The crash sensors are the most important component to success of the airbag system, and these sensors need to be verified by a certified car mechanic. Airbag sensors in cars mainly are of two types: electrical and mechanical.
Electrical sensors are different in design. “Ball and Tube” mechanism is used in some electro mechanical sensors, which mainly consists of a little tube containing a circuit switch and ball that are held by a small magnet collectively. If a crash occurs, the ball is removed from the magnet and move forward in the tube, striking a switch that completes the electrical circuit. The other electrical designs are similar in principle in which a metal roller or spring loaded weight is used on place of a ball. Mechanical sensors work autonomous of the electrical system and react in the same way to the electrical sensors, with a design that starts a firing pin triggering a small blast after collide. A mechanical sensor does not need a power resource, it cannot be switched off like an electrical sensor can when the battery is detached or cut off.
Source: Wrightimport