TECHNICAL FIELD The present invention relates to a method and apparatus for determining a proportion of faulty light emitting diodes (hereinafter referred to as "LEDs") in a traffic light display or lantern. BACKGROUND ART

In the use of LEDs in traffic light displays, it is considered useful to know if more than a specified proportion of the LEDs in a display are faulty.

Experience has shown that open circuit failures are the most common type of failure of an LED.

Knowledge that more than an allowable proportion of the LEDs in a display are faulty may enable traffic control equipment to turn off the faulty display, and for subsequent servicing of the traffic light display to occur.

Although a preferred embodiment of the invention will be described later in the specification with reference to its application to determining a proportion of LEDs with open circuit failures in an array of LEDs arranged as a plurality of sections in parallel and in which each section contains two LEDs in series, it will be apparent to persons skilled in the art that the invention is not limited thereto but has wider application to other types of failures and to other arrays of

LEDs.

SUMMARY OF INVENTION

Accordingly, it is an object of the present invention to provide a simple and reliable way of determining a proportion of faulty LEDs in a traffic light display having a plurality of LEDs and, in particular, of determining a proportion of LEDs with open circuit failures in an array of LEDs in a traffic light display connected to traffic control equipment.

It is another object of the present invention to provide a simple and reliable way of controlling the operating status of a traffic light display having a plurality of LEDs.

According to one aspect of the present invention, there is provided an apparatus for determining a proportion of faulty LEDs in a traffic light display having a plurality of LEDs, the apparatus comprising:-

(a) means for communicating the presence of one or more faulty LEDs to a common sensing point,

(b) means for feeding a signal to the common sensing point for sensing the one or more faulty LEDs, and

(c) means for processing the signal indicative of the one or more faulty LEDs to determine the proportion of faulty LEDs in the traffic light display.

Preferably, the processing means includes a threshold device that is activated when the proportion of faulty LEDs is more than an allowable proportion.

It is preferred that the apparatus further includes means for informing traffic control equipment of the activation of the threshold device.

The traffic light display may be turned off by the traffic control equipment when the threshold device is activated.

In a preferred form, the processing means includes a differential amplifier which connects to both the common sensing point and to a voltage divider circuit in series with the common sensing point, wherein the potential difference between the common sensing point and the voltage divider circuit is dependent on the proportion of faulty LEDs in the traffic light display, the potential difference being converted to a signal that is indicative of the proportion of faulty LEDs in the traffic light display.

According to another aspect of the present invention, there is provided an assembly for controlling the operating status of a traffic light display having a plurality of LEDs, the assembly comprising:-

(a) a traffic light display that includes the above described apparatus,

(b) traffic control equipment,

(c) means for informing the traffic control equipment of the proportion of faulty LEDs in the traffic light display so that, when the proportion of faulty LEDs is more than an allowable proportion, the threshold device is activated to turn off the traffic light display. According to a further aspect the present invention, there is provided a method for determining a proportion of LEDs with open circuit failures in an array of LEDs in a traffic light display, the array of LEDs being arranged as a plurality of sections in parallel and in which each section includes a pair of LEDs in series, the method comprising the steps of:-

(a) providing a fault sensing resistor which connects between each pair of LEDs in series and a common sensing point,

(b) feeding a sensing signal of a predetermined frequency to the common sensing point,

(c) providing a differential amplifier which connects to both the common sensing point and to a voltage divider circuit in series with the common sensing point, wherein the potential difference between the common sensing point and the voltage divider circuit is dependent on the proportion of LEDs in the array having open circuit failures, the potential difference being converted to a signal for determining the proportion of LEDs with open circuit failures by the differential amplifier.

Preferably, the proportion of LEDs with open circuit failures to be determined is more than an allowable proportion, and the method includes the further steps of: -

(d) feeding the signal from the differential amplifier to a rectifier and filter,

(e) providing a threshold device to which the rectified and filtered signal is fed, the threshold device being activated when more than an allowable proportion of LEDs in the array have open circuit failures, and

(f) providing traffic control equipment which is informed of the activation of the threshold device.

According to a still further aspect of the present invention, there is provided a method for controlling the operating status of a traffic light display having an array of LEDs arranged as a plurality of sections in parallel and in which each section includes a pair of LEDs in series, the method comprising the steps of:-

(a) determining a proportion of LEDs with open circuit failures in the array of LEDs in the traffic light display according to steps (a) to

(c) of the above described method,

(b) feeding the signal indicative of the proportion to a rectifier and filter,

(c) providing a threshold device to which the rectified and filtered signal is fed, the threshold device being activated when more than an allowable proportion of LEDs in the array have open circuit failures, and (d) providing traffic control equipment which is informed of the activation of the threshold device, and which turns off the traffic light display.

BRIEF DESCRIPTION OF THE DRAWING In order that the invention may be readily understood and put into practical effect, reference will now be made to the accompanying drawing of Figure 1 which is a block circuit diagram representing a method and apparatus according to a preferred embodiment of the present invention. MODES FOR CARRYING OUT THE INVENTION As shown in Fig 1 , each LED section 12, 14, 16 consists of two LEDs and optionally a current limiting resistor in series. A multiplicity of fault sensing resistors 18, 20, 22 is used to connect the centre point of each LED section to a common sensing point A. A sensing signal that can conveniently be a 1 kHz square wave with a peak value corresponding to the total voltage applied to the LED sections is fed to point A through a resistor B. A differential amplifier C is connected to point A and to a voltage divider circuit at D so arranged that, when all LEDs are operating correctly, the potential difference between A and D is small. This is a bridge circuit. A characteristic of a bridge circuit is that, when balanced, the output is at a minimum (about zero) and if the bridge is unbalanced then the output at E increases.

In the arrangement shown in Fig 1 , the degree on unbalance of the bridge is proportional to the number of LED sections that have an open circuit LED. The output of the amplifier C will be a 1kHz square wave whose amplitude is proportional to the number of LED sections with a faulty open circuit LED. The square wave from C is fed to rectifier F and filter G and then to threshold device H which will be activated only when more than a chosen proportion of the LED sections are faulty. The output of the threshold device H is used to inform traffic control equipment that the lantern being monitored may be faulty. The output from H may also be used to turn off the faulty lantern.

Although the invention has been herein shown and described in what is conceived to be the most practical and preferred embodiment, it is recognised that departures can be made within the scope of the invention, which is not limited to the details described herein but is to be accorded the full scope of the appended claims so as to embrace any and all equivalent devices and apparatus.