This Attachment Makes a Truck a Dot-Matrix Water Printer

I think that a lot of us would enjoy the ability to print messages onto public roads, but without the fear of receiving a citation for vandalism. Maybe that message could be a quote from a favorite book or a proposal — like skywriting on the ground. Ryder figured out a practical way to achieve that. He built an attachment that turns his truck into a massive dot-matrix printer that uses water as ink.

This works best on faded asphalt, which darkens enough when wet to get visible contrast. It operates like an old dot-matrix printer, with the vertical Y axis being several nozzles and the horizontal X axis being time/distance. So to write the letter "I" over and over again, all of the nozzles would open for a short period of time, close for a moment, and then repeat. Writing more complex characters requires the coordination of the nozzles opening and closing as the truck moves.

A Raspberry Pi single-board computer controls the water flow with an array of solenoid valves through a relay board. It hosts a web interface that the user can access from a computer or smartphone. The user enters the message they want, the speed they plan to drive at, and selects a font (though there is only one choice right now). The Raspberry Pi then creates a bitmap image of the message and converts the pixels into a consecutive series of states for the solenoid valves.

Those solenoid valves mount onto wood attached to a trailer hitch cargo rack, with water coming through hoses from a reservoir above and feeding with gravity. When the user is ready, they can start driving at the predetermined speed and press the "print" button. The Raspberry Pi will then start opening and closing the valves at the proper times to reproduce the image.

On the right surfaces, this leaves a large message that is quite visible. And because it writes with water, the message is only temporary and will evaporate quickly.