Abstract:
An electronic apartment station intercom unit, housed in a vandal proof enclosure, providing a means for a tenant to communicate with apartment building main entrance door panel and means to unlock said door. This unit has a power storage unit that enables this unit to be compatible with existing apartment building switching units. This power storage unit can be in the form of capacitors. In addition, this unit also contains a timer to shut the intercom unit off after a preset period of time.

Description:
BACKGROUND  
         [0001]    This invention relates to a vandal proof system for installation in public/low cost housing where existing intercom units are typically telephone handsets.  
           [0002]    In such installations, landlords find intercoms costly, lacking durability, and require routine maintenance. One of the more significant problems encountered with telephone apartment stations of the foregoing type has arisen from vandalism. With increasing frequency, the handsets have been ripped off, the cords cut, and the dials or buttons rendered inoperative such as by inserting various instruments or a strong adhesive between the moving parts of the apparatus. In some cases, the cover has been pried loose and the interior components damaged or removed. These problems have necessitated frequent monitoring and maintenance of the individual stations which landlords are not able to address.  
           [0003]    The following references relating to intercom systems are known in the art: U.S. Pat. Nos. 353,282 to Holm; 3,665,116 to Holstrom; 4,196,317 to Bartelink 4,764,953 to Chern; and 5,966,432 to Buckler.  
           [0004]    In the past, some apartment switching units were installed where apartment intercoms were furnished and maintained by the Bell telephone company. Thus, many elements of apartment station intercom units and apartment switching units functionally correlate to telephone sets and a telephone network. Since the break-up of Bell Telephone in 1983 into regional operating companies, (RBOCs), responsibility for intercom maintenance has gradually fallen to landlords. Now, landlords would prefer to replace these telephone units with stand alone intercoms requiring minimal or no maintenance, as well as being durable enough to withstand tenant abuses and vandalism.  
           [0005]    The interface between the system apartment station intercom unit and the apartment building central intercom switch is similar to a POTS system, wherein POTS is an acronym for a “plain old telephone” system. This POTS system is essentially a twisted pair with a 48 Volt open loop potential. In a phone system, when the receiver is lifted (off hook), the local telephone set is engaged or turned on, thus drawing power from the twisted pair. This off hook condition signals the telephone central office, which then, in turn supplies a dial tone. This process and system is called a loop start system, and has been the basis of telephony for nearly a century. Since the apartment station intercom unit interfaces with a similar system, it is required to perform many of the same functions as a telephone.  
           [0006]    The apartment station intercom unit has a ring signal detector and an electronic ringer. This ring signal detector alerts a tenant that someone in the entrance has dialed his or her apartment. To establish communication with the call, the tenant presses the Push-To-Talk(PTT) button on the Apartment Station Intercom Unit. This action does three things, first it signals the apartment building central switch that the system or an apartment station intercom unit is “off hook”. In addition, it also enables the transmit circuit and starts a timer.  
           [0007]    When the tenant releases the PTT switch, he or she can hear the caller. Once the tenant decides that the caller may enter the building, the door release button is pressed, signaling the apartment building central switch to release the door lock. This is a the Bell Standard Dual Tone Multi-Frequency(DTMF) signal.  
           [0008]    The timer that was initiated when the tenant first pressed the PTT button, and prevents the tenant from engaging the caller in a prolonged conversation, which is considered an abuse of the system.  
         SUMMARY  
         [0009]    Therefore, an object of the invention, is to provide a more suitable instrument for use in public and low cost housing.  
           [0010]    More specifically, it is an object of this invention to provide an Apartment Station Intercom Unit, which is highly resistant to vandalism, very reliable and low in cost.  
           [0011]    Another object of the invention is to provide a communications station of the character indicated in which the buttons or other moving portions of the apparatus can not be rendered inoperative through the use of various instruments, such as adhesive.  
           [0012]    It is further an object of the invention to prevent tenants and callers from engaging in lengthy conversation.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings which disclose at least one embodiment of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention.  
         [0014]    In the drawings, wherein similar reference characters denote similar elements throughout the several views:  
         [0015]    [0015]FIG. 1A is a front view of the system known as an apartment station intercom unit;  
         [0016]    [0016]FIG. 1B is a side view of the system shown in FIG. 1A;  
         [0017]    [0017]FIG. 2 is an electrical schematic circuit diagram of the system shown in FIG. 1A;  
         [0018]    [0018]FIG. 3 is a system block diagram of the system as it is coupled to other electronic devices in a building;  
         [0019]    [0019]FIG. 4 is a cross sectional view taken thru line I-I of FIG.  1 , showing tamper resistant screw fastener, which is used to secure the apartment station intercom unit to wall mounting bracket;  
         [0020]    [0020]FIG. 5 is a perspective view of a tamper resistant screw;  
         [0021]    [0021]FIG. 6 is a large side cross sectional view shown in FIG. 4;  
         [0022]    [0022]FIG. 7A is a front view of a wall mounted plate shown in FIG. 4;  
         [0023]    [0023]FIG. 7B is a top cross-sectional view of the wall mounted plate shown in FIG. 7A; and  
         [0024]    [0024]FIG. 8 is a front view of a back plate shown in FIG. 4. 
     
    
     DETAILED DESCRIPTION  
       [0025]    Referring in detail to the drawings, FIG. 1A shows an external view of system  100  according to the invention. System  100  is intended for use in apartment buildings with a signal entrance. Not shown are models for building with a plurality of entrances. These differences are defined by the number of door release buttons.  
         [0026]    System  100  has a push to talk button or PTT  7 , and a door release button  19  that are shown accessible to a user. There is also shown a tamper proof screw  105  which fits through hole  105 ′, which is used to secure face plate  103  to a back wall. FIG. 1B shows a side view of system  100  which also shows door release button  19  and tamper proof screw  105  secured into housing  103 .  
         [0027]    [0027]FIG. 2 is an electrical schematic diagram of system  100  shown in FIG. 1A. To alter system  100  to be coupled to almost any power supply including a telephone line, it contains a plurality of filter capacitors  12  and  28  that are of a sufficiently high value as to hold an adequately long charge during this period of use.  
         [0028]    When the system receives a ring signal from the apartment building intercom after button  7  is depressed in system  32  as shown in FIG. 3, a ring detector  8  will detect a level between 120-280 volts peak to peak at a frequency of between 15 to 30 Hertz, measured at conductors  22   a  and  22   b  of FIG. 2. Ring detector  8  then causes ringer or bell  9  to emit an audible tone. This tone is in the frequency range of 400 to 2,000 Hz with a minimum amplitude of 80 dB SPL  measured at a distance of  18  inches from the apartment station intercom unit  100 .  
         [0029]    While ringer  9  is enabled, a DC voltage, equal to the peak voltage on  22   a  and  22   b,  flows from bridge rectifier  2 , across line  53 . This causes a current to flow through resistors  25  and diode  26 , charging capacitor  28  to a voltage equal to the zener breakdown voltage of diode  27  at line  46 .  
         [0030]    Line  46  becomes the supply voltage to a hook switch latch and time-out circuit  5 . The values selected for resistor  25  and capacitor  28  allow sufficient energy transfer on the first half cycle of the ring signal to place hook switch latch and time-out circuit  5  in operational mode. During this power-up, a capacitor  44  stores the power on a reset line  45 , below its threshold voltage for a sufficient period to allow hook switch latch and time out circuit  5  to start in the proper state. In this state, lines  42  and  50  are held at logic “0”, while line  52  is held at a logic “1”.  
         [0031]    With line  50  held “low”, the electronic “hook switch”, consisting of resistors  47  and  49  and MOSFETs  3 , and  4 , is held in the “on hook” or open circuit state. In this state, the gate voltage of the N-channel MOSFET  4  is below its threshold voltage. Therefore, the drain current of MOSFET  4  is 0. This results in a zero current flow in resistors  47  and  49 . With no current in resistor  49 , the gate-drain voltage of the P-Channel MOSFET  3  is zero. Therefore, MOSFET  3  has zero drain to source current flow, and the remaining circuits are not activated.  
         [0032]    When the user elects to respond to a ringing apartment station intercom unit or system  100 , he presses push to talk (PTT) switch  7 . Upon doing so, line  52  is set to ground potential through the closed PTT switch  7 . This sets hook switch latch and timer-out circuit  5 . This action releases a clamping circuit internal to hook switch latch and time-out circuit  5 , and connected line  42 , allowing capacitor  41  to start charging by the current supplied through resistor  40 . Also, line  50  is set “high” forcing the MOSFET  4  to saturate. This causes current to flow through resistors  47  and  48  that in turn, turns on MOSFET  3 .  
         [0033]    Once MOSFET  3  is on, current will flow through inductor  11  charging capacitor  12  and forming a DC voltage supply to amplifiers  13  and  10 , DTMF generator  18  and all other circuits requiring DC power. Since line  54  contains both an audio signal and DC power, it is important that line  54  have a high AC impedance. Inductor  11  and filter capacitor  12  form the DC power source for the intercom. This combination is the classic L-C filter for power supplies. Line  54  has a high AC impedance node, while line  29  has a low AC impedance.  
         [0034]    While PTT switch  7  is depressed, and line  52  is grounded, the gate of MOSFET  6  is also grounded to turn the device off. This condition allows line  61  to go high, thus disabling speaker amplifier  13 . With line  61  high (or at a Vcc, the potential of line  29 ) NPN transistor  16  is saturated when current flows thru resistor  62 . Transistor  16  is a high gain device therefore, and the voltage drop through resistors  60  and  62  due to the base current is negligible in this application. When transistor  16  is saturated, the collector to emitter current flow supplies a bias current to emitter follower  17 , and electret microphone  15 . In the transducer of electret microphone  15 , a vibrating diaphragm constitutes one plate of a capacitor, while the charge stored on the capacitor varies in response to fluctuations in sound impinging upon the diaphragm. This varying stored charge appears as voltage variations (referenced to ground) at line  64 , in the microphone&#39;s output.  
         [0035]    The user, while holding down PTT switch  7  may talk to the caller at the apartment building lobby entrance. The audio from microphone  15  carried by line  64  to the base of emitter follower  17 . The signal is carried from emitter follower  17  to microphone amplifier  10  through capacitor  66  and resistor  67 . The output of amplifier  10  is coupled to line  54  through capacitor  56  and resistor  55 . With the proper component selection, the microphone circuitry delivers a −14 dBm to 0 dBm signal to lines  22   a  and  22   b  through MOSFET  3  and diode bridge  2 , for a 75 dB SPL  acoustic source when measured 18 inches from the front of the apartment station intercom unit  100 . The frequency response of the microphone circuit is ±3 dB in the frequency range of 300 Hz to 3300 Hz, a standard in the telephone industry.  
         [0036]    When the user releases PTT switch  7 , the voltage in line  52  rises to the potential on line  29  through resistor  51 . This condition forces MOSFET  6  to turn on, pulling the potential on  61  low. This condition enables speaker amplifier  13 , and turns off transistor  16 , thereby disabling microphone  15 . The user can now listen to the caller located in the apartment building lobby entrance.  
         [0037]    It should now be apparent that this circuit represents a half duplex communication link. This communication link is defined as one where the transmit and receive functions are mutually exclusive. It should also be apparent that a user of the Apartment station intercom unit  100  cannot initiate communications since there is no power present in the unit while idle.  
         [0038]    The user has a communication link until the hook switch latch and timer circuit has not timed out. Typically, the time out is set for one minute. This occurs when the voltage at point  42  reaches 66% of the value at line  29 . Once this happens, the voltage in line  50  decreases, disabling the electronic hook switch. This opens the circuit at lines  22   a  and  22   b,  and this circuit opening is sensed by apartment building intercom system  32  as a disconnect or on hook condition.  
         [0039]    When the circuit times out, the current in line  54  rapidly falls to zero, and the collapsing magnetic field in inductor  11  tries to force the potential negative on line  54 . To prevent damage to MOSFET  3 , diode  59  shunts the resulting negative flow to ground.  
         [0040]    If the user decides to allow entry to the caller, and the system has not timed out, he may press door release button  19  or  20 . This action enables the DTMF generator  18  to send a DTMF  9  or a DTMF  5  signal to the apartment building intercom system  32  of FIG. 3. These two numbers are the primary standard for signaling a door release in apartment complexes. However, any DTMF tone can be used. Once the apartment building intercom system  32  receives this signal, it will release the electric door lock  35 .  
         [0041]    An over voltage device  1  is placed across conductors  22   a  and  22   b.  Over voltage device  1  may be of conventional construction, and comprises a pair of semi conductive components that “breakdown” under an over voltage condition. Usually the arrangement is such that one over voltage device  1  protects against over voltages of one polarity, while another over voltage device  1  is arranged to protect against over voltages of the opposite polarity. Another suitable device is a voltage dependent resistor which has a resistive value that decreases rapidly when an over voltage condition exists.  
         [0042]    A diode bridge or ring bridge  21 , comprising  4  diodes is inserted between the apartment station intercom unit interface, conductors  22   a  and  22   b,  ring detector  8  and ringer  9 . Bridge  21  insures that the proper polarity of the DC voltage to the ring detector is maintained regardless of the polarity of  22   a  and  22   b.  Thus, apartment station intercom unit  100  is protected from reverse polarity even if the wiring  34 , which includes lines  22   a  and  22   b  from  33  to  30  as shown in FIG. 3 have been inadvertently reversed upon installation.  
         [0043]    [0043]FIG. 4 shows a cross sectional view of the device shown in FIG. 1A taken through the line I-I. FIG. 4 essentially shows how device  100  including wall plate  103  that can be secured to a wall such that this device becomes tamper proof. This embodiment is equipped with tamper proof screws  105  and  106  (See FIG. 5). FIG. 5 shows a pin and hex screw, which has a head  122 , a hex shape  124  and a pin  126  extending inside of the hex shape. This type screw is disclosed in U.S. Pat. No. 5,497,533 to Karlsen issued on Mar. 12, 1996 incorporated herein by reference. Thus, no ordinary wrench can be used to turn these screws. Instead a particular hex wrench that has a central aperture to accommodate the pin is required. While it is not impossible to remove tamper resistant screws, it is an arduous task to do so without a special tool. This special hex wrench, is one having a central aperture, and must be used to provide clearance from a blocking pin that aligned with the axis of screw  105  and within the hex aperture and wherein this hex wrench extends the length of the aperture. Screw  106  is located concentric with screw  105  and is of the same type. Thus, to turn screw  106  one must first remove screw  105 . If someone is successful in removing screw  105 , it is a much more difficult task to remove screw  106 . Screw  106 , when tightened, locks into bushing  107  and drives into wall mounted plate or bracket  103  thereby locking the apartment station intercom unit housing including cover or front plate  101  to the wall mounting plate  103 . In addition screw  105  screws into bushing  104  to clamp cover plate  101  to an intermediate plate  108 . Also shown in this drawing are the speaker  14  and microphone  15 .  
         [0044]    [0044]FIG. 6 shows a full side cross sectional view as shown in FIG. 4. In this view, there is wall-mounted bracket  103  that is coupled to a wall, back plate  102  which couples to wall mounted bracket  103 , intermediate plate  108  which can be coupled to a front plate  101 . There is also shown spacers  109  and  110  for spacing a circuit board  115  away from back plate  102 . Additional spacers  113  and  114  are used to space circuit board  115  away from intermediate plate  108 .  
         [0045]    [0045]FIG. 7A shows a front view of wall-mounted bracket  103  while FIG. 7B shows a top cross-sectional view of this bracket. Wall mounted bracket  103  has side-walls  103 ′ which are covered by cover plate  101  when cover plate  101  is clamped down on wall mounted bracket  103 . As shown in FIG. 6, side walls of cover plate  101  cover side walls  103 ′ to form a tight cover. This tight cover makes the unit more vandal proof because a vandal would most likely not be able to fit a screwdriver or other prying device between the intersection of side walls  103 ′ and the side walls on cover plate  101 . In addition, FIG. 7A shows heads  111  and  112 , which can be screw heads or formed integral with wall mounted bracket  103  and can be used to clip an additional plate to wall-mounted bracket  103 .  
         [0046]    [0046]FIG. 8 shows a front view of the back plate  102  which contains a securing hole  114  which is used to secure over heads  111  or  112 . There is also another securing hole (not shown) disposd opposite securing hole  114  which allows back plate  102  to secure onto wall mounted bracket  103 .  
         [0047]    To install system  100 , it first comes pre assembled with cover plate  101  coupled to back plate  102  with all the necessary electronic components disposed inside. Back plate  103  is next secured on a wall, separate from the system. Back plate  103  is secured by any known means such as via screws. Next, hole  114  and an opposite spaced hole (not shown) is secured over heads  111  and  112  on back plate  103 . Next, screw  106  is fed through hole  105 ′ (See FIG. 1A) in cover plate  101  and tightened to secure back plate  102  to wall mounted plate  103  and thus to a wall. Next, screw  105  is fed through the same hole  105 ′ to further secure cover plate  101  to intermediate plate  108  so that cover plate  101  has protection from being removed from system  100 . Thus, to remove system  100  from a wall, both screws  105  and  106  would have to be removed.  
         [0048]    While the embodiment illustrated in the drawings and described above is for and intercom system designed to unlock a single door, it should now be apparent to those skilled in the art that the subject invention can be designed to unlock a plurality of doors. It is preferred, therefore, that the present invention extend to cover those modifications.  
         [0049]    Though there are many similarities with telephony, there are significant differences. Since being dropped from the telephone network, many vendors have introduced their own interpretations of the Bell Standard. One method is where no voltage is applied to TIP and Ring during non use, contrary to the 48 VDC that is always present on phone lines. Another is the lack of zero voltage crossing circuitry in their ring signal generators. This gives the system a ring signal that can start and stop at any point in the ring cycle. The result is extremely high voltage slew rates or transient voltages (dv/dt) that are harmful to electronic circuits not properly protected. The last significant difference is that the 48 VDC component appears 150 milliseconds after the ring signal has ceased. This is problematic when initializing intercom circuitry, since this appears to such circuitry as a disconnect. Thus, capacitors  12  and  28  are introduced into the system to control the flow of power, and to store power in the system.  
         [0050]    Accordingly, while at least one embodiment of the present invention has been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention as defined in the appended claims.