GB2390494A

GB2390494A - A monitoring circuit for determining the state of a bi-state device

Info

Publication number: GB2390494A
Application number: GB0314702A
Authority: GB
Inventors: Patrick Gerard Mcdaid; Paul Michael Connors
Original assignee: Hi Key Ltd
Current assignee: Hi Key Ltd
Priority date: 2002-06-24
Filing date: 2003-06-24
Publication date: 2004-01-07
Anticipated expiration: 2023-06-24
Also published as: IE20030465A1; IES20020511A2; GB0314702D0; GB2390494B

Abstract

A monitoring circuit 1 for monitoring the state of a bi-state device such as a hall effect sensor 2 for determining the state of a seat belt buckle of a motor vehicle comprises a regulated voltage power supply 7 for applying a five-volt supply to the sensor through a primary transistor 8 and a current sense resistor 10. A comparator 12 compares the voltage applied to the sensor with a threshold voltage reference for detecting an over current condition in the sensor. A secondary transistor 14 is responsive to the output of the comparator 12 for switching off the primary transistor 8 in the event of an over current condition in the device. A microprocessor disables the secondary transistor for a settling time period. The microprocessor 5 determines from the comparator 12 and the current sense resistor (10) an over current condition and the current being drawn by the sensor. The microprocessor may select one of four resistors R1 to R4 of a current emulating circuit for developing a current through the current emulating circuit (3) which emulates the current drawn by the sensor, and a monitoring circuit monitors the current. A seat sensor comprising a force sensitive resistor is provided for locating in the seat of a vehicle for determining the presence of an individual on the seat, and the force sensitive resistor is monitored by the microprocessor (5).

Description

"A monitoring circuit for determining the state of a device, and a current

emulating circuit for emulating current drawn by the device" The present invention relates to a monitoring circuit for determining the state of a s device having at least two states, and the invention also relates to a current emulating circuit for emulating current drawn by a device having at least two states, and the invention also relates to a monitoring circuit for monitoring the state of a device having at least two states, which incorporates the current emulating circuit.

lo Monitoring circuits fo r monitoring the state of a devhce by passing a current through the device or applying a voltage to the device are well known. A basic form of such device is a bi-state switch, which is operable in an open circuit state in which current flow through the switch is inhibited, and in a closed circuit state in which current flow through the switch is permitted. A typical monitoring circuit for monitoring the state 15 of such a bi-state switch comprises a current sense resistor coupled in series with the bi-state switch, and a voltage d Detector for detecting voltage developed across the resistor. When a voltage is applied across the bi-state switch and the current sense resistor, the voltage developed across the current sense resistor should be zero volts when the switch is in the open circuit state, and when the switch is in the closed 20 circuit state, a voltage should be detectable across the resistor. Such bi-state switch devices when used in a circuit for detecting the state of a component, while they can to some extent give an indication of the state of the component being monitored, nonetheless suffer from a number of disadvantages. In particular, such bi- state switch devices having only two states, one being open circuit, and the other being

closed circuit, are unable to distinguish between a fault in the component and a fault in a circuit in which the bi-state switch is located. For example, if a fault develops in the circuit which causes the circuit to go into an open circuit state, the monitoring circuit will be unable to determine whether the open circuit state is as a result of the s state of the component being monitored, or the state of the circuit. Additionally, if a fault develops in the circuit which short-circuits the bi-state switch device, the monitoring circuit will be unable to determine whether the short-circuit is as a result of the state of the component or the circu it. This, in many circuits, is a considerable problem. To overcome this problem, more sophisticated devices are used, which sink currents of different values, or of values within different ranges depending on the state of the device. A typical one of such devices is a hall effect sensor. Such sensors can be used to represent two or more states. When used with a 5 volt power supply, such 15 hall effect sensors are particularly suitable for representing two states, one state being represented when the current being sunk is approximately 6mA, or lies within a range of 4mA to 8mA, and a second state when the current being sunk is approximately 14mA and lies within a range of 12mA to 16mA. Currents outside these ranges in general are indicative of a fault in the circuit. For example, a zero 20 current condition would indicate a fault in the circuit, and would be indicative of an open circuit, while a higher current of or close to 20rnA would typically be indicative of a shortircuit. The advantage of such hall effect sensors is that signals resulting from a fault in the circuit can be identified as such' and distinguished from signals representing the state of the hall effect sensor.

Such hall effect sensors are commonly used in the motor industry for sensing the state of the buckle of a seat belt of a motor vehicle for indicating whether the seat belt buckle is fastened or otherwise. Typically, the hall effect sensor is arranged to 5 sink a current in the range of 4mA to 6mA when the buckle is in the unbuckled state, and to sink a current in the range of 12mA to 16mA when the buckle is in the buckled state.

Circuits for monitoring the current being sunk by such hall effect sensors when used lo in conjunction with a seat belt buckle of a vehicle are known, and in general, are incorporated within the electronic system and circuitry of the motor vehicle. In general, such circuitry in motor vehicles merely monitors the seat belt buckle for detemmining if the buckle is fastened or otherwise. While this is useful, it does not take account of whether a person is sitting in the seat corresponding to the seat belt 15 or otherwise. It is desirable that a monitoring circuit for monitoring the state of a buckle of a seat belt should also pemmit monitoring of the seat to which the seat belt corresponds for determining if the seat is occupied or otherwise, and the monitoring of the seat should not interfere with the monitoring of other parameters of the motor vehicle. However, due to the arrangennent of monitoring circuits in a motor vehicle, in general, the alteration of one of the monitoring circuits or the addition of an extra monitoring circuit tends to interfere with the other monitoring circuHs, which can lead to spurious results from some or all of the monitoring circuits, which is unacceptable.

This is largely due to the fact that monitoring circuXs are typically arranged in banks in the electronic circuitry of a motor vehicle, and altering or adding monitoring circuits in general causes a variation in the currents which are drawn by the existing circuits which leads to the spurious results from such existing circuits.

s Additionally, monitoring circuits for monitoring hall effect sensors, and indeed, other sensors having more than two states tend to be relatively complex and expensive.

Th ere is therefore a need for a monitoring circuit for monitoring a hall effect sensor, 10 or indeed, any other device having at least two states, which overcomes the problems of known monitoring sensors, and there is also a need for a monitoring circuit which is suitable for incorporating in along with other monitoring circuits of a motor vehicle. Further, there is a need for a current emulating circuit for emulating currents drawn by a hall effect sensor, or other devices having at least two states, 15 and there is a need for a monitoring circuit incorporating such a current emulating circuit. The present invention is directed towards providing a monitoring circuit for determining the state of a device having at least two states, and the invention is also 20 directed towards providing a current emulating circuit for emulating a current drawn by a device which has at least two states. Indeed, the present invention is also directed towards a monitoring circuit which incorporates an emulating circuit.

According to the invention there is provided a monitoring circuit for determining the

s state of a device having at least two states, the device sinking currents of values within respective predetermined ranges indicative of the respective states of the device, the monitoring circuit comprising a regulated voltage powersupply circuit for supplying a current to the device, a comparing means for comparing the voltage s developed across the device with a threshold voltage reference for detecting an over current condition, a primary switch means for switching the current from the power supply circuit to the device for facilitating monitoring of the state of the device, the primary switch means being responsive to the comparing means detecting an over current condition for isolating the device from the power supply circuit, a timing 10 means for timing a predetermined settling time delay, the primary switch means being non- responsive to the comparing means during the predetermined settling time delay for preventing isolation of the device from the power supply circuit during the predetermined settling time delay for allowing settling of the device and the monitoring circuit, and a first current sensing means for sensing the current being 15 sunk by the device.

In one embodiment of the invention a control circuit is provided for reading the first current sensing means for detemmining the state of the device.

20 In another embodiment of the invention the primary switch means is operable by the control circuit at predetemmined time intervals for supplying current to the device for facilitating monitoring the device for a predetemmined monitoring time period at each predetermined time interval.

Preferably, the control circuit is responsive to the comparing means detemmining that the voltage developed across the device has exceeded the threshold voltage reference for reading the first current sensing means.

In one embodiment of the invention the primary switch means comprises a primary transistor. Preferably, the primary transistor is responsive to an enable signal from the control circuit for switching the power supply circuit to the device.

Advantageously, the primary transistor is active low.

10 In one embodiment of the invention a secondary switch means is provided for applying a disable signal to the primary switch means for disabling the primary switch means in response to the comparing means detemmining that the voltage developed across the device is below the threshold voyage reference after the settling time delay. Preferably, the secondary switch means comprises a secondary 15 transistor.

In one embodiment of the invention the first current sensing means is located in series with the primary switch means. Preferably, the first current sensing means is located between the primary switch means and the device. Advantageously, the first 20 current sensing means comprises a first current sensing resistor.

In one embodiment of the invention the timing means for timing the predetemmined settling time delay is provided by the control circuit.

In another embodiment of the invention the predetermined settling Ume delay is sufficient for allowing the voltage developed across the device to exceed the threshold voltage reference, assuming normal operation of the device.

s In one embodiment of the invention the predetermined settling time delay lies in the range of 50 microseconds to 200 microseconds. Preferably, the predetermined settling time delay lies in the range of 75 microseconds to 150 microseconds.

Advantageously, the predetermined settling time delay is approximately 100 microseconds. In one embodiment of the invention each predetemmined time interval is in the range of 50 milliseconds to 500 milliseconds. Preferably, each predetemmined time interval is in the range of 75 milliseconds to 250 milliseconds. Advantageously, each predetermined time interval is approximately 100 milliseconds.

In another embodiment of the invention each predetermined monitoring time period is in the range of 75 microseconds to 500 microseconds. Preferably, each predetermined monitoring time period is in the range of 100 microseconds to 300 microseconds. Advantageously, each predetermined monitoring Ume period is 20 approximately 150 microseconds.

In a further embodiment of the invention a current emulating circuit for emulating current drawn by the device is provided, the emulating circuit comprising a power supply rail for receiving a regulated voltage power supply, and at least two current

sink impedance means for sinking current from the power supply rail of respective different values within respective different current value ranges corresponding to the currents drawn by the device which are indicative of the state of the device, and a switch circuit for selectively switching the current sink impedance means for sinking s respective currents from the power supply rail in response to the state of the device.

Preferably, the current sink impedance means are coupled to the power supply rail in parallel with each other.

In one embodiment of the invention the switch circuit is responsive to the control lo circuit for selecting the current sink impedance means for sinking a current from the power supply rail.

In another embodiment of the invention the switch circuit comprises a plurality of select switches, one select switch being provided for each current sink impedance 15 means.

In a further embodiment of the invention N current sink impedance means are provided for sinking N respective currents corresponding to currents drawn by the device which are indicative of N states of the device. Preferably, the switch circuit 20 comprises N select switches corresponding to the N current sink impedance means.

Advantageously, the switch circuit selectively couples the current sink impedance means between the popover supply rail and ground.

In one embodiment of the invention a second current sensing means is provided in

the current emulating circuit for facilitating monitoring of the current being sunk by the selected one of the current sink impedance means. Preferably, the second current sensing means is coupled in series with the current sink impedance means.

Advantageously, the second current sensing means comprises a second current 5 sense resistor.

in one embodiment of the invention a means for monitoring the voltage across the second current sense resistor is provided.

lo In another embodiment of the invention each select switch comprises a transistor switch. In a further embodiment of the invention each current sink impedance means comprises a current sink resistor.

In a still further embodiment of the invention the power supply rail is supplied with a regulated voltage power supply independently of the regulated power supply circuit.

In one embodiment of the invention the monitoring circuit comprises the device.

In another embodiment of the invention the device is adapted for monitoring the state of a seat belt buckle of a seat belt of a motor vehicle seat.

In a further embodiment of the invention the device is provided by a hall effect

sensor. In one embodiment of the invention the monitoring circuit further comprises a seat sensor for monitoring the presence of an individual sitting on a motor vehicle seat 5 associated with the seat belt.

In another embodiment of the invention the seat sensor comprises a force sensitive impedance element supplied with a regulated voltage from the regulated voltage power supply circuit, the voltage developed across the force sensitive impedance 10 element being monitored by the control circuit. Preferably, the force sensitive impedance element comprises a force sensitive resistor.

The invention also provides a current emulating circuit for emulating a current drawn by a device having at least two states, the device sinking currents of value within 15 respective predetermined ranges indicative of the respective states of the device, the emulating circuit comprising a power supply rail for receiving a regulated voltage power supply, and at least two current sink impedance means for sinking current from the power supply rail of respective different values within respective different current value ranges corresponding to the currents drawn by the device which are 20 indicative of the state of the device, and a switch circuit for selectively switching the current sink impedance means for sinking respective currents from the power supply rail in response to the state of the device.

The invention will be more clearly understood from the following description of a

preferred embodiment thereof, which is given by way of example only, with reference to the accompanying drawing, which illustrates a monitoring circuit according to the invention in block representation s Referring to the drawing, there is illustrated a monitoring circuit according to the invention, indicated generally by the reference numeral 1 for monitoring the state of a device, which in this embodiment of the invention is a hall effect sensor 2 capable of operating in four states. The hall effect sensor 2 is associated with a seat belt buckle (not shown) of a motor vehicle seat for detemmining the buckled or unbuckled 10 state of the seat belt. The monitoring circuit 1 also in this embodiment of the invention comprises a current emulating circuit which is also according to the invention and is indicated generally by the reference numeral 3 for emulating currents drawn by the hall effect sensor 2 which are indicative of the state of the sensor 2. The hall effect sensory is of the type which when a 5 volt supply is 15 applied to the sensor 2, the sensor 2 sinks one of two currents, namely, a 6mA current which is indicative of the seat belt buckle being unbuckled, and a 14mA current which is indicative of the seat belt buckle being buckled. These two currents may range within a predetermined range of 4mA to 8mA in the case of the seat belt being unbuckled, and may range within a predetermined range of 12mA to 16mA in 20 the case of the seat belt being buckled. Currents outside the ranges 4mA to 8mA and 12mA to 16rnA are deemed to be fault currents, which in general, would indicate a fault in the sensor 2, or a fault in the circuitry in which the hall effect sensor 2 is located. A current of zero amps drawn by the hall effect sensor 2 is indicative of an open circuit condition in the sensor 2 or in the circuitry in which the sensor 2 is

located, and a 20mA current drawn by the hall effect sensor 2 is indicative of a short-

circuit condition in the sensor 2 or in the circuitry in which the hall effect sensor 2 is located. s The monitoring circuit 1 comprises a control means, in this embodiment of the invention a microprocessor 5 which controls the operation of the monitoring circuit 1, and also the emulating circuit 3 as will be described below. A regulated voltage power supply circuit 7 powers the microprocessor 5, and outputs a regulated voltage of approximately 5 volts, which can vary between 4.9 volts and 5. 1 volts. The 5-voH 10 supply from the power supply circuit 7 is applied to the hall effect sensor 2 through a primary switch means, which in this embodiment of the invention is provided by a primary transistor 8 which is active low. A first current sensing means, namely, a first current sense resistor 10 is provided in series with the primary transistor 8 between the primary transistor 8 and the sensor 2 so that the current drawn by the 15 sensor 2 is drawn through the first current sense resistor 10 for monitoring by the microprocessor 5 as will be described below. The primal transistor 8 is enabled at predetermined intervals typically of 100 milliseconds for applying the regulated voltage from the power supply circuit 7 to the sensor 2 for periodically monitoring the state thereof. The primary transistor 8 is enabled at the predetermined intervals for 20 predetermined monitoring time periods typically of 150 microseconds by a low enable signal outputted from an enable pin of the microprocessor 5.

A comparing means comprising a comparator 12 which is powered by the power supply circuit 7 compares the voltage applied to the sensor 2 with a threshold

voltage reference of approximately 4.25 volts for detecting an over current condition in the sensor 2 or its associated circuitry, which typically, would be caused by a short-circuit. The threshold voltage reference is derived from the regulated voltage power supply circuit 7 but is independent of the current drawn by the sensor 2. A s short- circuit in the sensor 2 or its associated circuitry would cause an excessive current, typically, 20mA to be sunk by the sensor 2, which in turn would cause the voltage developed across the sensor 2 to drop below 4.25 volts. The comparator 12 outputs a logic high in response to the voltage developed across the sensor 2 being below 4.25 volts. A secondary switch means provided by a secondary transistor 14 10 applies the logic high from the comparator 12 to the primary transistor 8 for switching off the primary transistor 8, thereby preventing further current being drawn when the sensor 2 or its associated circuit is in an over current condition.

However, in order to allow a sufficient settling time for the sensor 2 after the voyage 15 from the power supply circuit 7 has been applied to the sensor 2, a timing means is provided for timing a settling time delay after the primary transistor 8 has been enabled for applying the voltage from the power supply circuit 7 to the sensor 2. In this embodiment of the invention the microprocessor 5 Ames the settling time delay, which in this case is approximately 100 microseconds. The microprocessor 5 20 outputs a disable signal to the secondary transistor 14 for the predetemmined settling time, thereby holding the secondary transistor 14 in an off state, and preventing the output of the comparator 12 being applied to the primary transistor 8. On the microprocessor 5 having timed out the settling time delay period, the microprocessor 5 outputs an enable signal to the secondary transistor 14 for enabling the secondary

transistor 14 Once the secondary transistor 14 is enabled, the output of the comparator 12 is applied to the primary transistor 8. For so long as the output of the comparator 12 remains low, the primary transistor 8 remains switched on, thereby continuing to apply the regulated voltage from the power supply circuit 7 to the s sensor 2, until the microprocessor 5 has timed out the predetemmined monitoring time period, at which stage the enable output of the microprocessor 5 goes high, thus switching off the primary transistor 8. If, however, at any time during the predetermined monitoring time period after the sealing time period has timed out the output of the comparator 12 goes high, the high is applied through the secondary 10 transistor 14 to the primary transistor 8, thereby switching off the primary transistor 8 This, thus, avoids excess current being drawn by the sensor 2 should an over current condition exist in the sensor 2 or its associated circuitry.

The voltage across the first current sense resistor 10 is applied to a pair of analogue 15 to digital input pins of the microprocessor 5, and the microprocessor 5 detemmines the state of the sensor 2 from the voltage across the first current sense resistor 10.

By reading the voltage developed across the first current sense resistor 10, an accurate determination of the current being drawn by the sensor 2 is made by the microprocessor 5, and thus its state. Additionally, the microprocessor 5 reads the 20 output of the comparator 12 on a line 13 for detemnining when the voltage developed across the sensor 2 exceeds the threshold voltage reference. On the microprocessor 5 determining that the output fron1 the comparator 12 is indicative of the voltage developed across the sensor 2 exceeding the threshold voltage reference, the microprocessor 5 commences to read the voltage across the first

current sense resistor 10 for determining the current being drawn by the sensor 2. It is only when the voltage applied to the sensor 2 has exceeded the threshold voltage reference that an accurate reading of the state of the sensor 2 can be made.

5 A seat sensing means comprising a force sensitive impedance element, namely, a force sensitive resistor 15 is provided for locating in the seat portion of a vehicle seat associated with the seat belt, the buckle of which is being monitored, for determining the presence of an individual in the seat. A regulated voltage supply of S volts, similar to that applied to the hall effect sensor 2 is applied to the force sensitive to resistor 15 from the power supply circuit 7 through a first switch means, namely, a first transistor 16 and a first series resistor Rs1. The voltage developed across the force sensitive resistor 15 is applied to a pair of analogue to digital inputs of the microprocessor 5 which reads the voltage developed across the force sensitive resistor 15 for determining the presence of an individual in the seat. A look-up table 15 of voltage bands corresponding to weights of deferent types of individuals, for example, adults, children and the like is stored in the microprocessor 5 so that the microprocessor 5 can compare the voltage developed across the force sensitive resistor 15 with the voltage bands in the look-up table, for determining if an individual is sifting in the seat, and the type of individual sitting in the seat. The first transistor 20 16 is enabled by an enable signal from the microprocessor 5 at the predetermined intervals, and for the predetermined monitoring time periods for applying the 5 volt regulated supply to the force sensitive resistor 15, while the state of the seat belt buckle is being monitored.

Tuming now to the current emulating circuit 3, the current emulating Circuit 3 is adapted to be powered from a regulated voltage power supply rail 20, which is powered independently of the monitoring circuit 1, and in this embodiment of the invention, is a regulated voltage power supply rail of the electronic monitoring and s control circuitry of the motor vehicle in which the monitoring circuit 1 is to be installed. The power supply rail 20 is typically held at 5 volts. The voltage from the power supply rail 20 is applied to the current emulating circuit 3 through a second series resistor Rs2. The current emulating circuit 3 comprises a plurality of current sink impedance means, in this embodiment of the invention four current sink 10 resistors R1 to R4 which are connected in parallel for sinking four currents 11, 12, 13 and 14 from the power supply rail 20 to ground. The resistance values of the four current sink resistors R1, R2, R3 and R4 are selected so that the four currents 11, 12, 13 and 14 are similar to the four currents which would be sunk by the hall effect sensor 2 corresponding to the four states of the sensor 2, if the sensor 2 were 15 connected to the power supply rail 20 in place of the current sink resistors R1 to R4.

This, thus, permits the monitoring circuit 1 to be installed in a motor vehicle without affecting the monitoring and control electronic circuitry of the motor vehicle, while at the same time providing the vehicle monitoring and control circuitry with the current signals expected from the sensor 2. The first resistor R1 is of value for sinking the 20 current 11 to be similar to the current sunk by the sensor 2 which is indicative of the unbuckled state of the seat belt buckle. The second resistor R2 is of value for! sinking the current 12 to be similar to the current sunk by the sensor 2 which is indicative of the buckle of the seat belt being in the buckled state. The third resistor R3 is of value for sinking the current 13 to be similar to the maximum current drawn

by the sensor 2 which is indicative of a shortmircuit or over current condition of the sensor 2, and the fourth resistor R4 is of value for sinking the current 14 to be similar to the current drawn by the sensor 2 when an open circuit state exists, which is substantially OrnA.

A switch circuit 21 which is responsive to the microprocessor 5 selectively switches one of the first to the fourth resistors R1 to R4 for sinking the appropriate current 11 to 14 in response to the state of the sensor 2. The switch circuit 21 comprises four transistor switches T1 to T4, corresponding to the resistors R1 to R4 for selectively lo switching the resistors R1 to R4 for sinking the currents 11 to 14 from the power supply rail 20 to ground. The transistor switch T1 is enabled by a select 11 output of the microprocessor 5 for selecting the resistor R1 for sinking the current 11. The transistor switch T2 is operated by a select 12 output from the microprocessor 5 for selecting the resistor R2 for sinking the current 12 while the transistor switches TO 15 and T4 are activated by the microprocessor 5 by enable signals select 13 and select 14 for selecting the resistors R3 and R4 for sinking the currents 13 and 14, respectively. A second current sensing means, namely, a second current sense resistor 22 is 20 coupled to the power supply rail 20 in series with the second series resistor Rs2 and the resistors R1 to R4 for facilitating monitoring of the current being sunk through the selected one of theresistors R1 to R4, namely, for monitoring the currents 11 to 14.

A current monitoring circuit 23 monitors the voltage developed across the second current sense resistor 22 for determining the current being sunk by the current

emulating circuit 3.

In use, in order to facilitate the inclusion of the seat sensor provided by the force sensitive resistor 15 and the monitoring thereof into the electronic circuitry of a motor 5 vehicle without having an effect on the existing monitoring and control circuitry in the motor vehicle, the monitoring circuit of the motor vehicle for monitoring the state of the hall effect sensor 2 is replaced by the current emulating circuit 3. Since the current emulating circuit 3 emulates the currents drawn by the hall effect sensor 2, the vehicle monitoring and control circuitry is unaffected by the inclusion of the 10 monitoring circuit 1 in the vehicle monitoring and control electronic circuitry. The force sensitive resistor 15 is located in the seat portion of a vehicle seat for monitoring the presence of an individual thereon, and the hall effect sensor 2 which is already associated with the buckle of the seat belt is connected to the monitoring circuit 1. The monitoring circuit 1 then operates under the control of the IS microprocessor 5 for determining the state of the hall effect sensor 2 for in turn detemmining the state of the seat belt buckle, and also for determining the -voltage developed across the force sensitive resistor 15 for determining the presence of an individual in the seat.

20 Depending on the monitored results at the predetermined intervals, the microprocessor 5 outputs an appropriate select signal on the appropriate select 11 to select 14 outputs which then activates the appropriate transistor switch T1 to T4 for switching the appropriate one of the first to the fourth resistors R1 to R4 to ground.

The microprocessor 5 typically holds the selected one of the transistor switches T1

to T4 in the on state until the state of the hall effect sensor 2 has been detemmined during the next predetermined monitoring period. The microprocessor 5 also by determining the presence of an individual sitting in the seat or the seat belt buckle in the buckled state can then make a determination in the event of an accident whether 5 to inflate a corresponding airbag or otherwise. Additionally, the operation of the current emulating circuit 3 continues to provide the existing monitoring and control circuitry of the motor vehicle with an appropriate signal indicating the state of the hall effect sensor, and in turn the seat belt buckle.

Io The monitoring circuit 1 operates as follows. At periodic intervals the monitoring circuit 1 monitors the state of the sensor 2 for determining the state of the buckle of the seat belt. The length of the intervals between each monitoring of the sensor 2 may be any desired interval, but as discussed above are typically, in the order of 100 milliseconds. A monitoring cycle commences with the microprocessor 5 outputting IS an active low enable to the primary transistor 8 and operating the primary transistor 8 in the on state for applying the regulated voltage from the power supply circuit 7 to the sensor 2. Current from the power supply circuit 7 passes through the first current sense resistor 10 to the sensor2. Immediately on the microprocessor 5 outputting the active low enable to the primary transistor 8, the microprocessor 5 20 commences to time the settling time delay, and while the microprocessor 5 is timing the settling time delay, the microprocessor 5 holds the secondary transistor 14 in the off state. The comparator 12 compares the voltage applied to the sensor 2 with the threshold voltage reference, and outputs a signal indicative of whether the voltage developed across the sensor 2 is above or below the threshold voltage reference at

the end of the settling time delay. On the microprocessor 5 determining from the output of the comparator 12 that the voltage applied to the sensor 2 has exceeded the threshold voltage reference, the microprocessor 5 commences to read the voltage developed across the first current sense resistor 10 for determining the state 5 of the sensor 2, and in turn the state of the seat belt buckle. At the end of the settling time delay period the secondary transistor 14 is enabled by the microprocessor 5. if at the end of the settling time period or at any time thereafter, the output from the comparator 12 is indicating that the voltage applied to the sensor 42 has not yet exceeded the threshold voltage reference, the logic high from the 10 comparator 12 is applied to the primary transistor 8 through the secondary transistor 14 for operating the primary transistor 8 in the off state. As discussed above, in general, it is envisaged that each monitoring cycle will last approximately 150 microseconds, which is sufficient time to allow the voltage developed across the first current sense resistor 10 to give an accurate indication of the current being drawn by 15 the sensor 2, and in turn the state of the sensor 2, and in turn the state of the seat belt buckle. The next monitoring cycle commences after the appropriate interval has been timed out by the microprocessor 5.

Simultaneously, as the sensor 2 is being monitored by the monitoring circuit 1, the 20 force sensitive resistor 15 is also monitored.

Accordingly, the monitoring circuit 1 incorporating the current emulating circuit 3 can be interposed in the monitoring and control electronic circuitry of a motor vehicle between the seat belt buckle sensor and the monitoring electronics of the motor

vehicle without interfering with the motor vehicle electronics, and furthemmore, independent monitoring of the seat belt buckle and the seat sensor can be carried out while at the same time providing currents to the vehicle monitoring and control electronic circuitry indicative of the state of the sensor 2.

While the monitoring circuit has been described for determining only four states of the hall effect sensor 2, it will be readily apparent to those skilled in the art that the monitoring circuit could be adapted for determining many more or less than four states of a sensor. It will also, of course, be appreciated that the sensor 2 may be to any other type of device besides a hall effect sensor. Additionally, while the hall effect sensor has been described as being operable with a 5 volt supply, the hall effect sensor may be provided with any other voltage rating.

While the current emulating circuit has been described as comprising four resistors 15 R1 to R4 for emulating four currents, any other number of resistors R1 to Rn may be provided for emulating more or less than four currents.

It will of course be appreciated that current sink impedance means besides resistors R1 to R4 may be used for emulating the currents in the current emulating circuit.

It will also be appreciated that the current emulating circuit may be provided without the second current sense resistor. Any other suitable means may be provided for sensing the current being emulated by the current emulating circuit. It will also of course be appreciated that the first and second current sense resistors may be

provided by any other suitable current sensing means. Indeed, it is also envisaged that the second current sense resistor instead of being provided in the current emulating current circuit may be provided adjacent the power supply rail 20 which powers the current emulating circuit. It is also envisaged that the second current s sense resistor may be provided by a current sense resistor already located in the monitoring and control circuitry of the motor vehicle.

It will also be appreciated that any other suitable switch circuit besides that described for selectively switching the resistors R1 to R4 to ground for emulating the lo respective currents may be used.

While the seat sensor has been described as comprising a force sensitive resistor, any other suitable means for monitoring the presence of an individual in a seat may be provided.

It is also envisaged that in certain cases, the seat sensor may be omitted without departing from the scope of the invention.

While the monitoring circuit has been described as being provided in conjunction 20 with the current emulating circuit, it will be readily apparent to those skilled in the art that the monitoring circuit may be provided on its own without the current emulating circuit, and indeed, without the seat sensor. Further, it is envisaged that the current emulating circuit may be provided on its own without the monitoring circuit, and without the seat sensor, and it is also envisaged that the current emulating circuit

may be provided with other monitoring circuits besides that described, and it is also envisaged that the monitoring circuit may be provided with current emulating circuits other than that described.

s

Claims

1. A monitoring circuit for detemmining the state of a device having at least two states, the device sinking currents of values within respective predetermined ranges indicative of the respective states of the device, the monitoring circuit comprising a 5 regulated voltage power supply circuit for supplying a current to the device, a comparing means for comparing the voltage developed across the device with a threshold voltage reference for detecting an over current condition, a primary switch means for switching the current from the power supply circuit to the device for facilitating monitoring of the state of the device, the primary switch means being 10 responsive to the comparing means detecting an over current condition for isolating the device from the power supply circuit, a timing means for timing a predetermined settling time delay, the primary switch means being non-responsive to the comparing means during the predetermined settling time delay for preventing isolation of the device from the power supply circuit during the predetermined settling time delay for 15 allowing settling of the device and the monitoring circuit, and a first current sensing means for sensing the current being sunk by the device.

2 A monitoring circuit as claimed in Claim 1 in which a control circuit is provided for reading the first current sensing means for determining the state of the device.

3 A monitoring circuit as claimed in Claim 2 in which the primary switch means is operable by the control circuit at predetermined time intervals for supplying current to the device for facilitating monitoring the device for a predetermined monitoring time period at each predetermined time interval.

4. A monitoring circuit as claimed in Claim 2 or 3 in which the control circuit is responsive to the comparing means detenrnining that the voltage developed across the device has exceeded the threshold voltage reference for reading the first current 5 sensing means.

5. A monitoring circuit as claimed in any preceding claim in which the primary switch means comprises a primary transistor.

lo

6. A monitoring circuit as claimed in Claim 5 in which the primary transistor is responsive to an enable signal from the control circuit for switching the power supply circuit to the device.

7. A monitoring circuit as claimed in Claim 5 or 6 in which the primary transistor Is is active low.

8. A monitoring circuit as claimed in any preceding claim in which a secondary switch means is provided for applying a disable signal to the primary switch means for disabling the primary switch means in response to the comparing means 20 detemnining that the voltage developed across the device is below the threshold voltage reference after the settling time delay.

g. A monitoring circuit as claimed in Claim 8 in which the secondary switch means comprises a secondary transistor.

10. A monitoring circuit as claimed in any preceding claim in which the first current sensing means is located in series with the primary switch means.

s

11. A monitoring circuit as claimed in any preceding claim in which the first current sensing means is located between the primary switch means and the device.

12. A monitoring circuit as claimed in any preceding claim in which the first current sensing means comprises a first current sensing resistor.

13. A monitoring circuit as claimed in any preceding claim in which the timing means for timing the predetermined settling time delay is provided by the control circuit. 15

14 A monitoring circuit as claimed in any preceding claim in which the predetermined settling time delay is sufficient for allowing the voltage developed across the device to exceed the threshold voltage reference, assuming nommal operation of the device.

20

15 A monitoring circuit as claimed in any preceding claim in which the predetemmined settling time delay lies in the range of 50 microseconds to 200 microseconds.

16. A monitoring circuit as claimed in Claim 15 in which the predetemmined

settling time delay lies in the range of 75 microseconds to 150 microseconds.

17 A monitoring circuit as claimed in Claim 16 in which the predetermined settling time delay is approximately 100 microseconds.

s

18. A monitoring circuit as claimed in any preceding claim in which each predetermined time interval is in the range of 50 milliseconds to 500 milliseconds.

19. A monitoring circuit as claimed in Claim 18 in which each predetermined time 10 interval is in the range of 7S milliseconds to 250 milliseconds.

20. A monitoring circuit as claimed in Claim 19 in which each predetermined time interval is approximately 100 milliseconds.

IS

21. A monitoring circuit as claimed in any preceding claim in which each predetermined monitoring time period is in the range of 75 microseconds to 500 microseconds.

22. A monitoring circuit as claimed in Claim 21 in which each predetermined 20 monitoring time period is in the range of 100 microseconds to 300 microseconds.

23. A monitoring circuit as claimed in Claim 22 in which each predetemmined monitoring time period is approximately 150 microseconds.

24. A monitoring circuit as claimed in any preceding claim in which a current emulating circuit for emulating current drawn by the device is provided, the emulating circuit comprising a power supply rail for receiving a regulated voltage power supply, and at least two current sink impedance means for sinking current from the power s supply rail of respective different values within respective different current value ranges corresponding to the currents drawn by the device which are indicative of the state of the device, and a switch circuit for selectively switching the current sink impedance means for sinking respective currents from the power supply rail in response to the state of the device.

25. A monitoring circuit as claimed in Claim 24 in which the current sink impedance means are coupled to the power supply rail in parallel with each other.

26. A monitoring circuit as claimed in Claim 24 or 25 in which the switch circuit is 15 responsive to the control circuit for selecting the current sink impedance means for sinking a current from the power supply rail.

27 A monitoring circuit as claimed in any of Claims 24 to 26 in which the switch circuit comprises a plurality of select switches, one select switch being provided for 20 each current sink impedance means.

28. A monitoring circuit as claimed in any of Claims 24 to 27 in which N current sink impedance means are provided for sinking N respective currents corresponding to currents drawn by the device which are indicative of N states of the device.

29. A monitoring circuit as claimed in Claim 28 in which the switch circuit comprises N select switches corresponding to the N current sink impedance means.

s

30. A monitoring circuit as claimed in any of Claims 24 to 29 in which the switch circuit selectively couples the current sink impedance means between the power supply rail and ground.

31. A monitoring circuit as claimed in any of Claims 24 to 30 in which a second 10 current sensing means is provided in the current emulating circuit for facilitating monitoring of the current being sunk by the selected one of the current sink impedance means.

32. A monitoring circuit as claimed in Claim 31 in which the second current IS sensing means is coupled in series with the current sink impedance means.

33. A monitoring circuit as claimed in Claim 31 or 32 in which the second current sensing means comprises a second current sense resistor.

20

34. A monitoring circuit as claimed in Claim 33 in which a means for monitoring the voltage across the second current sense resistor is provided.

35. A monitoring circuit as claimed in any of Claims 24 to 34 in which each select switch comprises a transistor switch.

36 A monitoring circuit as claimed in any of Claims 24 to 35 in which each current sink impedance means comprises a current sink resistor.

5

37. A monitoring circuit as claimed in any of Claims 24 to 36 in Which the power supply rail is supplied with a regulated voltage power supply independently of the regulated power supply circuit.

38. A monitoring circuit as claimed in any preceding clain1 in which the monitoring 10 circuit comprises the device.

39. A monitoring circuit as claimed in any preceding claim in which the device is adapted for monitoring the state of a seat belt buckle of a seat belt of a motor vehicle seat.

40. A monitoring circuit as claimed in any preceding claim in which the device is provided by a hall effect sensor.

41. A monitoring circuit as claimed in any preceding claim in which the monitoring 20 circuit further comprises a seat sensor for monitoring the presence of an individual sitting on a motor vehicle seat associated with the seat belt.

42. A monitoring circuit as claimed in Claim 41 in which the seat sensor comprises a force sensitive impedance element supplied with a regulated voltage

from the regulated voltage power supply circuit, the voltage developed across the force sensitive impedance element being monitored by the control circuit.

43. A monitoring circuit as claimed in Claim 42 in which the force sensitive s impedance element comprises a force sensitive resistor.

44. A monitoring circuit substantially as described herein with reference to and as illustrated in the accompanying dravving.

lo

45. A current emulating circuit for emulating a current drawn by a device having at least two states, the device sinking currents of value within respective predetermined ranges indicative of the respective states of the device, the emulating circuit comprising a power supply rail for receiving a regulated voltage power supply, and at least two current sink impedance means for sinking current from the power 15 supply rail of respective different values within respective different current value ranges corresponding to the currents drawn by the device which are indicative of the state of the device, and a switch circuit for selectively switching the current sink impedance means for sinking respective currents from the power supply rail in response to the state of the device.

46. A current emulating circuit as claimed in Claim 45 in Which the current sink impedance means are coupled to the power supply rail in parallel with each other.

47. A current emulating circuit as claimed in Claim 45 or 46 in which the switch

circuit is responsive to a control circuit for selecting the current sink impedance means for sinking current from the power supply rail.

48. A current emulating circuit as claimed in any of Claims 45 to 47 in which the s switch circuit comprises a plurality of select switches, one select switch being provided for each current sink impedance means.

49. A current emulating circuit as claimed in any of Claims 45 to 48 in which N current sink impedance means are provided forsinking N respective currents 10 corresponding to currents drawn by the device which are indicative of N states of the device.

50. A current emulating circuit as claimed in Claim 49 in which the switch circuit comprises N select switches corresponding to the N current sink impedance means.

51. A current emulating circuit as claimed in any of Claims 45 to 50 it; which the switch circuit selectively couples the current sink impedance means between the power supply rail and ground.

20

52. A current emulating circuit as claimed in any of Claims 45 to 51 in which a current sensing means is provided in the current emulating circuit for facilitating monitoring of the current being sunk by the selected current sink impedance means.

53 A current emulating circuit as claimed in Claim 52 in which the second

current sensing means is coupled in series with the current sink impedance means.

54 A current emulating circuit as claimed in Claim 52 or 53 in which the second current sensing means comprises a second current sense resistor.

s

55. A current emulating circuit as claimed in any of Claims 52 to 54 in which a means for monitoring the voltage across the second current sense resistor is provided. 10

56. A current emulating circuit as claimed in any of Claims 45 to 55 in which each switch comprises a transistorswitch.

57. A cu rrent emulating circuit as claimed in any of Claims 45 to 56 in which each current sink impedance means comprises a current sink resistor.

is

58..4 cu rrent emulating circuit substantially as described herein with reference to and as illustrated in the accompanying drawings.

20 F.F. GORMAN & CO.