When HAC:Manchester were asked in December last year to create an awesome exhibit to show off at the Madlab Playhouse, as part of the Playspace Unconference, I joked 'Lets build a giant etch-a-sketch'... So we did!

As we were going GIANT, we decided it would be best to upgrade the technology controlling it. We settled on using a projector, which pretty much forced us to use the name project-a-sketch!

How we made it

Frame

Step one was to build the frame. This was done on the Shopbot CNC router at FabLab Manchester. The frame was cut out of 20mm MDF, then contoured around the top to give it that oldschool etch-a-sketch shape. We sanded the edges, painted it red and attached some legs back at our home base, MadLab.

The knobs were built next, two CNCed MDF circles were center-drilled, and had a channel cut into them to allow an M8 coach bolt head to be held in place. Acrylic washers and Skate bearings were used to make sure the knobs can spin freely in the holes cut for them

Electronics

Third up were the electronics. The brain of the system is an Arduino Duemilenove, with two mechanical quadrature encoders to read the knob rotation and an accelerometer attached. The arduino reads the accelerometer ever 50ms, and takes input from the quadratures when they are turned. This data is then sent over a serial link to the controlling PC, which runs a processing script to display the dot.

Next, we had to attach the knobs to the encoders, and get everything running. The quadrature encoders used were low resolution, at only 16 notches per rotation, so I returned to FabLab to build a gearbox. Using the laser cutter, i cut a 16 tooth gear to fit on the quadrature encoder, and a second, 64 tooth one to attach to the knob itself. Raiding the scrap bin for more chunks of acrylic, we also cut out a frame to hold all the gears in place. This gives a 1:4 gear ratio, for each turn of the knob, the quadrature encoder outputs 64 state changes, a far more acceptable resolution.

Screen

Lastly, we added a screen to rear-project onto. This was made out of a simple piece of white cotton sheet, stapled to the back of the device. Rear projecting means the person using the project-a-sketch doesn't block the projector, which would defeat the point!

Video

Maria at littlestar.tv has used the project a sketch as a prop in her video 'Project-a-sketch your love'! Watch it here