are used. A visible red LED and photodetector pair detects motion on the horizontal axis while a non-visible infrared LED and photodetector pair detects motion on the other axis. The openings for the LEDs are visible on the underside of the mouse.

The circuitry inside the mouse gives you a better idea of how the LEDs work. Both LEDs are angled such that reflected light from the special mousepad comes back through the clear plastic balls aligned with each LED. A mirror above each ball then reflects the light onto the photodetector for that LED. (Not the alignment of the two photodetectors - one is horizontal and the other is vertical.)

Using LEDs and photodetectors that are sensitive to different wavelengths prevents inteference and give you more accurate motion detection. Besides being reflective the mousepad is printed with grey and blue tracking lines which enhance the ability to detect motion. But it also has the side effect of not being able to detect any motion if the mouse pad is not oriented correctly.

Early versions of these mice required a separate power supply to operate. Some came with adaptor cables to "borrow" power provided by they keyboard port. The PCjr version of this mouse borrowed power from the lightpen port. Later versions like the one pictured here are able to run using power from unused signal lines on the serial port.

LEDs, mirrors and photodetectors inside of the mouse. The photodetectors are the two small "chip" like devices at the center of the right edge of the image.

Note that the height of the mirrors is slightly different for the two photodetectors.

Making an optical mouse pad

Without the special mousepad these mice can not be used. However, it is possible to construct something that works well enough.

The homemade mousepad below was created by printing a grid of lines on special ink-jet transparency film and then laying the printed side over a sheet of aluminum foil. The transparency film is textured to aborb ink on the printing side so I laid that side face down, using the smooth non-printing side as the top that the mouse glides on.

This mousepad uses black lines and it works well enough to make the mouse usable. If I were to do another version I would experiment with different colored lines for the vertical and horizontal lines. The original mousepad used grey and blue; presumably using those colors would improve the accuracy of the mouse.

Close-up of the mouse on a homemade optical mouse pad. This version of the mousepad uses all black grid lines. Using grey and blue lines would probably improve the accuracy of the mouse.

Created November 24th, 2011
(C)opyright Michael B. Brutman, mbbrutman at gmail.com