There is a motion sensor that triggers the show over an RF link.
Here is a short breakdown of how it works:
Pumpkin Player: uses a Wiring S board, a Rogue Robotics rMP3 Playback Shield, an nRF24L01+ radio, and a hand built LED driver board to control the LEDs.
Motion Sensor: uses a Wiring S board, a PIR module (from Seeed Studio, IIRC), and an nRF24L01+ radio.
Remote Control and Display: uses a Rogue Robotics Rapscallion board, and an nRF24L01+ radio.
The Pumpkin Player receives messages from both the Motion Sensor and the Remote Control. It plays music using the rMP3 and changes the facial expressions to match the music. The facial expressions are stored as an accompanying lyrics file for the current song being played. The rMP3 plays the music, and the Wiring S reads the lyrics file at the same time to get the expressions.
The Motion Sensor sends messages to both the Pumpkin Player and the Remote Control. It uses a typical PIR sensor module.
The Remote Control receives messages from the Motion Sensor. If the Motion Sensor is triggered, then the RGB LED on the Rapscallion flashes (and a tone on a speaker is played, if a speaker is connected). Playback can be started and stopped using the buttons on the Rapscallion.
Wheeeee! Another fun Halloween.