Importance

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Abstract

Warning lights at a railroad grade crossing are energized at a predetermined flash rate when a train is approaching through the use of an essentially fail-safe solid state crossing controller employing circuitry that is active in creating an off condition of the warning lamps. A controller unit operates a pair of warning lamps alternately in synchronism with a warning rate (flash rate) signal having a predetermined repetition rate, such as 55 pulses per minute. When the crossing is safe, an RC circuit in the lamp driver responds to a 500 Hz safe status signal by providing continuous control excitation to a normally conductive lamp switching circuit to maintain the lamps deenergized. In a failure mode, the normal condition of the switching circuit maintains one lamp of each pair continuously energized. Alternate flashing of a lamp pair is accomplished in synchronism with the warning rate signal by alternating an oscillator at the flash rate between two output frequencies and applying the oscillator output as modulation on the supply current to the lamps. Additional lamp pairs are operated by independent control units all responsive to the alternating oscillator output.

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First Claim