Abstract: An object sensor (or event detector) apparatus detects distinctive characteristics (such as color) of otherwise physically similar objects. Respective indices or counters are incremented in response to the various detections. The event detector (object sensor) may take various forms such as a conveyor belt production monitor, but is especially useful as an electronic scorekeeper for a competitive game which simultaneously keeps score for two or more different players by distinguishing between the different colors (or other characteristics) of balls or other objects each player uses to play a given game. A solitary mode also keeps score for a single player within a given time period. In this selectable alternative mode, the event detector apparatus records accumulated points and displays the time remaining in a given time period which defines a game time period.