I went to a place that installed car alarms and asked to purchase just a shock sensor, it was 20$. It has 4 leads; 12v, Gnd, Warn, and Alarm. Warn and Alarm are pulled low when a vibration is detected. Warn being a slight vibration and Alarm being a stronger one. The sensitivity is adjustable by potentiometer. There looks to be a magnet suspended by both sides by a type of rubber with what I guess to be a hall sensor in front of it. When the magnet is vibrated out of position it detects it.