THIS INVENTION relates to meter testing circuit. In particular, the invention concerns a meter testing circuit and a transducer head for determining accurate operation of watt/hour power consumption meters. Watt/hour meters for domestic and other purposes are of two general types. A first type of meter is generally electromechanical in its operation and has a rotating disk. The disk has an index mark and the number of rotations per unit time is indicative of power consumed by the consumer whose premises are supplied with power monitored by that meter. The meter usually has a glass cover extending over a front face of the meter through which the disk and other parts of the meter are visible.

A wide variety of meters of this type exist and the type of glass cover, spacing between the glass cover and the disk and other structural features vary considerably from one brand of meter to another. In addition, these meters operate in varying ambient conditions such as heat, light and electrical noise.

Another type of meter is electronic in its construction and has a flashing light or Light Emitting Diode (LED) which provides a visual indication of the rate of power consumption. It is often necessary to log data to provide an indication of power consumed over a predetermined time period for testing or monitoring purposes. A known device currently employed for this purpose includes an emitter for directing radiation, typically light or infra red radiation, onto the index mark on a rotating disk of a meter and has a detector which then provides a response each time the presence of the index mark is detected. The output from the device may be coupled to a data logger. Where the operation of an electronic-type meter is being monitored an emitter is not required in the monitoring or testing device.

These known devices are cumbersome and require precise alignment and accurate attachment to the meter being tested. This was often difficult to achieve because of the wide variety of physical structural differences between various brands of meters. The emitter typically emitted

steady and constant intensity light and its function was adversely affected by variations in ambient conditions.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a meter testing circuit and a transducer head which at least minimise the disadvantages referred to above.

According to one aspect of the invention, there is provided a testing transducer head for a meter testing circuit, the head having a rear face, an emitter and a first receiver in the rear face and a covered receiver responsive to effects of ambient conditions to allow at least some of the ambient effects to be removed from the signal received by the first receiver, and an indicator for providing an indication, whereby when the head is properly aligned and secured to a meter being tested.

According to another aspect, the invention provides a meter testing circuit including a transducer head with an emitter and a receiver, a transmitter controller for controlling the intensity of the output of the emitter depending upon the meter under test and for pulsing the output of the emitter and a signal processor for receiving an output from the receiver and for determining from that output the detection of the presence of either an index mark on a rotating disk of the meter or the flashing of an indicator on the meter.

The emitter and receiver may emit and receive radiation of any suitable type. Preferably, infra red radiation is involved. The emitter may comprise an infra red emitting diode while the receiver may comprise a photo transistor.

The covered receiver or "blind" receiver is an identical device to the receiver previously mentioned and both receivers are in close proximity to one another so that both are subjected to identical noise sources. In this way the first mentioned receiver provides an output responsive both to the presence of noise and the presence of detected infra red radiation while the blind receiver is responsive only to noise. The outputs of these receivers may be subtracted from one another to

substantially eliminate the effects of noise.

The head includes an indicator which provides a tell tale visual output whenever the presence of the index mark or flashing indicator is detected. This allows for easy and accurate alignment of the transducer head relative to the meter under test.

The signal processor preferably includes a high gain amplifier and a differential amplifier. The high gain amplifier amplifies the outputs from the first mentioned receiver and the blind receiver while the differential amplifier subtracts one of the outputs from the other to lessen the effects of noise.

Preferably the signal processor includes a filter and circuitry for analysing the filtered output to allow the signal to indicate the detection of the presence of the index mark or flashing indicator on the meter. The processor may include a rectifier, level hold circuit and a peak and trough detector.

The transmitter controller not only ensures that the transmitter provides a pulsed output but also controls the intensity of the output in dependence upon the meter under test. In this way power consumed by the emitter may be controlled. The duty cycle of emitter pulsing is preferably small and typically about 0.8%. The pulse frequency is relatively high in relation to the rotational speed of the rotating disk or flashing frequency of the indicator on the electronic meter.

The processor preferably compares the magnitude of the received signal from the receiver with a reference to thereby provide a signal for controlling the emitter intensity.

BRIEF DESCRIPTION OF THE DRAWINGS A particular preferred embodiment of the invention will now be described by way of example in which

Figures 1 , 2 and 3 are front, side and rear elevational views of a transducer head of an embodiment of the invention;

Figure 4 is a block diagram of a meter testing circuit which incorporates the transducer head of Figures 1 , 2 and 3;

Figure 5 is a detailed circuit diagram of circuitry employed in the transducer head;

Figure 6 is a detailed circuit diagram of a further part of the detailed circuitry employed in the block diagram of Figure 4; Figure 7 is a detailed circuit diagram of a further part of the detailed circuitry employed in the block diagram of Figure 4; and,

Figure 8 is a circuit diagram of a microcomputer circuit used in the block diagram of Figure 4.

DESCRIPTION OF PREFERRED EMBODIMENT With reference to Figures 1 , 2 and 3 a transducer head 10 is shown. The head 10 has a rear face 11 to which are secured two suction cups 12, 13 which facilitate the attachment of the head to a cover glass of a watt/hour meter. A radiation transmitter and a radiation receiver are present in the rear face and are directed towards the cover glass of the meter to which the head may be attached. The transmitter and receiver are not shown in these views.

The head has a cylindrical body 14 and a front face 15. The front face 15 has an indicator, typically LED 16. When the head 10 is correctly positioned relative to a meter the LED 16 flashes each time the receiver detects the presence of the index mark on a rotating disk of the meter. Where the meter is of an electronic type rather than one having a rotating disk, the receiver may detect flashing of an indicator on the meter and when the head is correctly aligned relative to such a meter, the indicator 16 flashes each time the indicator on the meter flashes. An electrical lead 17 couples the head to other circuitry of the meter testing circuit.

Figure 4 shows a block diagram of the meter testing circuit of the invention. Receiver 20, typically a photo transistor with an amplifier is responsive to radiation from a rotating disk or LED indicator of a watt/hour meter. The amplified output secured from receiver 20 is indicative of the detection of the index mark on the rotating disk or the flashing of an LED on an electronic watt/hour meter.

The circuit has two modes of operation:

1. Mechanical meters

2. Electronic meters.

The differential output from the receiver is conditioned by the differential amplifier and filter 21. The filter is typically a high pass filter and the output from the filter is applied to a gain control module 22. The output from module 22 is applied to the peak hold circuit 23 and to the comparator. The output of the peak hold 23 is fed into an analog to digital convert (ADC) 24. The ADC output is a digital signal representative of the output from the peak hold circuit 23.

The following describes the operation for mechanical meters.

A comparison is made between the output from analog to digital converter 24 and a reference signal to control the output from digital to analog converter 26 for an initial period to calibrate the circuit to the meter under test. The output from the DAC 26 is converted into a current pulse by the voltage to current converter 27. Converter 27 is enabled by the microcomputer 25. The output from voltage to current converter 27 drives the emitter circuit 28 with a pulse having a 0.8% typical duty cycle.

Pulsing of the transmitter occurs at a frequency much greater than the normal rotational speed of the disk in a mechanical watt/hour meter.

The following describes the operation for an electronic meter.

The voltage to current converter 27 is disabled by the computer 25 via an output at pin PEO to stop any transmission from emitter 28. A comparison is made in the output from analog to digital converter 24 and a reference which controls the input to digital to analog converter 26.

The output from digital to analog converter 26 is compared with the output from gain control module 22 by comparator 29. Output PCO from the computer 25 controls the output amplitude of the gain control module 22. An output from comparator 29 is applied to terminal PA2 of the computer 25 where it is compared with programmed limits and the output of the analog to digital converter at terminal PD2 of the computer 25.

The computer is programmed to discriminate between peaks

and troughs in the signals resulting from the receiver and when the difference between the peaks and troughs exceeds a predetermined value an output is provided by the computer at terminal PC3 to indicate that the index mark on the disk or a flashing signal provided by an LED of an electronic watt/hour meter has been detected. This output may be coupled to a data logger (not shown).

Figure 5 shows greater detail of circuitry employed in the transducer head. The transmitter consists of a light emitting diode (LED) L1 where the circuit of the invention is used with a watt hour meter having a rotating disk, a transmitter of this type is employed to direct radiation at the disk. This allows the presence of the index mark on the disk to be detected at each revolution of the disk. Where the meter being tested is an electronic meter having an LED indicator, the frequency of flashing of which indicates the power being consumed, a transmitter L1 is not required. Two receiving phototransistors Q1 and Q2 are employed to detect the index mark on the rotating disk in a meter or the flashing LED of an electronic meter. Receiver Q2 is covered over and is responsive only to noise and ambient conditions and other factors not associated with the presence of an index mark or flashing LED. Receivers Q1 and Q2 are connected as shown to provide signals SIGL and SIGH which may be subtracted to eliminate the effects of noise. Amplifier U11A is connected as a comparator which derives its inputs from Q1 and Q2 and controls Q1 so as to not allow Q1 to saturate under ambient light conditions. Voltage followers U12A and B provide a low impedance differential output. The output of follower U12A is high when transistor Q2 tends to turn off and the output of follower U12B is low when transistor Q1 tends to turn on. Transistor Q3 and its related components functions as a constant current source and ensures that transistors Q1 and Q2 do not simultaneously turn on. Figure 6 shows the outputs from followers U12A and B applied to respective high pass filters consisting of capacitor C1 and resistor R1 and capacitor C2 and resistor R2. The outputs from the filters are applied

to differential amplifier U1A. These filters and amplifier U1A provide the block 21 shown in Figure 4. Field effect transistor TR4 and amplifier U1 B provide the gain control block 22 shown in Figure 4. Amplifier U1 B provides a gain of 10. When FET TR4 is conducting it provides a gain of 1/10. FET TR4 is rendered conductive by signal PCO from computer 25 (see Figure 4).

Amplifier U2A, diode D1 and capacitor C5 function as a peak detector and provide the peak hold block 23 of Figure 4. Capacitor C10 and resistor R14 decouple the peak detector from amplifier U1B. Transistor TR1 is used to periodically discharge capacitor C5. This occurs when signal PC1 turns transistor TR1 on. Amplifier U2B is configured as a buffer to couple the output from the peak detector to the analog to digital converter 24 in Figure 4.

Figure 6 shows signal PC2 from the computer 25. That signal drives transistor TR2 to provide a signal INDLED for controlling the LED 16 in Figure 1 for indicating that the head 10 has been properly positioned relative to a meter. Signal PC3 controls transistor TR3 to provide an output indicating that the index mark on the disk or a flashing signal provided by an LED of an electronic watt/hour meter has been detected. U2C is the comparator 29 of Figure 4. Connectors 1 and 2 shown diagrammatically in Figure 6 have the connections shown in the lower left hand corner in Figure 6.

Figure 7 shows power supply circuits for the meter testing circuit of the invention. Regulator U3 provides regulated +10V and -10V as shown. Regulator U5 provides regulated -5V. Regulator U6 and U4 provide regulated 10V and 5V respectively. Filter capacitors C28, C29, C17, C18 and C19 are associated with the voltage regulators.

Figure 8 shows detail of a microcomputer U10. A reset signal for computer U10 is secured from device U8. Amplifier U2D and transistor TR5 provide a voltage to current source. Inputs PBO to PB7 to resistors R19 and R24 to R31 are derived from computer U10. This resistor network provides the digital to analog converter 26 shown in Figure 4. The voltage

to current source is representative of block 27 in Figure 4. Output IRLED controls transmitter LED L1 in Figure 5 and provides a variable intensity output dependent upon the meter under test. Transistor TR6 is controlled by input PEO derived from the computer U10 which transistor TR6 is conducting the voltage to current source is rendered operative. In this way, the source is controlled so that the transmitter L1 (Figure 5) may be rendered inoperative to allow the circuit of the invention to be used with an electronic watt/hour meter.

The computer 25 shown in block diagram form in Figure 4 and in greater detail as U10 in Figure 8 operates as follows. Terminal PEO is coupled via pull up resistor to +5V. In order to operate the circuit of the invention in its mechanical meter mode, connection PEO to FET TR6 is at +5V and FET TR6 couples the ground potential to one end of resistor R30 to allow the voltage to current converter to function. When PEO is coupled to ground potential TR6 does not conduct and the voltage to current converter is disabled.

The computer operation is initially calibrated so that the magnitude of the troughs (for mechanical meter mode) and peaks (for electronic meter mode) is established. Likewise the duration of a trough or peak is also established. When the computer is then set to function for a test only when peaks or troughs within desired acquisition limits occurring for desired time periods result in an output at terminal PC3 for application to a data logger. This information provides an indication of the accuracy of the reading of the meter.