Under-Couch Zombie Arm

This is a trick from Halloween '08. We hid this gizmo under the couch, and the arm would reach out and touch people's ankles, causing fear and amusement. Anyone with a web-enabled phone at the party could bring up a web page with controls to operate the arm.

The arm is stuck onto an old camera tripod with a crank to extend and retract the camera mount. I replaced the crank arm with a little dc motor. The motor was controlled by an Arduino micro controller through an H-bridge. The Arduino has to know when the arm is fully extended or retracted so that it won't keep trying to move it and burn out the motor. To make that possible, I embedded magnets in the extension arm and put a Hall-effect sensor on the tube. The sensor knows when a magnet is right under it, indicating that the full range of motion has been reached. The magnets are placed with different poles facing the sensor, so it can tell which end it is at and refuse to turn the motor any more in the that direction.

The Arduino communicated with a computer hidden behind the couch. The computer was serving a web page on the local wireless network with button controls for the arm, so anyone with a web-enabled phone could bring it up and operate the arm remotely form anywhere in the apartment.

The arm itself is a latex cast gone wrong that I had left over from a previous Halloween. The mold was contaminated with soap residue, so the casts came out with a lot of imperfections. With the right paint, it made a good zombie arm.

Full extension:

Wiring: