Abstract:
An electronic animal containment or confinement system that contains a transmitter which generates a regulated signal through a boundary antenna made from a buried wire. The regulated signal creates a field signal of a predetermined width regardless of the length of the wire used to define the boundary antenna. The field signal is received by a receiver worn by a dog. If the dog gets too close to the field signal, the receiver generates a current that passes across a pair of probes that contact the skin of the dog in order to give the dog a shock. This represents a form of training to teach the dog to respect an electrified boundary formed by the buried wire. There is also provided a system for protecting a transmitter in a pet confinement system against lightning strikes.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an electronic animal containment or confinement system and, more specifically to a pet confinement system that contains a transmitter which generates a regulated signal through a boundary antenna made from a buried wire. The regulated signal creates a field signal of a predetermined width regardless of the length of the wire used to define the boundary antenna.  
         [0003]     2. Description of Prior Art  
         [0004]     Over the years, electronic animal containment systems have become very popular because they use electronic signals traveling though a buried wire to create a boundary defining a predetermined area within a property. In this way, the property owner does not have to erect a fence in order to ensure that an animal is confined within a desired space.  
         [0005]     In a typical electronic animal confinement system, a boundary signal emitter wire in the form of a boundary antenna is buried along the perimeter of a predetermined area of real property or ground in which a pet, such as a dog, is to be confined. A transmitter typically placed in a house or garage is electronically connected directly to the boundary antenna to energize the antenna with a magnetic field signal generated in the transmitter. The wire radiates the magnetic field signal to electronically define an imaginary “boundary” coincident with the wire. A receiver worn about the neck of the dog responds to the radiated signal as the dog approaches the perimeter boundary set up by the boundary antenna. The receiver includes circuitry designed to provide a shock to the dog to cause the dog to move away from the perimeter boundary. As a result, the dog may be kept in the yard without the need for a fence.  
         [0006]     One well-known pet containment system is manufactured by Woodstream Corporation of Lititz, Pa. The pet containment system comes in two embodiments. The first is known as the 5140 Fence Free Pet Containment System and the second is known as the 5132 Fence Free Deluxe Pet Containment Systems. Both of these systems provide humane electrical stimulation in a safe and effective way to train a dog to stay within the boundaries of a yard.  
         [0007]     While systems such as the ones mentioned above have met with great success, there is nevertheless room for improvement. One area where improvement is needed is in creating a magnetic force field of constant magnitude. In prior art systems, signal field width varies according to several factors, including the length and gauge of the wire defining the boundary antenna. In order to normalize the signal field width for installations of any size, a variable resistance must be added to the loop wire that defines the boundary antenna. This resistance must be added in series with the loop wire and the terminals of the transmitter. In prior art systems, the resistance may be added by use of discrete resistive elements or thorough incorporation or a potentiometer into the transmitter circuitry. No automatic adjustment system exists.  
         [0008]     Also in stormy weather, the wire boundary antenna acts not only as a emitter of the magnetic field boundary signal but also as a vehicle for attracting lightening. Should lightening strike at or near the wire, the transmitter circuitry may be damaged or destroyed.  
         [0009]     The present invention is directed toward solving these problems.  
       SUMMARY OF THE INVENTION  
       [0010]     The basic elements of the inventive pet confinement system consist of a receiver that is secured to a collar that is worn by a dog around its neck. The receiver contains two probes that make contact with the skin of the dog so that when the dog gets too close to an electrified boundary, a shock is created within the receiver and passed through the probes into the dog as way to alert the dog that behavior such as approaching the fence should not be done. The receiver also contains a charger receptacle that mates with a complementary connection provided in a transmitter. It is the transmitter which is the subject of this invention.  
         [0011]     The installation of a pet confinement system according to the subject invention begins by creating a boundary providing an area within which a dog is free to roam. In its simplest form, the pet confinement system comprises a wire that is buried in the ground in a loop with the ends of the wire being connected to the inventive transmitter. In this way, the boundary is defined about the entire property and the dog is free to roam anywhere within that boundary.  
         [0012]     The inventive transmitter contains an input jack which accommodates a connection plug from a portable transformer in order to provide an input voltage for the system. Through the use of a full wave bridge rectifier, the input voltage can be AC or DC, but is preferably DC. The rectifier yields a DC voltage that passes through a voltage regulator which has an output port that provides operating voltage to circuit elements within the system.  
         [0013]     The heart of the inventive transmitter is a microcontroller that provides various control signals to a voltage adjuster. An EEPROM is used to program a desired identification code (ID) which matches a comparable identification code set in the receiver that is worn by the dog. This identification code is constantly transmitted to a boundary antenna under the direction of the controller so that the receiver is able to detect and pick up the information that is also contained within the signal that carries the ID.  
         [0014]     Also forming part of the invention is the boundary antenna which is made up of a wire that is buried in the ground. This boundary antenna transmits information from the microcontroller that includes the ID along with other information to cause the receiver to respond in different ways. One end of the boundary antenna is connected to a switch that passes through an arrangement of resistors to signal ground (a common electrical point on the Printed Circuit Board). The other end of the boundary antenna is connected to the output of an adjustable voltage regulator. This regulator has an input for receiving an input voltage Vcc from the full-wave bridge rectifier. The regulator also contains an adjustment port for receiving a signal from the voltage adjuster in order to change the magnitude of the output voltage appearing at the output of the regulator. The inventive circuit also includes an operational amplifier (OP AMP) that is used to amplify a voltage signal which represents the voltage passing through the resistor arrangement when the switch is on. The voltage signal is amplified by the OP AMP to a usable value and then placed into an A-to-D converter housed in the microcontroller. The transmitter also contains a switch array made up of a series of six push-button switches that are used to cause the microcontroller to ultimately adjust the current passing through the boundary antenna to yield a magnetic field of desired width appearing around and about the wire forming the boundary antenna.  
         [0015]     In order to constructively measure the current flowing through the wire, set points are used. A set point is simply an A-to-D converter count which represents the desired loop current for a field width setting. Each of the five field width settings has its own set point. The set points were calculated using a spreadsheet and are based on an amplifier gain of 40. The set points are 54 for a field width of 1.5 ft. and an average loop current of 80 milliamps, 95 for a field width of 2.5 ft. and an average loop current of 140 milliamps, 124 for a field width of 3.25 ft. and an average loop current of 180 milliamps, 149 for a field width of 4.0 ft. and an average loop current of 220 milliamps, and 190 for a field width of 5.0 ft and an average loop current of 310 milliamps.  
         [0016]     When a push-button switch is selected, evidencing a desired field width across the wire making up the boundary antenna, the microcontroller puts out a signal into the voltage adjuster which in turn causes the voltage regulator to put out an output current that passes through the boundary antenna. The desired set point count is monitored in the microcontroller and the microcontroller looks to see if the count represents a desired field width. If the count is too low, then the microcontroller causes the voltage adjuster to increase the output voltage from the voltage regulator. This interaction takes place until the microcontroller gets a set point reading from the data passing through the A-to-D converter that equals or exceeds the predetermined set point value for a desired field width. At that point, the microcontroller no longer yields signals for changing the voltage output of the voltage regulator and the field width passing through the wire of the boundary antenna remains at the desired value.  
         [0017]     By virtue of the foregoing, there is thus provided an electronic animal confinement system with an improved transmitter that provides advantages in performance and utility over prior art systems.  
         [0018]     Thus, it is an object of the present invention to provide an electronic animal confinement system that has an improved transmitter for providing a signal through a boundary antenna so that the field strength emanating from the boundary antenna is at a predetermined value regardless of the length of the antenna.  
         [0019]     It is another object of the present invention to provide an improved system for protecting a transmitter in a pet confinement system against lightning strikes.  
         [0020]     These and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The accompanying drawings which are incorporated in and constitute a part of the specification illustrate embodiments of the invention and together with a general description of the invention given above and the detailed description given below serve to explain the principles of the invention.  
         [0022]      FIG. 1  is a diagrammatic view of a property showing an installation of the subject invention.  
         [0023]      FIG. 1A  is a diagrammatic view of the dog shown in  FIG. 1 .  
         [0024]      FIG. 2A  is a perspective view of a receiver used in the inventive pet containment system.  
         [0025]      FIG. 2B  is a plan view of the receiver of  FIG. 2A  mounted on a collar worn by a pet.  
         [0026]      FIG. 3  is a plan view of the casing for a transmitter incorporating the present invention.  
         [0027]      FIG. 4  is a block diagram of an embodiment of the present invention.  
         [0028]      FIG. 5  is a circuit diagram of the embodiment of the present invention shown in  FIG. 4 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     With reference to  FIGS. 1 through 3 , the basic elements of the inventive pet confinement system are shown.  FIGS. 1A, 2A ,  2 B and  3  generally show a receiver  10  that is secured to a collar  12  that is worn by a dog D around its neck. The receiver contains two probes  14  and  16  that make contact with the skin of the dog so that when the dog gets too close to an electrified boundary, a shock is created within the receiver  10  and passed through the probes  14  and  16  into the dog as way to alert the dog that behavior such as approaching the fence should not be done. The receiver  10  also contains a charger receptacle  18  that mates with a complementary connection provided in the transmitter  20 , which is the subject of this invention. The receiver  10  contains a light emitting diode (LED)  22  which indicates when the receiver is functioning and an on/off switch  24  for placing the receiver in an on position and in a desired mode of operation. One such receiver that is contemplated to be used in the present invention is that provided in the Woodstream 5140 Fence Free system. The receiver is discussed in greater detail at the Havaheart website having URL: www.havahart.com, the discussion of which is incorporated by reference herein.  
         [0030]      FIG. 1  shows a diagram for the installation of a pet confinement system according to the subject invention. Most often the pet confinement system is used in the context of creating a boundary  100  for providing an area  102  within which a dog D is free to roam. Most often the boundary is set up about a structure such as a house  104 . Frequently, a boundary area  102  contains additional areas such as  106  which are areas where the animal should not roam.  
         [0031]     In its simplest form with reference to  FIGS. 1 and 3 , the pet confinement system comprises a wire  110  that is buried in the ground in a loop with the ends of the wire  112  and  114  being connected to a wire pair  116  and  118  that is twisted in the manner shown in  FIG. 3  and identified as  120  and terminating in ends  116  and  118  secured to input terminals provided on the transmitter  20 . The twisted wires are used to cancel the magnetic field that normally flows through the wire when it is being used as a single strand to define the boundary  100 . The same concept of the twisted wire can be used such as shown by the use of twisted pair  122  to cancel the field which then makes conductive contact with the wire  124  that surrounds a pool  126  in area  106 . In this way, the boundary  100  is defined about the entire property and the dog is free to roam anywhere within that boundary. At the same time an inner boundary  124  is defined within the boundary  100  to provide an area  106  that the dog is not permitted to enter.  
         [0032]     The transmitter  20  also contains six push-button switches S 1  through S 6  that are used to control the amount of the current passing through the boundary wire  110 . Switch S 6  turns the system on and off and causes an LED  130  to light when the system is on. Switches S 1  through S 5  place the transmitter into a mode where a signal is passed through the wire  110  to generate a magnetic field having a predetermined width. In the present invention, pushing switch S 1  causes the transmitter  20  to emanate a signal that creates a field width with a radius of 1.5 ft. In turn, pushing switch S 2  yields a field width of 2.5 ft. whereas pushing switch S 3  yields a field width of 3.25 ft. Finally, pushing switch S 4  yields a field width of 4.0 ft. and pushing switch S 5  yields a field width of 5.0 ft.  
         [0033]      FIG. 4  shows a general block diagram of the elements that constitute the inventive transmitter  20 . The transmitter contains an input jack  210  which accommodates a connection plug from a portable transformer (not shown) in order to provide an input voltage for the system. Through the use of a full wave bridge rectifier  212 , the input voltage can be AC or DC, but is preferably DC. The rectifier yields a DC voltage, Vcc, that passes through a voltage regulator  214  which has an output port  216  that provides operating voltage to circuit elements within the system. The voltage regulator also has an output  218  connected to a charge jack  220  for mating with the charger receptacle  18  in the receiver  10  in order to charge a rechargeable battery (not shown) housed in the receiver  10 .  
         [0034]     The heart of the inventive transmitter  20  is a microcontroller  300  that provides various control signals on lines  310  to a voltage adjuster  312 . EEPROM  314  is used to program a desired identification code which matches a comparable identification code set in the receiver  10  that is worn by the dog D. When the EEPROM is operating, the identification code (ID) is transmitted to the microcontroller  300  through data lines  316 . This identification code is constantly transmitted to boundary antenna  320  under the direction of controller  300  as will be explained hereinafter so that the receiver is able to detect and pick up the information that is also contained within the signal that carries the ID.  
         [0035]     Also forming part of the invention is the boundary antenna  320  which is made up of a wire that is buried in the ground in the manner shown in  FIG. 1  with reference to the wire  110 . This boundary antenna  320  transmits information from the microcontroller  300  that includes the ID along with other information to cause the receiver  10  to respond in different ways. One end of the boundary antenna is connected to a switch  322  that passes through an arrangement of resistors  324  to signal ground  326 . The other end of the boundary antenna is connected to the output of an adjustable voltage regulator  330 . This regulator has an input for receiving an input voltage Vcc of predetermined value from the output of the full wave bridge  212 . The regulator  330  also contains an adjustment port for receiving a signal from the voltage adjuster  312  in order to change the magnitude of the output voltage appearing at the output of the regulator  330 . The inventive circuit also includes an operational amplifier (OP AMP)  340  that is used to amplify a voltage signal appearing on line  342  which represents the voltage passing through resistor arrangement  324  when the switch  322  is on. The voltage signal appearing on line  342  is amplified by OP AMP  342  to a usable value and then placed into the microcontroller and passed through an A-to-D converter housed in the microcontroller. The transmitter also contains a switch array  350  made up of a series of six push button switches that are used to cause the microcontroller  300  to ultimately adjust the current passing through the boundary antenna  320  to yield a magnetic field of desired width appearing around and about wire  110 . Switch S 6  is used to turn the circuit on and off. Switches S 1  through S 5  are used to cause the microcontroller  300  to control the transmitter circuit in order to create a desired boundary field width around wire  110  that defines the boundary antenna  320 .  
         [0036]     Basically, when switch  322  is on, the boundary antenna  320  receives a current that passes through the antenna through the switch and then through the resistor arrangement  324  to signal ground. The flow through the boundary antenna creates a magnetic field around the wire that constitutes the boundary antenna. The OP AMP  340  amplifies the voltage across the resistor arrangement  324  to increase the voltage amplitude to a usable value for conversion by the A-to-D converter in the microcontroller  300 . For each field width requested by switches S 1  through S 5 , there is a unique current flowing through the wire that is the boundary antenna  320 .  
         [0037]     In order to constructively measure the current flowing through the wire, set points are used. A set point is simply an A-to-D converter count which represents the desired loop current for a field width setting. Each of the five field width settings has its own set point. The set points were calculated using a spreadsheet and are based on an amplifier gain of 40. The set points are 54 for a field width of 1.5 ft., 95 for a field width of 2.5 ft.,  124  for a field width of 3.25 ft., 149 for a field width of 4.0 ft., and 190 for a field width of 5.0 ft.  
         [0038]     As will be explained in greater detail hereinafter, when a push-button switch is selected, evidencing a desired field width across the wire making up the boundary antenna, the microcontroller  300  puts out a signal on lines  310  into a voltage adjuster  312  made up of resistors and transistor switches. The adjuster, in turn, causes the voltage regulator  330  to put out an output voltage that passes through the boundary antenna  320 . The actual loop current is monitored in the microcontroller and the microcontroller looks to see if the count represents a desired field width. If the count is too low, then the microcontroller causes the voltage adjuster  312  to increase the output voltage from the voltage regulator  330 . This interaction takes place until the microcontroller A-to-D converter reading equals or exceeds the predetermined set point value for a desired field width. At that point, the microcontroller no longer yields signals for changing the voltage output of the voltage regulator and the field width passing through the wire of the boundary antenna is at the predetermined desired value.  
         [0039]     Having covered the general operation of the transmitter, the detailed operation of the circuitry that constitutes the  FIG. 4  embodiment of the present invention will now be described with reference to  FIG. 5  with like reference numerals denoting like elements to those described in the other figures.  
         [0040]      FIG. 5  is a schematic diagram of a preferred embodiment of the present invention. At the heart of the system is microcontroller  300 . Six push-button switches S 1 -S 6  are connected to the microcontroller  330 . Switches S 2 , S 3  and S 4  share a common connection with pin  15  of controller  300 , whereas, switches S 1 , S 5  and S 6  share a common connection with pin  9  of controller  300 . In turn, the other connections of the switches are as follows: switches S 4  and S 5  are connected to pin  8 ; switches S 3  and S 6  are connected to pin  7 ; and switches S 1  and S 2  are connected to pin  12 .  
         [0041]     EEPROM  314  has pins  1 - 4  connected to ground. The EEPROM also has pin  7  connected to ground. Pin  8  is connected to the output  216  of voltage regulator  214 . Pins  5  and  6  of EEPROM  314  through resistors R 18  and R 19 , respectively, are connected to output  216  which in turn are connected to ground through capacitor C 11 . Pins  5  and  6  are connected to pins  1  and  2 , respectively, of controller  300 .  
         [0042]     With regard to the power supply aspects of the present invention,  FIG. 5  shows diodes D 1 -D 4  arranged as a full wave bridge rectifier. DC jack  210  has one conductor  60  connected to node N 1  formed between the cathode of diode D 1  and the anode of diode D 2  and the other conductor  62  connected to node N 2  formed between the cathode of diode D 3  and the anode of diode D 4 . The anodes of diodes D 1  and D 3  are connected to ground. The cathodes of diodes D 2  and D 4  are connected to the input pin  1  of rectifier  214 . Pin  1  of rectifier  214  is also connected to ground through capacitor C 1 .  
         [0043]     Output pin  3  of rectifier  214  is connected to ground through two parallel capacitors C 2  and C 3 . Pin  3  also provide a 5 V DC power source. Pin  2  of rectifier  214  is connected to ground. Pin  3  of rectifier  214  is connected in series with resistor R 16  and LED D 8  to pin  11  of controller  300 . Finally, pin  3  is connected to conductor  70  of charge jack  220  through resistor R 31 . The other conductor  218  of jack  220  is connected to ground.  
         [0044]     Turning now to transistors Q 2 -Q 6 , these transistors have their bases connected to controller  300  in the following way: transistor Q 2  has a base connection to pin  20  of controller  300  in series with resistor R 26 ; transistor Q 3  has a base connection to pin  13  of controller  300  in series with resistor R 25 ; transistor Q 4  has a base connection to pin  4  of controller  300  in series with resistor R 23 ; transistor Q 5  has a base connection to pin  10  of controller  300  in series with resistor R 30 ; and transistor Q 6  has a base connection to pin  6  of controller  300  in series with resistor R 14 .  
         [0045]     The emitters of transistors Q 2 -Q 6  are connected to ground. The bases of transistors Q 2 -Q 6  are connected to pin  1  of the regulator  330  via series resistors R 5 , R 21 , R 22 , R 28 , and R 1 , respectively. Resistor R 4  is connected to ground and pin  1  of regulator  330 .  
         [0046]     Pin  2  of regulator  330  is connected to pin  1  of regulator  330  via resistor R 2 . Pin  2  is also connected to ground via capacitor C 5  and to the anode of diode D 6 . The cathode of diode D 6  is connected to terminal  321  of boundary antenna  320  via series resistor R 3 . Terminal  321  is also connected to ground via transzorb D 5 . In like manner, terminal  323  of boundary antenna  320  is connected to ground via transzorb D 7 . Terminal  323  is connected to ground via resistor R 24  and to the collector of transistor  322 . In turn, transistor has its base connected to pin  14  of controller  300  via resistor R 15 , and its emitter connected to ground via the parallel arrangement of resistors R 6 -R 8 .  
         [0047]     Transzorbs D 5  and D 7  are transient voltage suppressors such as those bearing product number SMBJ 12 A made by Fairchild Semiconductor Corporation, www.fairchildsemi.com. The transzorbs provide protection for the transmitter circuitry from transients caused by lightning strikes.  
         [0048]     Pins  17  and  18  connect to crystal XT 1  and to ground via capacitors C 9  and C 10 , respectively. Pin  16  is connected to voltage source  216  and to ground via capacitor C 8 . Also pin  19  is connected to the cathode of LED D 9  whose anode is connected to voltage source  216  via resistor R 20 . Finally, pin  12  of controller  300  is connected to ground via resistor R 17 .  
         [0049]     The emitter of transistor  322  is connected to pin  3  of OP AMP  340  via resistor R 9 . Pin  2  of OP AMP  340  is connected to ground via resistor R 10  and to pin  1  via resistor R 11 . Pin  5  of OP AMP  340  is connected to ground via capacitor C 7  and to 5 V source  216 . Pin  1  of OP AMP  340  is connected to pin  3  of controller  300  and to ground via resistor R 12 .  
         [0050]     As explained before, the heart of the inventive circuit is the microcontroller  300 . The microcontroller is an eight bit microcontroller developed with low power and high speed CMOS technology. One such controller is that made by Elan Micro Electronics Corp., Hsinchu City, Taiwan and bearing product designation number EM78P458.  
         [0051]     As stated before, six push buttons S 1  through S 6  are connected to the microcontroller  300  to provide controlling inputs. Switch S 6  turns the system on and off. Switches S 1  through S 5  provide an input to cause the microcontroller to adjust the field width around the boundary antenna  320 . The microcontroller  300  executes a program to carry out specific actions based upon which of the buttons is pressed. When a button is pressed, the microcontroller runs a program and outputs analog control signals to the five transistors Q 2  through Q 6 , which are in the voltage adjuster. In a preferred embodiment the transistors are NPN type switching transistors.  
         [0052]     A preferred embodiment of EEPROM  314  bears Model Number AT24C04 made by Atmel Corp., San Jose, Calif., www.atmel.com. The EEPROM is an electrically erasable and programmable read only memory organized as 512 words of eight bit each.  
         [0053]     Light emitting diodes D 8  ( 132 ) and D 9  ( 130 ) are contained on the exterior shell of the transmitter  20  and are used to indicate certain conditions of the transmitter. In particular, LED D 8  lights to indicate that the boundary antenna is connected to the transmitter and operative. LED D 9  indicates whether the transmitter is on or off Both of the LEDs are activated by signals emanating from the microcontroller  300 .  
         [0054]     The voltage regulator  214  in a preferred embodiment bears product designation number LM7805CT and is manufactured by National Semiconductor Corp., www.national.com. Voltage regulator  214  regulates the power to power up the remaining devices in the circuit such as OP AMP  340 , the microcontroller  300 , the EEPROM  314 , and other items in the circuit.  
         [0055]     Looking now at the programmable voltage regulator  330 , this device in a preferred embodiment bears product designation number LM 317  and is made by Motorola, Inc., www.motorola.com-. The output voltage of the regulator designated as V out  can be solved by the formula: 
 
 V   out =1.25v((1+(Variable Resistance/Resistor  R 2)). 
 
 The variable resistance is determined in the voltage adjuster by which of the transistors Q 2  through Q 6  are conducting in order to selectively place one or more of resistors R 4 , R 5 , R 21 , R 22 , R 1  and R 28  in parallel. As can be seen from  FIG. 5 , the formula in its basic form, when all transistors are not conducting, is: 1.25v (1+(R 4 /Resistor R 1 )). This represents the highest voltage output of the voltage regulator  330 . 
 
         [0056]     By way of example, let&#39;s assume that the microcontroller  300  wishes to provide a certain voltage out from the programmable voltage regulator  330 . Further, assume that the desired voltage relies on the parallel arrangement of resistors R 4 , R 22 , and R 28  in voltage adjuster  312 . To accomplish this, the microcontroller puts out activation signals to cause resistors Q 4  and Q 5  to conduct; thus, placing the resistors R 4 , R 22  and R 28  in parallel arrangement and creating an output ratio in order to arrive at the calculation of V out . Through the use of the five transistors Q 2  through Q 6  and the permanent resistor R 4 , thirty-two different values are possible for V out . In this way, for an initial value, all of the transistors are conducting so that all of the resistors are arranged in parallel to produce the lowest V out . Through the output of the opt amp  340 , the ADC rating is read in the microcontroller and compared with a desired reading for a given field width in the manner described above concerning the use of set points.  
         [0057]     As explained before, transistor Q 1  performs two primary functions. First, it is used to complete a path for the boundary antenna and second, to transmit data from the transmitter to the collar worn receiver by making and breaking the loop voltage. This arrangement provides one way communication. This is accomplished by opening and closing the circuit at a specific frequency. In essence, a binary signal is sent to the receiver  10  by the on/off operation of transistor  322 . The output port PWM 2  of controller  300  turns transistor  322  on and off at a specific rate to create the digital signal passing through the loop antenna  320 .  
         [0058]     Transistor  322  can also be used in the second mode for calibration to adjust the current passing through the boundary antenna  320  to produce a magnetic field of a predetermined width. In this way, a desired field width can be created. The field width can be known precisely each time a calibration is accomplished. Whether or not the boundary antenna is connected or is in an open circuit condition can also be detected and in this way, control the lighting of the loop LED D 9 .  
         [0059]     Calibration is carried out under several circumstances. The first is when any button is pressed. The second is when the unit is powered up. The third is at regular preset intervals. In a preferred embodiment, the intervals are every five seconds.  
         [0060]     When entering the calibration phase the desired field width must be known. The user can manually select it by pressing any of the field width buttons ( 1 - 5 ) or by using the default setting when the transmitter is powered up. The microcontroller then goes through the following routine: 
    1. Set the programmable voltage source to the lowest output voltage (Q 2 - 6  ON). Turn the LOOP LED off.     2. Turn Q 1  ON. This completes the loop circuit that forms the boundary antenna  320 .     3. Take 16 ADC readings and average them. This significantly increases the accuracy of the reading by reducing induced noise picked up by the loop wire from nearby sources.     4. Compare the averaged ADC reading to the set-point:     a. If greater than or equal to the set-point the calibration is successful and complete. Go to Step 5.     b. Increase the programmable voltage source output voltage to the next higher voltage. 
        I. Go to Step 3 if all 32 levels have not been tried.     II. ERROR—All 32 levels have been tried and the set-point has not been reached. The loop is either broken or not connected. Turn the LOOP LED off.    
        5. Done! Turn Q 1  off.    
 
         [0070]     Loop detection is simplified by implementing it in firmware. The loop is either present or not present, based on the existence of loop current. On a periodic basis, perhaps once every five seconds, when data is not being transmitted, the loop can be powered on for a brief instant to determine if loop current is present or not. The loop power would then be turned off by setting Q 1  to the off state, and then normal operation can continue. A calibration phase shall begin if the loop present state changes from not present to present.  
         [0071]     It is to be understood that the present invention is not limited to the illustrated user interfaces or to the order of the user interfaces described herein. Various types and styles of user interfaces may be used in accordance with the present invention without limitation.  
         [0072]     Modifications and variations of the above-described embodiments of the present invention are possible, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described.