Traffic signal loop monitoring system

Abstract

A monitoring system is provided for use in conjunction with a wire loop which is embedded in a roadway, and which senses the presence of vehicles in the roadway to control a traffic signal. The monitoring system senses the output of the loop oscillator which is included in the traffic control circuitry and which generates a loop signal and whose frequency-determining circuit includes the wire loop to indicate immediately any failure in the loop circuit. If for any reason the loop oscillator fails to operate, that fact will be sensed immediately by the monitoring system of the invention, and such a failure will be equated to a failure in the embedded loop, or in the lead-in circuit which connects the loop to the loop oscillator. A digital embodiment of the monitoring system of the invention includes a reference oscillator which generates a reference signal whose frequency is lower than the frequency of the loop signal, and it also includes a two-count binary counter which is clocked by the reference signal, and which is cleared by the loop signal so that the counter cannot exceed a count of one, so long as the loop oscillator is functioning. However, should the loop oscillator fail, indicating a failure condition in the loop, the counter will then proceed to the count of two, and that count will set a flip-flop to activate an appropriate indicator. The flip-flop will remain set and the indicator activated, until reset by appropriate manual means. An analog embodiment of the monitoring system of the invention is also provided.