“Way back” in 2009, students in the UC Berkley “ME 102” class came up with this excellent automatic chalk-spraying machine. It uses 8 cans of spray-chalk to spray the message of your choosing onto the sidewalk or street as you push it along. This device is controlled by two Arduino Duemilanove boards, which apparently base the chalk “dot” timing on encoders sensing movement in the two wheels that the cart rides on. I suppose it could be done without the encoder, but based on the consistency of the print, these students are either extremely steady walkers, or have some mechanized help.
As this is an “ME” class, the mechanism that causes the chalk spray is quite clever as well. A piece of wood forms a lever arm, which is linked up to the activating servos by a bent piece of wire. This seems to work quite well, and can easily be adjusted by screws. Also, the handle is made out of PVC pipe, which I can definitely appreciate.
Be sure to watch the video of it in action all the way to the end to see some strange text produced by it. Maybe that will make you feel better about your puny freshman engineering project. Sure, you passed the class, but your project probably wasn’t quite as “epic” as this one. At least mine wasn’t!