Abstract:
A communications station for use in a distributed audio system having two Hall-effect integrated circuits mounted on a printed circuit hook switch board, the hook switch board mounted within a handset cradle of the communications station. The Hall-effect integrated circuits place the communications station on-hook or off-hook, depending on the amount of ambient magnetic flux. The ambient magnetic flux varies with the presence or absence of a handset in the cradle. Thus, if a handset containing a magnet is in the cradle, and the applied magnetic flux is greater than the predetermined flux necessary to affect the Hall-effect integrated circuit, the communications station will go on-hook. When the handset and the resultant flux is removed from the cradle, the Hall-effect integrated circuit will change state and the communications station will go off-hook. Two unipolar Hall-effect integrated circuits are opposedly attached to the switch hook board in order for one of the circuits to change state when a magnetic flux of sufficient level is applied.

Description:
FIELD OF THE INVENTION 
     The invention relates to a communications station having a Hall effect IC for placing the station on-hook or off-hook when the station handset is in the handset cradle or absent from the cradle. The invention also relates to a communications station for use in a distributed audio system having a Hall-effect integrated circuit (IC) mounted within a handset cradle of the communications station for placing the communications station on-hook or off-hook. The communications station can be used as a dedicated field party line telephone, page party or intercom system, and can employ carrier techniques for transmitting and receiving audio signals. 
     BACKGROUND OF THE INVENTION 
     A need exists for communication between personnel in large industrial and commercial facilities such as warehouses, power generating plants, refineries, manufacturing facilities, rail yards and sea ports, among other facilities. The use of radio communications is common; however, radio communications is not always effective in environments where the range of radio signals is limited by underground passageways, for example. Additionally, the ability to deliver emergency voice communications to a variety of personnel throughout a facility is difficult absent an accessible public address system. 
     One common system employs a page line and an audio line for station-to-station communications. In this manner, communications stations can be distributed throughout a facility and connected using page lines and audio party lines. Typically, only one conversation per party line can occur at a time, thereby limiting the number of simultaneous users to those who are parties to the conversation. 
     A user must first go off-hook to access the page line, page the desired party, tell the other party which alternate line to switch to, and release the page switch. Both parties must then switch to the same party line and go off-hook in order to converse. When the parties have completed their conversation, they must hang up the handset. Examples of known distributed audio systems are disclosed in U.S. Pat. No. 4,847,888 to Cox et al., U.S. Pat. No. 5,014,347 to Cox et al., U.S. Pat. No. 4,206,318 to Steely, U.S. Pat. No. 5,179,588 to Nowicki, U.S. Pat. No. 5,559,625 to Smith et al., and U.S. Pat. No. 5,033,080 to Deane. 
     For existing communications stations, on-hook/off-hook switching is accomplished using mechanical switches. For example, when the handset of a communications station is placed off-hook, a mechanical switch is activated to connect the telephone to the line. Such mechanical switches can be unreliable, large in size and expensive. Additionally, mechanical switches are generally exposed to environmental contaminants, for example, particulates or gases, which can corrode the mechanical switching mechanism and contacts. Other examples of harsh environmental conditions include exposure to moisture, corrosive vapors such as industrial chemicals or salt air, temperature extremes, particulates, radiation, electrical fields, stray magnetic fields, and the like. When exposed to these types of environments, the mechanical switches may corrode, become encrusted or otherwise become inoperable. 
     Alternatively, non-contact, magnetically-operated, reed switches have been used. The non-contact means are generally costly, more susceptible to stray electromagnetic fields, and less robust than mechanical switches. Reed switches, while generally sealed in an inert gas ampoule, may become inoperable when, for example, a fracture forms when exposed to temperature extremes, which may result in corrosion of the contacts. In another example, a stray magnetic field may cause the switch to become biased and therefore not respond properly. 
     Thus, there is a need for improved communications stations having reliable on-hook and off-hook switches that are small in size, low in cost and able to withstand harsh environmental conditions, and therefore are not susceptible to the corrosion of the contacts or weak stray magnetic fields. 
     SUMMARY OF THE INVENTION 
     Accordingly, an aspect of the present invention is a communications station having reliable on-hook and off-hook switches that can withstand harsh environments. A communications station such as a telephone has a handset support cradle which need not be physically a part of or connected to the body of the telephone. The handset support cradle can be disposed alongside the telephone body, or in the general proximity of the telephone body. This facilitates rapid replacement of components if necessary. A hook-switch board is preferably a part of the handset support cradle, and is electrically connected to the circuitry of the telephone. 
     In accordance with another aspect of the present invention, a communications station comprises a Hall-effect integrated circuit for on-hook and off-hook switching. A printed circuit board equipped with the Hall-effect device, HED, is mounted inside the communications station, or more specifically the handset cradle, in order to protect it from environmental effects. Additionally, the HED, and optionally the associated printed circuit board, can be coated or otherwise encapsulated in a protective material, for example, a solithane or a sealed plastic sheathing, for protection from harsh environmental conditions. Two HEDs are mounted on the printed circuit board for redundancy. Alternatively, two HEDs can be mounted on the printed circuit board so as to be inverted relative to each other. Each HED is directional. By utilizing inverted placement, a communications handset containing a magnet of unknown polarity can still reliably achieve on-hook/off-hook switching when placed near the HEDs. 
     In accordance with yet another aspect of the present invention, a field telephone is provided for use in a distributed audio system. The field telephone comprises a handset connected to a main telephone body. The handset generates a magnetic field caused by leakage flux from a magnet. A handset support cradle is attached to the telephone body and comprises a hook-switch board having a Hall-effect integrated circuit which reacts with the magnetic field when the handset is placed in the handset support cradle. The magnetic field affects the Hall-effect integrated circuit to place the field telephone in an on-hook state. Similarly, when the handset is removed from the handset support cradle, the magnetic field is removed from the Hall-effect integrated circuit, and the field telephone enters an off-hook state. The magnet can be a component of a receiver contained within the handset, or a separate component attached to the handset. 
     The distributed audio system includes a plurality of telephone lines, including a page line pair and a party line pair. Means are included in the telephone for selecting a telephone line pair and transmitting audio signals over the selected line pair to a receiving station. Each handset acts as a means for transmitting user audio signals to another connected telephone, and for receiving audio signals transmitted from another telephone. The Hall-effect integrated circuit performs the on-hook/off-hook switching of any selected and connected telephone line. 
     Other advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawings which form a part of the original disclosure: 
     FIG. 1 depicts a communications station constructed in accordance with an embodiment of the present invention; 
     FIG. 2 is a side view of the communications station in FIG. 1; 
     FIG. 3 is a schematic diagram of a field telephone assembly in accordance with an embodiment of the present invention; 
     FIG. 4 is a schematic diagram of the field telephone assembly in FIG. 3; and 
     FIG. 5 depicts the schematic diagram of an HED printed circuit board for use in a communications station and constructed in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 illustrate a communications station  10  in accordance with an embodiment of the present invention. For illustrative purposes, the communications station  10  is a field telephone assembly for use in a distributed audio system. It is to be understood, however, that the aspect of the present invention relating to a Hall-effect integrated circuit for on-hook/off-hook switching can be implemented in other types of telephones or base stations. Examples of distributed audio systems without Hall-effect integrated circuits are described in U.S. Pat. Nos. 4,847,888 and 5,014,347, incorporated herein by reference. Distributed audio systems, which are also known as intraplant voice communications systems, and unitized or baseband audio systems, are characterized as having no operational relationship or connection to a central office or subscriber loop. 
     The field telephone assembly  10  includes a telephone body or housing  12  and a handset  14  which is connected to the body  12  by way of a handset connector cord  16 . The handset  14  can be any conventional telephone handset having a microphone/transmitter  18  and a speaker/receiver  20 , and designed so that a user can place the speaker/receiver  20  in the vicinity of the user&#39;s ear and the microphone/transmitter  18  in the vicinity of the user&#39;s mouth. Additionally, the handset can be replaced by or supplemented with a teletypewriter, an interactive video display terminal, a personal communications device, a laptop computer, a modem containing device, or other compatible communications receiving and transmitting device. The connector cord  16  is preferably any known multiwire insulated cable. 
     The handset  14  has a magnet  22  that generates a magnetic field about the handset. The magnet  22  can be integral with the speaker/receiver  20  or attached separately to the handset  14 . 
     The field telephone assembly  10  is connected to a plurality of telephone lines through cable  24  and connector  26 . The connector  26  is capable of interfacing with any type of telephone line, which could be copper pairs, fiber, coaxial or any other known type of telephone transmission line for baseband communications. 
     In accordance with an embodiment of the present invention, a handset support cradle  28  is incorporated into the body or housing  12  of the field telephone assembly  10 . When not in use, the handset  14  is held in place in the handset support cradle  28 . The handset support cradle  28  is preferably fabricated from molded plastic, and is shaped to retain at least a portion of the handset  14 . In the preferred embodiment, the handset support cradle  28  is integral with the front panel  30  of the telephone body  12 . 
     The field telephone assembly  10  also has a volume control knob  32  preferably located on the front panel  30 . The volume control knob  32  allows a user to control the volume being output by a speaker  34  located in the field telephone. A party line knob  36  is also preferably located on the front panel  30 . The party line knob  36  is connected through the front panel  30  to a selector switch  38 . The party line knob  36  and selector switch  38  allow a user to select one of several telephone lines connected to the field telephone assembly  10  through the connector  26 . The speaker  34 , the party line knob  36 , the selector switch  38 , and other components can also be mounted to a mounting chassis  40  located behind the front panel  30 . The mounting chassis  40  can be attached to the front panel  30  or the telephone body or housing  12 . 
     A light emitting diode (LED)  42  can also be provided on the front panel  30  of the telephone body  12  to indicate when the field telephone assembly  10  is on-hook or off-hook. For example, when the field telephone assembly  10  is off-hook and connected to a telephone line, the LED  42  is illuminated. When the field telephone assembly  10  is on-hook or otherwise not connected, the LED  42  is not powered. 
     In accordance with the present invention, a Hall-effect printed circuit hook switch board  44  having at least one Hall-effect device (HED)  46  is provided behind the front panel  30  of the telephone body  12  and in the vicinity of the handset support cradle  28  is. The HED  46  places the telephone assembly  10  on-hook or off-hook, respectively, when the magnetic field generated about the handset  14  by the magnet  22  is present or absent with respect to the handset support cradle  28 . In the preferred embodiment, two HEDs  46 , 48  are provided on the printed circuit hook switch board  44 . Each HED  46 , 48  has a predetermined polarity, and consequently are inverted with respect to each other so that one of the HEDs  46 , 48  will activate when the handset  14 , and therefore the magnet  22 , is present in the handset support cradle  28 . Inversion of the two HEDs  46 , 48  allows for situations where the polarity of the magnet  22  is not known. The HEDs  46 , 48  are preferably attached to the printed circuit hook switch board  44  so that they are optimally positioned, that is, the HEDs  46 , 48  are positioned relative to the expected location of the magnetic flux generated by the speaker/receiver  20  such that one of the HEDs  46 , 48  is affected by the flux from the magnet  22 . 
     The HEDs  46  and  48  are preferably a Hall-effect IC model HAL506UA-E manufactured by Micronas of Frieburg, Germany. The preferred operating junction temperature range is −40° C. to +60° C.; however, this range can vary with the type of Hall-effect IC selected for the HEDs  46  and  48 . Other common Hall-effect ICs which can be used have expanded temperature ranges from −40° C. to +170° C., and narrower ranges from 0° C. to +50° C. The preferred temperature range is selected to accommodate the environment where the HEDs  46 , 48  are to be deployed. 
     The Hall-effect IC is preferably unipolar, with the magnetic south pole on the branded side, or top, of the package. The output of the Hall-effect IC preferably is low when an applied magnetic flux is greater than 77 gauss. In accordance with the present invention, one Hall-effect IC is mounted facing upward on the printed circuit hook switch board  44 , while a second Hall-effect IC is mounted facing downward on the printed circuit hook switch board  44 , allowing for a magnet  22  with unknown polarity to generate a magnetic flux capable of affecting one of the HEDs  46 , 48 . Similarly, the output of the Hall-effect IC turns high when the magnetic field is removed, or the magnetic field is reduced to below 21 gauss. The output high/low gaussian levels of the Hall-effect IC can be selected for the environment where the field telephone assembly  10  is to be employed. Thus, that is, the sensitivity levels described above (i.e., 21 to 77 gauss) characterize the most sensitive Hall-effect device that is presently commercially available. The desired high and low levels of a selected range for magnetic flux density can be other selected levels and are a function of the magnet  22  in the handset  14 , or of the ambient magnetic flux in the environment where the field telephone assembly  10  is being used. For example, it may be desirable to have sensitivity levels that differ by plus or minus 50 gauss, so long as there is a clear threshold between output high and output low levels. Other makes and models of Hall-effect ICs may be used in the present invention. Thus, the selected HEDs  46 , 48  preferably have magnetic flux density sensitivities maximized to the environment where they are used, minimizing the effects of stray or otherwise ambient magnetic flux. 
     FIG. 3 is a schematic diagram of the field telephone assembly  10  shown in FIGS. 1 and 2. Wire connections between the handset  14 , the speaker  34 , the selector switch  38  and the printed circuit hook switch board  44 , as they are mounted on the mounting chassis  40 , are shown. The pre-amp board  52  can be mounted either in the housing  12  or in a wall box  27 . When the pre-amp board  52  is mounted in wall box  27 , connector  26  interconnects the housing  12  with the wall box  27 . The default telephone line connection is the page line pair  100  when on-hook so that pages can be received. When the page switch  50  located on the handset  14  (FIG. 1) is depressed, and the field telephone  10  is placed off-hook, the speaker/receiver  20  is powered and a user can send an audio page to other field telephone assemblies  10  connected to the page circuit  100 . When a user rotates the selector switch  38  to a first party line  102  while the field telephone  10  is off-hook, the speaker  34  is disabled and the user can communicate with another field telephone assembly  10  that is also off-hook with the first party line  102  selected. The LED  42  is also powered when the field telephone  10  is off-hook, indicating that a page line  100 , or a party line  102 , is connected and ready for audio transmission. The HEDs  46  and  48  are advantageous for use in the field telephone assemblies of a distributed audio system because they can facilitate on-hook/off-hook switching at the field telephone stations. This represents an advantage for intraplant voice communications systems which do not employ central office monitoring of off-hook conditions as in a conventional public switched telephone network. 
     FIG. 4 is a schematic diagram of components of the field telephone assembly  10 . When the printed circuit hook switch board  44  sends an off-hook signal to TC 7  on the pre-amp board  52 , transistor Q 7  is biased to a conductive state to energize a coil K 3 C of a relay K 3 . The relay contacts K 3 A and K 3 B are thereby switched to connect the page line  100  or a selected party line  102  to a power supply and pre-amplifier circuits  74 . Page relay K 1 C responds to the page switch  50 , connecting the handset  14  to a page line  100  while off-hook. 
     FIG. 5 is a circuit diagram for a communications station having a Hall-effect device for on-hook/off-hook switching in accordance with an embodiment of the present invention. Two HEDs  46 ,  48  are connected to a first terminal strip  54 . Each HED  46 ,  48  has a ground terminal  56 ,  58 , a collector voltage supply terminal  60 ,  62  and an output terminal  64 ,  66 . Each voltage supply terminal  60 ,  62  is connected to a 12-volt power supply line  68 . Each output terminal  64 ,  66  is connected to a transistor Q 1   70  for controlling the energization of the coil in a control relay  72 . Ground terminals  56 ,  58  are connected to a common ground. 
     When a magnetic flux of proper polarity is placed near an BED  46 ,  48 , a state change occurs, and an output voltage is generated by one of the HEDs  46 ,  48  and is applied to the coil of the control relay  72 . Since the polarity of the magnet  22  or the polarity of a magnet contained in the speaker/receiver  20  may not be known, the two HEDs  46 , 48 , are placed back-to-back and with opposing polarities on the printed circuit hook switch board  44 . In this manner, one of the HEDs  46 , 48  changes state when the handset  14  is placed in the handset support cradle  28 , thereby presenting a magnetic flux. One of the HEDs  46 ,  48  operates with the transistor  70  and the relay  72  to place the communications station on-hook. Thus, when the HED in ON, the communications station is on-hook, transistor  70  is off and the relay  72  is deenergized. Similarly, when the handset  14  is removed from the handset support cradle  28 , thereby removing the magnetic flux from the vicinity of the HEDs  46 ,  48 , the HED  46 ,  48  that was affected by the magnetic flux changes state, again placing the communications station off-hook. 
     If the communications station described in connection with FIG. 5 is implemented as a field telephone assembly as described in connection with FIGS. 3 and 4, on-hook for a field telephone  10  is when a communication signal cannot be transmitted to a pre-selected telephone line pair  102 . Off-hook is when a communication signal can be transmitted because the field telephone  10  is actively connected to a telephone line pair  102 . When powered, the normally open contacts of the control relay  72  are closed, and the communications station  10  is off-hook and connected to a telephone line. When the control relay  72  does not receive an output voltage, that is, when an HED  46 ,  48  is not biasing the transistor Q 1   70  do to no magnetic flux from the handset  14 , the normally closed contacts of the control relay  72  are closed, resulting in the communications station being on-hook and not connected to a party line  102 . 
     In accordance with the embodiment of the present invention described in connection with FIGS. 3 and 4, the field telephone assembly  10  operates by having a caller first page another location, and then having the caller and any person hearing the page on another field telephone assembly  10 , switch to one of several other channels provided to each field telephone assembly  10  for conversing. A page is initiated by lifting handset  14  from the nominal storage position in the handset support cradle  28 . The lifting of the handset  14  removes the magnetic field from the HEDs  46 ,  48 , resulting in the field telephone assembly  10  going off-hook on party line  102 . The page switch  50  operates in a push-to-talk (PTT) mode similar to two-way radio communications. A user removes the handset  14  from the handset support cradle  28 , presses the page switch  50 , and talks into the handset microphone  18 . The microphone  18  transmits the audio to all other field telephone assemblies  10  connected to the page line  100 , where the audio is received at their respective speakers  34 . The caller sending the page announces which party line  102  another user should access. Users wishing to reply to the caller at the receiving field telephone assemblies  10  must remove their respective handsets  14  from their handset support cradles  28 , adjust switch  36  to the designated party line  102 , and talk. The page function and the voice communications can be implemented using conventional transmission techniques. 
     The HEDs  46 , 48  can withstand harsher environments than mechanical switches, such as increased temperatures, increased temperature ranges, corrosive environments, and the like. Following integration of the HED ICs  46 ,  48  on the printed circuit hook switch board  44 , the entire printed circuit hook switch board  44 , with all components, can be sealed from environmental contaminants such as particulates, water, chemical vapors, and the like. The sealing means can be by encapsulation using, for example, solithane or other sealants to form a hermetic or nearly hermetic seal. Other forms of sealing or encapsulating a printed circuit board known in the art can be used, including, but not limited to, spraying on a protective coating, electrostatically applying a protective coating, laminating, and placing the board in a container having O-ring seals and interface connectors. 
     While a number of advantageous embodiments have been chosen to illustrate the present invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.