I built the functions of a mouse into a footrest so I could compose and edit text without breaking my train of thought and taking my hands off the keyboard to fumble with a conventional mouse or trackball. Commercial foot operated mouses are available for up to $1500, but I do not care for the way they are implemented. My version can be built for under $30. Disclaimer: I am not actually a computer geek, I just play one on the Internet.
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Step 1: components and tools
PVC architectural plastic sheet for foot board, 1/2 inch thick (actual 0.470), 17 inches by 22 inches, chosen because it is slick and because I had some scraps.
1 optical mouse; Targus model PAUM004U because it was on sale nearby for $10; reconditioned ones are available for that price via the internet.
2 roller lever switches, Radio Shack #275-017A, $4 each, order or find walk-in store that stocks them at radioshack.com.
7 feet clear plastic tubing, 1/2-inch inside dia, 3/4-inch outside dia.
Screws for fastening tubing to foot board, 1-inch long #6 phillips pan head, and washers.
Woodworking tools. If motorized tools are used on plastic, they should be run at very slow speed.