As well known, traffic control lamps, or traffic signal banks (or simply "traffic signals") are provided for traffic control on streets and highways, especially at intersections. The traffic signals are cyclically displayed through a suitable timing and control mechanism. Such traffic control lamps are usually provided with green, red or amber lenses or sometimes with lenses having arrows to indicate direction.
Heretofore, such as shown in U.S. Pat. No. 2,166,721 dated Jul. 18, 1939, a traffic signal system has been provided to disconnect a traffic signal from regular operation if one of the lamps of the signal becomes burned out and to display either a steady or flashing light. As well known, when a lamp or bulb in the green, amber or red signal burns out, driver confusion may result because traffic on one street receives a green or go signal, and traffic on the other street receives no stop or red signal. Possible collisions are the result.
U.S. Pat. No. 4,135,145 dated Jan. 16, 1979 shows an error detecting circuit for a traffic control system which senses the operating status of a plurality of traffic signal lights controlled by a group of load switches which produce an error signal when certain predetermined conditions exist for more than a predetermined period of time. The error detection circuit provides a visual indication of the traffic signal lights and their associated load switches. A visual signal identifies the load switches and signal lights controlled by the switches in which one of the predetermined conditions exist. This patent does not show or disclose a detection system for sensing and detecting a burned out or inoperable traffic signal lamp.
U.S. Pat. No. 4,495,010 dated Jan. 15, 1985 discloses sensing apparatus that permits sensing of a change in a load condition so that the voltage level is increased immediately after additional lights are turned on in a fluorescent load bank. As a result there is no inconvenient delay and a predetermined threshold of voltage is maintained. Such patent is not directed to a traffic signal system.
It is an object of this invention to provide a lamp sensing system for traffic lights at an intersection for determining if one or more lamps are burned out or inoperable.
Another object of the invention is to provide such a lamp sensing system which makes a series of two measurements, one for the intersection voltage and another for the intersection current, and compares these measurements against an established predetermined pattern of operation of the lamps for the intersection in order to determine if a signal lamp is burned out or inoperable.