Importance

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Abstract

A system and method of monitoring light emitted from a light emitting diode (LED) display device, which typically has a plurality of selectively energizable display segments which are grouped to form desired shapes and/or alphanumeric characters, is provided, particularly for use in an Aspect Display Unit (ADU) that requires vitality when utilized in the railroad industry as a component of an Automatic Train Protection system utilized in the railroad industry. In a preferred embodiment, each particular segment of the LED display device is independently monitored by an accompanying independent dual photo-transistor circuit when a non-vital output periodically drives each segment to a known electrical state and then back to the original state. A first and a second photo-transistor is used for each segment of the LED display. Each segment and the accompanying first and second phototransistor is surrounded by a shield which prevents light from other segments from being detected by the particular first and second photo-transistor. The first photo-transistor is directed toward the particular segment to sense light emitted therefrom and ambient light thereabout, while the second photo-transistor is directed away from the particular segment sense only ambient light thereabout that segment. The first and second photo-transistors generate electrical signals which are representative of the light detected by each of the first and second photo-transistors. The generated electrical signals drive a monitoring circuit which generates error signals representative of malfunctioning LED display segments, based on a comparison to a fixed voltage of a difference in voltages provided from the first and second photo-transistors.

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First Claim