Abstract: An apparatus 10 has an inertia assembly 14 which includes a moveable ball 60 for movement from a rest position under the influence of a predetermined change in vehicle inertia. A cap 64 is mounted to engage the ball 60 and to be displaced upon movement of the ball 60 from its rest position. A sensor assembly 16 detects movement of the cap 64. The sensor assembly 16 includes a photodiode 90 which directs a beam of light 92 to impinge upon a phototransistor 96. The cap 64 has a projection member 80 which is moved to interrupt the beam of light 92 when the ball 60 moves from its rest position. In the preferred embodiment, the sensor assembly 16 provides a signal which is used to control activation of a vehicle occupant restraint 18. In the preferred embodiment, the vehicle occupant restraint 18 is a seat belt retractor 18, which has a rotatable spool 134 and ratchet wheels 138.