an sir has an and gate for selectively passing a deployment signal from a deployment circuit to a firing circuit . an arming circuit includes a logic circuit containing a small normally closed arming sensor which disables the and gate until a prescribed acceleration level occurs to open the arming sensor . a pulse stretcher ensures that if the sensor opens only briefly , the and gate will remain enabled long enough to complete deployment . the arming sensor comprises a plastic housing with a cylindrical cavity , terminals closing opposite ends of the cavity , and a spring biased mass in the cavity which electrically connects the terminals until an acceleration moves the mass to break the connection .