Abstract: A device is provided for indicating the occurrence of rate of change of velocity of a body equal to or in excess of a predetermined value. An inertial mass is yieldably biased toward a reference position, but moves to a second position when the body is subjected to rates of change of velocity in excess of the predetermined value. A light source is provided, and when the inertial mass is in its reference position, a first light responsive means is out of light communication with the source, while a second light responsive means receives light from the light source. When the inertial mass is in its second position, the first light responsive means receives light from the light source and the second light responsive means is out of light communication with the source. Upon receipt of light, the first light responsive means produces an electric signal, but the second light responsive means no longer produces a signal.