one hex head 1/4-20x 1/2" bolt
one 1/4-20 nut
seven #4 x 3/8" wood screws
a print of each stl file below
A standard-size hobby servo (I used a Power HD HD2360A servo, but they should all work aight)
A can of silly string (we got ours for $1 from a dollar tree)
A distance sensor (I used a Sharp GP2D120 from Pololu: http://www.pololu.com/catalog/product/1136)
You'll want to press fit the nut into the hexagonal hole on the plunger (the square piece) I had great success placing the nut over the hole and pressing it gently into place (it'll just kind of jam in there a little bit) with my fingers, then flipping the plunger over and pressing the thing against the floor with the heel of my hand. It's a tight fit, but it goes in like a charm with a bit of force.
Now, press the bolt into the hexagonal hole on the servo adapter. You can use two plies to force it into place, or use your hand like you did with the plunger.
Push the plunger nut-side-up into the square hole in the cap. The servo adapter goes in bolt-side down from the other end, and you spin the adapter to screw the two together.
Snap the cap assembly onto the top of a can of silly string. It should hold on tightly. Using your thumb, turn the adapter counter-clockwise until the silly string starts to burble and spray. Turn it back a fraction of a turn until the burbling stops.
Looking down onto the servo, turn the head all the way to the right. Take any horns and screws off of the servo. Flip it upside-down and press-fit the bare plastic cog of the servo into the hole in the servo adapter. Once the servo is flush with the mounting surface, secure it in place by screwing four #4 screws into the mounting holes.
Install your arduino into the base and screw it onto the standoffs using three #4 screws.
For now, hot-glue the distance sensor to the outside of the base, and solder a couple wires to connect it to +5V, GND, and connect the output wire to analog pin 0 of the arduino.
Wire the servo up to 5V, GND, and connect the signal wire to digital pin 9 of the arduino.
Load silly_string.pde onto the arduino.
Wave your hand in front of the sensor. The servo should turn, pressing down the plunger and causing the can to spray.
Unleash your inner 14 year-old!
Check out stories about the development of this thing and more at pocketfactory.org. We'll be offering kits soon!