An accessory module which plugs into the front of the Stopclock and allows Light Gates to control the timing in either of two ways. In mode A, the clock measures between successive changes on one input (minimum time of 0.12s). This can be used to time the interruption of the light beam by a vehicle of known length. In Mode B, the clock times between successive changes on either input, so that the time between two Light Gates can be measured. Batteries available on page 76 - see F4A54161 - Button Cell, Silver Oxide.