Abstract:
A tire condition monitoring system includes a monitoring device securely positioned within a tire and in electronic communication with a receiver and a cab mounted visual display. In one embodiment, the monitoring device includes a battery, an inductive pick-up coil, pressure and/or temperature sensors, a microprocessor, and a data transmitter. Each monitoring device has a unique multi-bit identification code. The monitoring device is shipped in an energy conserving dormant state until activated either by pressurization of the tire or by use of a portable hand-held wand transmitter. The hand-held transmitter assigns the monitoring device a relative tire position. After activation, the monitoring device periodically senses tire condition. This information is stored and compared to preset parameters and the last stored tire condition information. The tire information is periodically sent to the receiver and visually displayed. If the sensed tire condition information deviates from preset parameters, the sensed tire information is immediately telemetered to the receiver and an alarm is activated. During prolonged periods of vehicle and tire inactivity, the monitoring device measures and transmits less frequently to preserve power. The monitoring device automatically reactivates upon vehicle reactivity. In another embodiment, a monitoring device having a mechanical air pressure sensor, a motion detector, a battery and a transmitter is positioned within the tire. Once the tire is traveling at a predetermined velocity, power is supplied to the mechanical air pressure sensor. When the tire pressure drops below a predetermined level, the mechanical air pressure sensor provides power to the transmitter which generates a signal to a receiver and cab mounted alarm.

Description:
BACKGROUND OF THE INVENTION 
     The present invention pertains to a tire monitoring system. More particularly, the present invention relates to a tire monitoring system which monitors tire engineering conditions, including pressure and temperature, using a monitoring device which is generally installed on the interior portion of a pneumatic tire or tire rim and in electronic communication with a receiver. 
     The need to maintain tires at the correct pressure level to eliminate driving on under-inflated tires is fundamental in preventing undue tread wear, increased fuel consumption and flat tire accidents. If the average passenger car tire pressure decreases from 32 p.s.i. to 25 p.s.i., the life of the tire is reduced by 20% due to uneven tread wear and fuel consumption increases by up to 10%. A vehicle&#39;s handling and braking are also adversely affected by tires having low air pressure. The U.S. National Highway Traffic and Safety Administration has reported that almost half of the tires on the road are under-inflated and may account for as many as 250,000 annual accidents. It has also been estimated that over five million gallons of gasoline are wasted each year due to tire under-inflation. 
     Tires known as “run-flat” tires have recently been developed which have reinforced side walls so that the tire can be driven on for a certain number of miles with little or no physical manifestation even though the tire is completely deflated. Without a tire monitoring system, the driver will not be aware that he or she has a flat tire and may destroy the tire before having it repaired or replaced. 
     An operational and practical design for remote tire pressure and/or temperature measuring devices has been attempted for many years. Unfortunately, none of these devices has achieved acceptance for many reasons including the unreliability and fragility of the components. Not until the invention of miniature solid state sensors and microprocessors has any degree of success been achieved. However, these systems can also be unreliable and very expensive. 
     Many modern tire monitoring systems consist of pressure and temperature transducers as well as a transmitter. Power is supplied by utilizing a battery, inductive coils or piezo-electric power. Although there are a myriad of combinations of these components, considerable effort is required to design a system that is reliable, easily used by non-technical personnel, and cost effective. 
     Accordingly, what is needed is a tire pressure monitoring system utilizing modern electronics which are internally mounted within a tire. What is further needed is a tire pressure monitoring system which is reliable, cost-effective and easily used by non-technical personnel. The present invention fulfills these needs and provides other related advantages. 
     SUMMARY OF THE INVENTION 
     A tire condition monitoring system is provided which informs the driver of the condition, including air pressure, of one or more of the tires so that the driver is alerted when a tire is deflated. Other engineering data, including tire temperature, can also be relayed to the driver or maintenance personnel. The system is cost-effective and reliable and can be used by non-technical personnel for the life of the tire. 
     In accordance with the invention, a tire condition monitoring device having a unique identification code is positioned within a pneumatic tire. The tire condition monitoring device includes a battery, at least one sensor in electrical circuit with the battery, a programmable microprocessor in circuit with the battery, a transmitter in circuit with the battery and a pick-up coil in electrical circuit with the microprocessor. The sensors include a pressure sensor and a temperature sensor. The transmitter includes a SAW filter for pulse modulated transmissions. The unique identification code comprises a multiple bit code which specifically identifies the device. The monitoring device is securely attached to either a rim for the tire or an inner surface of the tire itself. 
     The tire condition monitoring device is shipped in a dormant mode. After installation, the monitoring device can be activated and assigned a tire location by pressurizing each tire to a predetermined level to activate the monitoring device from a dormant state to an operational state. Alternatively, the monitoring device is activated and assigned a tire location by use of a portable hand-held wand transmitter. This is accomplished by holding the hand-held transmitter close to the tire and actuating a switch of a keypad on the hand-held transmitter and subsequently transmitting a signal to the monitoring device through the tire. The keypad has at least one switch to assign a tire location for every tire on the vehicle. 
     The tire condition monitoring device periodically senses a condition within the tire, including measurement of tire temperature and/or air pressure. The sensed condition information is then electronically stored and the sensed condition information is compared with preset parameters based on previously stored condition information. 
     If the sensed tire condition data falls within the preset parameters, the transmitter of the monitoring device periodically telemeters the sensed condition information and the monitoring device identification code in pulse modulated signal to a receiver in electronic communication with the monitoring device. The receiver is usually mounted in the cab of the vehicle. The receiver communicates this information to a cab mounted visual display unit which visually displays the information. 
     The monitoring device immediately telemeters the sensed condition information and identification code to the receiver if the sensed condition information falls without the preset parameters. The visual display unit includes an audible alarm for alerting a vehicle passenger of a change of sensed tire conditions. 
     The sensed condition information is telemetered less frequently during periods of low vehicle and tire activity. The monitoring device is reactivated upon tire or vehicle reactivity. 
     In another embodiment of the present invention, a monitoring device having a battery, an air pressure sensor in circuit with the battery, a transmitter in circuit with the air pressure sensor and the battery, and a motion detector in circuit with the battery is positioned within the tire. The air pressure sensor comprises a mechanical sensor which is positioned entirely within the tire. The mechanical air pressure sensor includes a housing having a conductive portion, a pressure sensitive diaphragm positioned within the housing, a first conductive terminal in contact with the battery and the conductive diaphragm, and a second conductive terminal in physical contact with the conductive portion of the housing and the transmitter. 
     A receiver is in electronic communication with the transmitter and a cab mounted alarm. An encoder can be included in the monitoring device for generating a vehicle specific signal. 
     The motion detector is used to determine whether the tire is rotating at a predetermined velocity. Once the predetermined velocity is reached, power is supplied from the battery to the first terminal and the diaphragm. Tire pressure which has dropped below a predetermined level is determined when the ambient pressure within the tire decreases to the point where the diaphragm expands to physically contact the conductive portion of the housing, creating a conductive relationship between the diaphragm and the conductive portion of the housing, and thus the second terminal. Electrical power is transferred from the diaphragm to the transmitter via the conductive portion of the housing and the second terminal. The transmitter generates a signal to the receiver to activate the alarm and alert the driver of existence of a low pressure tire. 
     Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate the invention. In such drawings: 
     FIG. 1 is a schematic representation of the system of the present invention wherein a partially fragmented tire having a tire condition monitoring device embodying the present invention secured therein is receiving a signal from a hand-held transmitter held close thereto, the monitoring device being in electronic communication with a receiver and display unit; 
     FIG. 2 is an exemplary electronic schematic of the hand-held transmitter used in conjunction with the monitoring device; 
     FIG. 3 a  is an exemplary electronic schematic of a microprocessor and sensors of the monitoring device; 
     FIG. 3 b  is an exemplary electronic schematic of a data transmitter of the monitoring device; 
     FIG. 3 c  is an exemplary electronic schematic of a signal pick-up coil of the monitoring device; 
     FIGS. 4 a-   4   d  are exemplary electronic schematics of a receiver and visual display used in conjunction with the monitoring device; 
     FIG. 5 is a flowchart illustrating the steps of powering on and initializing the monitoring device prior to shipment; 
     FIG. 6 is a flowchart illustrating the steps taken during shipment and storage of the monitoring device; 
     FIG. 7 is a flowchart illustrating the steps taken when the monitoring device is activated by the hand-held transmitting wand; 
     FIG. 8 is a flowchart illustrating the steps taken to set the pressure gain after activation by the hand-held transmitter wand; 
     FIG. 9 is a flowchart illustrating the steps taken to assign tire position to the monitoring device by the hand-held transmitter; 
     FIG. 10 is a flowchart illustrating the steps taken during normal operation of the monitoring device; 
     FIG. 11 is a flowchart illustrating the steps taken during measurement of tire conditions by the monitoring device; 
     FIGS. 12 a  and  12   b  are flowcharts illustrating the steps taken during comparison of the measured conditions to preset limits and expiration of internal timers; 
     FIG. 13 is a flowchart illustrating the steps taken during transmission tire condition data by the data transmitter of the monitoring device to the receiver of the monitoring system; 
     FIG. 14 is a functional block diagram of another embodiment of the system of the present invention utilizing a mechanical pressure sensor; and 
     FIG. 15 is an exploded perspective view of the mechanical pressure sensor of the system of FIG.  14 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in the drawings for purposes of illustration, the present invention is concerned with a tire condition monitoring system generally referred to by the reference number  10 . 
     A first embodiment of the system  10  is schematically illustrated in FIG.  1  and is generally comprised of a hand-held transmitting wand  12  which, when held in relatively close relation to a tire  14  containing a tire condition monitoring device  16 , transmits information to a monitoring device  16  through a wall of the tire  14 , the monitoring device  16  in turn transmits information to a receiver and display unit  18 , usually positioned in a cab portion of the vehicle. 
     The hand-held transmitter wand  12  includes a keypad  20  having a plurality of keys  22 . As a key  22  is pressed, tire position data is assigned to the monitoring device  16  for that particular tire  14 . In the case of a four-wheel vehicle, the hand-held transmitter  12  has at least four keys  22 , one for each tire position. For vehicles having eighteen wheels, the hand-held transmitter  12  has ten keys and an outside/inside switch to denote each tire position. The hand-held transmitter  12  may also include a small display  24  to visually verify the inputted information. An electronic schematic of an exemplary hand-held transmitter  12  is illustrated in FIG.  2 . 
     The monitoring device  16  is comprised of an etched circuit board made of epoxy glass to hold various components including a long-life lithium battery, various sensors (including pressure and temperature sensors), an inductive pick-up coil, a microprocessor, and a transmitter. The inductive coil needs no power from the battery and receives signals sent from the hand-held transmitter  12 . The microprocessor processes the electrical signals from the sensors and hand-held transmitter  12  and stores data and the algorithm of the system. The sensors can also be implemented for other tire conditions including tire revolution and/or mileage, tire identification information such as serial number, manufacturing data, tire size or retread information. The transmitter signals information to the receiver/display unit  18 . The transmitter of the monitoring device  16  includes a SAW filter for production of pulse modulated transmission signals. Exemplary electronic schematics of the circuitry of the monitoring device  16  are illustrated in FIGS. 3 a-c.  Each monitoring device  16  is given a unique identifying serial number, comprising up to a twenty-four bit code, when manufactured. This identification code is programmed into the memory of the monitoring device  16  so that no man readable labels are necessary. The monitoring device  16  is encapsulated in a durable molded rubber casing which can be securely attached to an inner tire wall or tire rim. 
     The monitoring devices  16  are calibrated at the factory and shipped in a low power consumption dormant state. The monitoring devices  16  are selected at random for installation. After installation, the monitoring devices  16  are activated by assigning a tire position using the hand-held transmitter  12  as described above. The monitoring device  16  awakes from its dormant state and transmits its identification number and tire location to the receiver/display unit  18 . The monitoring device identification number and tire location are stored in the receiver/display unit  18  and an audible tone is emitted to confirm the reception of the information. The stored tire locations corresponding to the monitoring device identification numbers can be changed as tires are replaced or the tires are moved to different locations on the vehicle. 
     Once activated the monitoring devices  16  periodically self-activate at random intervals to sample and store engineering information, such as pressure and temperature readings. The monitoring device  16  transmits data, including temperature and pressure readings, to the receiver/display unit  18  after predetermined time intervals (usually fifteen minutes) to confirm system integrity. The receiver/display unit  18  stores and displays the relevant pressure and tire information according to tire position. The receiver/display unit  18  can also collect operational data from the transmissions which can be downloaded to a database for tire service history and preventative maintenance assistance. 
     If the pressure and temperature readings vary from previous readings by preset limits the information is transmitted immediately, even if the fifteen minute interval has not elapsed. If the receiver/display unit  18  does not receive a transmission from a monitoring device  16  during a predetermined period, such as thirty minutes, it will alarm to indicate a failure of that monitoring device  16 . The operation and logic of the system  10  are more fully described in the flow charts of FIGS. 5-13. 
     With reference to FIG. 5, the monitoring device  16  is calibrated at the factory by first powering on the monitoring device  16  ( 100 ). The hardware and software are then initialized ( 102  and  103 ). The wand input for the hand-held transmitter wand is then disabled ( 104 ) so that the calibration sequence can proceed without interference from inadvertent commands issued by hand-held transmitters in the area. The monitoring device  16  stands by for power according to a delay time ( 106 ) and once the delay time has passed ( 108 ) the analog circuit is powered for an initialize time period ( 110 ). Once the initialize time period has elapsed ( 112 ) pressure and battery readings are taken ( 114 ) and the analog circuit is powered off ( 116 ). 
     The pressure and battery levels represent atmospheric pressure and current battery level measurements which are stored in memory as pressure offsets ( 118 ) and fresh battery levels ( 120 ). A default pressure gain setting and default durations for all timers are then loaded ( 122  and  124 ). The monitoring device  16  is set in an uncalibrated status ( 126 ), the hand-held transmitter wand input is re-enabled ( 128 ) and the monitoring device  16  enters a shipping mode ( 130 ). 
     Referring to FIG. 6, while in the shipping mode ( 130 ) the monitoring device  16  remains in a dormant sleep state until input is received by the hand-held transmitter wand ( 132 ). After verifying that the input received was from the hand-held transmitter wand ( 134 ), the monitoring device  16  enters a wand input routine ( 136 ). 
     With reference to FIG. 7, the want input routine ( 136 ) includes the first waking the monitoring device from its dormant sleep state ( 138 ). The hand-held transmitter wand command code is then read and validated ( 140  and  142 ). A determination is made as to whether the wand command code is valid or not ( 144 ). If it is determined not to be valid, the monitoring device  16  returns to its dormant state ( 146 ). If the command code is determined to be valid, the command code which was invoked is determined ( 148 ) and the appropriate command code routine is accessed ( 150 ). Several key command code routines, such as pressure, tire diagnostics, and other features can be accessed at this point. Once finished, the monitoring device  16  returns to the appropriate routine ( 152 ). 
     A command code which is given at the factory is the set pressure gain command code ( 154 ), illustrated in FIG.  8 . The hand-held transmitter wand input is disabled ( 156 ) so as not to interfere with the routine and a true pressure value from the wand command code is read ( 158 ) by powering on the analog circuit ( 160 ), and reading the pressure ( 162 ) and then powering off the analog circuit ( 164 ). This is accomplished in the factory by placing the monitoring device  16  in a chamber which is pressurized to a level approximating the anticipated tire pressure which the monitoring device  16  is to be exposed to. For example, in a standard passenger vehicle the monitoring device is subjected to 32 psi. Increased values, such as 50, 90 or 100 psi may also be used as appropriate. The pressure gain is calculated ( 166 ) using the pressure offset ( 118 ) and the true pressure value ( 158 ). The pressure gain value is then stored in the memory ( 168 ). 
     The analog circuit is then powered on ( 170 ), pressure, temperature, and battery readings are made ( 172 ), and the analog circuit is then powered off ( 174 ). The correct pressure reading is stored using offset and gain values ( 176 ) and the uncalibrated status is cleared ( 178 ) as a quality check and final step in calibration. The pressure, temperature and battery readings are transmitted ( 180 ) (and more specifically described in FIG.  13 ), the hand-held transmitter wand input is re-enabled ( 182 ), and the monitoring device  16  returns to its dormant state ( 184 ). 
     The command code routine for tire position assignment ( 186 ) is illustrated in FIG.  9 . After disabling the hand-held transmitter wand input ( 188 ), the position value from the wand command code is read ( 190 ) and the tire position assignment status is set ( 192 ). The monitoring device  16  then sets the pressure and/or temperature values to zero ( 194 ). These measurements and other relevant data are transmitted ( 196 ) to the receiver/display unit  18  (as more fully described in FIG.  13 ). The hand-held transmitter wand input is then re-enabled ( 198 ), the monitoring device  16  enters a sleep mode for a predetermined duration ( 200 ) after which it wakes up ( 202 ) and enters the normal mode of operation ( 204 ). 
     In the normal mode of operation ( 204 ), illustrated in FIG. 10, the hand-held transmitter wand input pin is disabled ( 206 ) and the short-term, long-term and deep sleep timers are incremented ( 208 ) as necessary. These timers may be altered due to vehicle and/or tire inactivity. For example, if the vehicle is parked overnight the sensors and circuitry within the monitoring device  16  will detect that there is little activity overtime. In order to conserve battery power and also to avoid unnecessary transmissions, the monitoring device  16  enters an inactive state where fewer transmissions are made over time. 
     Upon vehicle reactivity, such as the entering of a passenger in the car or the rotation of the tires which is sensed as pressure and/or temperature changes, the monitoring device  16  is returned to the active state. After adjusting the timers, analog measurements are taken ( 210 ) and the limits and changes in the measurements are checked ( 212 ). If the measurements are over the limit status ( 214 ) a transmission ( 216 ) is immediately made and then the hand-held transmitter wand input pin is re-enabled ( 218 ). If the measurements are not over the limit status, no immediate transmission is made and the input pin is re-enabled ( 218 ). 
     The monitoring device  16  then enters a dormant sleep mode for a predetermined time duration ( 220 ) after which it automatically awakes from the dormant mode ( 222 ) and initiates the process again. It should be noted that this routine covers the short-term sleep timer scenario where a transmission is made only if there is a change in readings or the limits are exceeded. Once the long-term timer elapses, a transmission of relevant data is made regardless of whether the limits are exceeded or not. 
     Referring now to FIG. 11, the routine for analog measure ( 224 ) includes powering on the analog circuit ( 226 ), making pressure, temperature and battery readings ( 228 ) and powering off the analog circuit ( 230 ). The pressure reading is corrected using the stored offset and gain data ( 232 ). It is then determined whether the battery reading is less than 80% of the fresh battery level ( 234 ). If the battery reading is less than 80%, a low battery status is set ( 236 ), the low battery information is transmitted to the receiver/display unit  18  and then the routine returns either to the dormant state or the next step of the appropriate routine ( 238 ). Due to the on/off nature and low power consumption of the monitoring device  16 , the battery will continue to operate for several weeks or more even after the low battery signal is sent. If the battery reading is not less than 80%, the routine returns either to the dormant state or the next step of a larger routine ( 238 ). 
     Referring to FIGS. 12 a  and  12   b,  the routine for limit checking ( 240 ) includes comparing the pressure and temperature readings to the short-term limits and determining whether these readings exceed the predetermined limits ( 242  and  244 ). This typically occurs when the pressure changes by 1 p.s.i. or less. 
     If either the pressure or temperature readings exceed the limits, the over-limit status is set ( 246 ) as detailed in sub-flowchart  1  of FIG.  12 . After setting the over-limit status ( 246 ), which results in a transmission to the receiver and display unit  18 , the short-term and long-term pressure and temperature limits are reset using the current readings ( 248 ). As there has been a transmission and the long-term timers are intended to send transmissions at predetermined intervals to confirm integrity of the system and the deep sleep timers are intended to be used during prolonged periods of inactivity, the short-term, long-term and deep sleep timers are also reset to default durations ( 250  and  252 ) to lengthen the time between measurements and transmission to avoid unnecessary transmissions which shorten the life of the battery. The sleep time for the monitoring device  16  is also reset ( 254 ) and the system returns to the routine ( 256 ). 
     If the pressure and temperature readings do not exceed the limits, it is next determined whether the short-term timer has elapsed ( 258 ). If it has, current readings from the analog measure are used to reset the short-term pressure and temperature limits ( 260 ) and the short-term timer is reset ( 262 ). If the short term timer has not elapsed, it is determined whether the pressure and temperature readings exceed the long-term limits ( 264  and  266 ). This typically occurs when the tire pressure changes by approximately 4-5 p.s.i. over the length of the long-term timer. If either the pressure or temperature readings exceed the long-term limits, the routine of sub-flowchart  1  is followed as described above. 
     If the pressure and temperature readings do not exceed the long-term limits, it is determined whether the long-term timer has elapsed ( 268 ). If the long-term timer has elapsed, the over-limit status is set ( 270 ), a transmission is made, the short-term and long-term pressure and temperature limits are reset using current readings ( 272 ), the short-term and long-term timers are reset ( 274 ) and the system is returned to the routine ( 276 ) according to sub-flowchart  2 . 
     If the long-term timer has not elapsed, it is then determined whether the deep sleep timer has elapsed ( 278 ). During periods of inactivity, the deep sleep timer is the default timer and all other timers are adjusted accordingly. If the deep sleep timer has not elapsed the system returns to the routine ( 280 ). If the deep sleep timer has elapsed, the over-limit status is set ( 282 ) and a transmission is made. The pressure and temperature limits are reset using current readings ( 284 ), the short-term, long-term and deep sleep timers are adjusted for deep sleep values ( 286  and  288 ) and reset ( 290 ) and the system returns to the appropriate routine ( 292 ). 
     As illustrated in FIG. 13, the routine for data transmission ( 294 ) begins by reading in the status data ( 296 ) and sensor identification number ( 298 ). The temperature and pressure data are next read ( 300  and  302 ). The data validation codes are calculated and the data is formatted for transmission ( 304  and  306 ). The transmitter is powered on ( 308 ), the formatted data is sent to the transmitter ( 310 ) and the transmitter transmits this data and then powers off ( 312 ). The transmitter preferably sends either a pulse width or a frequency shift key (FSK) signal, collectively referred to as a pulse modulated signal, to avoid interference. The appropriate status information is reset ( 314 ) and the system returns either to its dormant state or the next step of the larger routine ( 316 ). 
     A second embodiment utilizes the same operating procedure described above, but eliminates the use of the hand-held transmitter  12 . In order to activate, the installed monitoring device  16  is subjected to a predetermined pressure. For example, the tire  14  may be filled to 50 p.s.i. at which point the monitoring device  16  will awaken from it factory dormant state and begin transmission and normal operation. The pressure can be adjusted to 32 p.s.i. for passenger car tires after activation. Tire position information is obtained by selecting the tire position on the receiver/display unit  18  and pressurizing that particular tire. The process is repeated after tire replacement or relocation. 
     A third embodiment of the present invention is illustrated in the functional block diagram of FIG.  14 . The monitoring device  16  includes a piezo motion detector  400  which is preset to close an electronic switch  402  and provide power from a battery  404  to a mechanical pressure sensor  406  when the tire  14  is rotating at a predetermined rate. Typically, the tire  14  must be traveling at least 5 m.p.h. to activate the switch  402  in order to conserve power and prolong the useful life of the battery  404 . If the tire pressure drops below a predetermined level, the mechanical sensor  406  conducts power to an encoder  408  which delivers a vehicle specific coded data stream to a transmitter  410  for transmission to the receiver/display unit  18  within the cab of the vehicle. Upon receiving the transmitted signal, the receiver/display unit  18  alarms to alert the driver that one of the tires  14  has low pressure. The driver can then inspect the tires  14  to determine which tire needs to be filled up or repaired. 
     The mechanical pressure sensor  406  is illustrated in FIG.  15 . The sensor  406  is comprised of a generally cylindrical conductive housing  412  having an internal non-conductive lip  414 . The lip  414  supports a conductive elastic diaphragm  416 , typically a metal aneroid diaphragm which expands and contracts according to pressure variations. A compression ring  418  separates the diaphragm  416  from a central contact plate  420  having a terminal pin  422  extending therefrom to connect to the battery  404  and circuitry of the monitoring device  16 . The terminal pin  422  is positioned so as to be in constant electrical connection with the diaphragm  416 . A conductive peripheral contact plate  424  is positioned over the central contact plate  420  so that there is no electrical contact between the terminal pin  422  and peripheral contact plate  424 . The peripheral contact plate  424  is sized so as to be in physical contact with the conductive housing  412 . The peripheral contact plate  424  has a terminal pin  426  which extends from the plate  424  to the encoder circuitry  408  of the monitoring device  16 . 
     The mechanical sensor  406  is an integral part of the monitoring device  16  and resides entirely within the tire  14  after installation of the monitoring device  16 . The diaphragm  416  of the sensor  406  is fabricated to be in a contracted state and not in physical contact with the housing  412  at a certain predetermined pressure level, for example 18 p.s.i. for passenger vehicles. Once the pressure within the tire  14  drops below this level, the diaphragm  416  expands to physically contact the housing  412 . If power is supplied to terminal  422  due to the rotation of the tires, the physical contact between the diaphragm  416  and the housing  412  will result in the power being conducted through the housing  412  and to terminal  426  through peripheral contact plate  424 . This acts to close or complete the circuit of the monitoring device  16  and activate the transmitter  410  which sends a signal to the receiver and alarm. When the pressure is equal to or greater than the predetermined pressure level, the diaphragm  416  is in a contracted non-contact state and the circuit remains open with no power being supplied to the monitoring device  16 . 
     While not providing specific tire information, this embodiment is much simpler in design and can be provided at a significantly reduced cost compared to existing tire monitoring systems. This embodiment is also particularly useful in the recently developed “run-flat” tires. As there are little or no adverse effects on the vehicle upon tire deflation, the driver is not aware of the deflated tire and may continue driving until the tire is destroyed. Therefore, the driver must be alerted when one or more tires deflate. 
     Although several embodiments have been described in detail for purposes of illustration, various modifications may be made to each without departing from the scope and spirit of the invention. Accordingly, the invention is not to be limited, except as by the appended claims.