I made this somewhat efficient pencil sharpener, it's on another thread here:
Fed up with how long traditional pencil sharpeners take to sharpen your pencils? Sharpens even the most stubbornly blunt pencils quicker than a very, very quick thing.
I made a series of pedal powered art works a few years ago including foolishness like this and a pedal powered meat mincer. The best was a pedal powered generator powering an array of 150 LEDs that projected an audience interactice projected installation. This is the only piece that survives although I still have the bits for the LED work, just no frame to nail it all to.
I've kept this because it is silly and does work rather well.
Frame: 1960s Triumph
Front tyre: Flat
Chainset: Raleigh cottered with a 1/8" chainring attacked with an angle grinder to enable it to be used with scrap 3/32"chains.
Bars: Bit of steel pipe from the scrap metal bin.
Sprocket: Off a 5 speed freewheel again attacked with an angle grinder, bodged on to a bit of steel plate, then welded onto the end of a desk(now handlebar)mounted pencil sharpener.
Handle bar mounting alows for easy chain tension adjustment, just rotate bars forward/back to get correct tension. No need for faffing with tensioners or the push up/down debate.