Monitoring circuit

Abstract: A monitoring circuit (10) comprising a first terminal (14) at a first voltage level (V s ); a second terminal (22) at a second, different, voltage level; a gauge (12) connected between the first terminal and a first junction (16); a sensor (18) having a variable resistance and a first resistor (20) having a fixed resistance connected in series between the first junction and the second terminal and defining a second junction (24) therebetween; a second and a third resistor (26,28) each having a fixed resistance connected in series between the first junction and the second terminal and defining a third junction (30) therebetween; a voltage comparator (36) having inputs (32,34) connected to the second and third junctions; and a warning device (40) connected to the output of the voltage comparator and actuated by the voltage comparator when the voltage at the second junction reaches or passes through a predetermined level in comparison with the voltage at the third junction. Provides accurate activation of the warning device independent of the resistance of the gauge.

Description: This invention relates to a monitoring circuit including a gauge and a warning light or other warning device. This invention has particular application in motor vehicles. It has become common practice to include warning lights, as well as gauges, in motor vehicles. The gauges give an indication of fuel level, engine coolant temperature, etc. and the warning lights provide an additional indication of an abnormal condition such as low fuel level, high coolant temperature, low oil pressure, etc. In situations where both a gauge and a warning light are provided for monitoring a particular condition, it is usual practice to use the conventional gauge circuit to provide the electrical signal for the warning light. For example, a conventional fuel level monitoring circuit has a sensor or sender having a resistance which varies with fuel level located in the fuel tank and a fuel gauge located in the instrument cluster of the motor vehicle. The warning light and associated circuitry is usually located close to the gauge. The associated circuitry monitors the voltage available at the gauge and compares it with a predetermined voltage level which is intended to represent the warning level. These known monitoring circuits rely on the resistance of the gauge being a known value. In practice, however, the gauge resistance varies from one gauge to another (due to production tolerances and other factors), and hence much testing is required before a final value of gauge resistance can be determined. Other factors (such as drift due to warm-up; temperature variations; etc.) also cause the gauge resistance to vary whilst in use, and hence can result in inaccurate triggering of the warning light. US-A-4,417,231 discloses a prior art arrangement in accordance with the preamble of claim 1. It is an object of the present invention to overcome the above mentioned disadvantages. To this end, a monitoring circuit in accordance with the present invention is characterised over US-A-4,417,231 by the features specified in the characterising portion of claim 1. The sensor resistance and the first, second and third resistor define a wheatstone bridge, and the voltage comparator detects changes in voltage difference at the centre junctions of each arm of the wheatstone bridge. The gauge does not form part of the wheatstone bridge. This invention therefore has the advantage that the resistance of the gauge is not influential in the activation of the warning device. Its value prior to installation does not have to be known, and any variations in its resistance during usage does not affect the activation of the warning device. The activation point of the warning device can therefore be accurately predetermined. The first voltage level is preferably a supply voltage, and the second voltage level is preferably electrical ground. Preferably, the first voltage level is provided from an ignition circuit of a motor vehicle. The sensor is preferably a temperature sensor in the form of a thermistor. Alternatively, the sensor may be a fuel level monitor or oil pressure monitor. Preferably, the warning device is a warning light. In this case, the monitoring circuit preferably includes a resistor which acts to provide hysteresis by feeding back a small voltage when the warning light is activated. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a schematic of a basic monitoring circuit in accordance with the present invention; and Figure 2 is a more detailed schematic of a monitoring circuit in accordance with the present invention. Referring to Figure 1, the basic monitoring circuit 10 shown therein comprises a gauge having a coil 12 connected between a first terminal 14 and a first junction 16. A sensor 18 having a variable resistance is connected in series with a first resistor 20 having a fixed resistance between the first junction 16 and a second terminal 22. A second junction 24 is defined between the sensor 18 and the first resistor 20. A second resistor 26 and a third resistor 28, each having a fixed resistance, are connected in series between the first junction 16 and the second terminal 22. The second and third resistors 26,28 define a third junction 30 therebetween. The second and third junctions 24,30 are connected to the inputs 32,34 respectively of a voltage comparator 36. The output 38 of the voltage comparator 36 is connected to a warning device 40, which may be a warning light and associated driving circuit. The first terminal 14 is connected to a substantially constant supply voltage V s , and the second terminal 22 is connected to electrical ground. During normal operation of the monitoring circuit 10, the resistance of the sensor 18 is directly related to the state of the condition it is monitoring. At any moment in time, the current through the coil 12 of the gauge is dependent on the resistance of the sensor 18, with the gauge providing a visible readout of the actual state of the condition being monitored. Any changes in the monitored condition are reflected by a change in the resistance of the sensor 18, with a consequent change in the current through the coil 12, and a change in the readout provided by the gauge. Further, the voltage comparator 36 monitors and compares the voltage difference between the second and third junctions 24,30. Whilst this voltage difference remains within a predetermined specified range, no output voltage is generated at the output 38 of the voltage comparator 36. If, however, an abnormal state arises, the voltage difference across the second and third junctions 24,30 falls outside the predetermined specified range (in this particular case, the voltage difference changes polarity), and an output voltage is generated at the output 38 of the voltage comparator 36. This output voltage triggers the warning device 40 to provide a visible or audible indication of an abnormal state of the condition being monitored. This arrangement has the advantages over previously known arrangements of providing an accurate warning of an abnormal condition which is not affected by, or dependent on knowing, the resistance of the coil 12 of the gauge. Sensor 18 and first, second and third resistors 20,26,28 define a wheatstone bridge circuit. Any change in the voltage level at the first junction 16, due to resistance changes in the coil 12, equally affect both sides of the wheatstone bridge, and hence do not affect the predetermined point at which an output voltage is provided at the output 38 of the voltage comparator 36. This arrangement also provides a simple, cheap arrangement for overcoming the problems associated with previously known arrangements. Knowing the resistive range of the sensor 18, the values of the first, second and third resistors 20,26,28 are selected to provide the predetermined point at which the warning device is required to be activated. In a preferred arrangement, the values of the first, second and third resistors 20,26,28 are selected such that during normal operation the voltage at the third junction 30 is less than the voltage at the second junction 24, and such that during abnormal conditions the voltage at the second junction 24 is less than the voltage at the third junction 30. The voltage comparator 36 compares the voltages at the second and third junctions 24,30, and activates the warning device when a "null" point is detected in the wheatstone bridge circuit. Referring now to the more detailed monitoring circuit 50 shown in Figure 2, this circuit is suitable for monitoring the temperature of the engine coolant of a motor vehicle. The monitoring circuit 50 comprises a temperature gauge 52, a first resistor 54, a temperature sensor 56, a second resistor 58, a third resistor 60, a voltage comparator 62, first and second terminals 64,66 respectively, and first, second and third junctions 68,70,72 respectively. All of these elements act in a similar manner to the correspondingly named elements in Figure 1. The output 74 of the voltage comparator 62 is connected to a warning light driver circuit 76 which activates a warning light (not shown) when an output voltage is generated at the output 74. The temperature gauge 52 is mounted in the instrument cluster (not shown) of the motor vehicle. The first terminal 64 is on the temperature gauge 52 and is connected with the ignition circuit 96 of the motor vehicle. A supply voltage V s is provided at the first terminal 64 when the ignition is switched on. The temperature sensor 56 is in the form of a thermistor, and is mounted on the engine block of the motor vehicle to monitor the temperature of the engine coolant. As the temperature of the coolant rises, the resistance of the thermistor falls, and vice versa. The value of the first resistor 54 is kept as low as possible so as to maintain an adequate supply voltage across the thermistor, but high enough to provide an adequate signal for the voltage comparator 62. In a typical example, the thermistor has a resistance of approximately 60 ohms at the temperature at which the warning light is to be activated, and the first resistor 54 has a value of 1.6 ohms. The second and third resistors 58,60 are selected to provide the same ratio at the warning light activation temperature. In a similar manner to the preferred arrangement of the voltage comparator 36 of Figure 1, the voltage comparator 62 provides an output voltage at its output 74 when the voltage difference across the second and third junctions 70,72 reaches a null. The monitoring circuit 50 also includes a resistor 78 which acts to provide hysteresis by feeding back a small voltage when the warning light is activated. This arrangement helps to provide positive activation/deactivation of the warning light, and prevent it flickering. A spark gap 80 is connected between the second junction 70 and electrical ground. A resistor 84 is connected between the second junction 70 and one of the inputs 86 of the voltage comparator 62. A diode 88 connects the other input 90 (which is connected to the third junction 72) of the voltage comparator 62 to the ignition circuit 96 by way of a voltage divider 92. A capacitor 94 is connected across the inputs 86,90 of the voltage comparator 62. These components are optional, but help to make the monitoring circuit 50 more tolerant to the effects of electro-static discharge, radio frequency interference, and abnormal supply voltages. The monitoring circuit 50 acts in the same manner as the preferred operation of the monitoring circuit 10 shown in Figure 1 and described above, and has the same advantages. When the engine coolant temperature reaches an abnormally high level, the voltage difference across the second and third junctions 70,72 passes through a null or changes polarity. The condition is detected by the voltage comparator 62, and an output voltage is generated at the output 74. The output voltage signal is detected by the warning light driver circuit 76, and the warning light is lit. In motor vehicle applications, similar monitoring circuits can be used to activate warning lights when abnormally low fuel level or abnormally low oil pressure in the engine is detected.

Claims: A monitoring circuit (10;50) comprising a first terminal (14;64) at a first voltage level (V s ); a second terminal (22;66) at a second, different, voltage level; a gauge (12;52) connected between the first terminal (14;64) and a first junction (16;68) a sensor (18;56) having a variable resistance and a first resistor (20;54) having a fixed resistance connected in series and defining a second junction (24;70) therebetween; a second and a third resistor (26,28;58,60) each having a fixed resistance connected in series and defining a third junction (30;72) therebetween; a voltage comparator (36;62) having inputs (32,34;86,90) connected to the second and third junctions (24,30;70,72) and a warning device (40;76) connected to the output (38;74) of the voltage comparator (36;62); characterised in that the sensor (18;56) and the first resistor (20;54) are connected between the first junction (16;68) and the second terminal (22;66); in that the second and third resistors (26,28;58,60) are connected between the first junction (16;68) and the second terminal (22;66); and in that the warning device (40;76) is actuated by the voltage comparator (36;62) when the voltage at the second junction (24;70) reaches or passes through a predetermined level in comparison with the voltage at the third junction (30;72), wherein the predetermined level is a null or a change in polarity in the voltage difference across the second and third junctions (24,30;70,72). A monitoring circuit as claimed in Claim 1, wherein the first voltage level is a supply voltage (V s ), and the second voltage level is electrical ground. A monitoring circuit as claimed in Claim 1 or Claim 2, wherein the first voltage level is provided from an ignition circuit (96) of a motor vehicle. A monitoring circuit as claimed in any one of Claims 1 to 3, wherein the sensor (18;56) is a temperature sensor (56) in the form of a thermistor. A monitoring circuit as claimed in any one of Claims 1 to 4, wherein the warning device (40) is a warning light. A monitoring circuit as claimed in Claim 5 including a resistor (78) Between the third resistor (60) and the second terminal (66) which acts to provide hysteresis by feeding back a small voltage when the warning light is activated.

Inventors: ALLAN DAVID STEWART, ALLAN, DAVID STEWART
Applicants: DELCO ELECTRONIC OVERSEAS CORP, DELCO ELECTRONICS OVERSEAS CORPORATION (A DELAWARE CORP.)
Application year: 1990
Main classification: B60R16
IPC codes: G08B21, G01D1, B60R16, G01R17