Abstract: 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.