Note: Descriptions are shown in the official language in which they were submitted.
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TEhEPHONE CONTROLLER APPAR~UB
BACKGROUND O~ ~HE INVENTIO~
Field of the Invention
This invention relates to telephone controller
apparatus and, more particularly, to such apparatus which
enables or disables a selected telephone.
Description of the Prior Art
In the prior art, telephone signals are transmitted
through telephone lines to a desired telephone which may be
located in a home or office. More specifically, as shown in
Fig. 1, a telephone signal is transmitted from a sending
telephone (not shown) through a telephone line 8, a local
telephone line 20 (contained within the home or office), a
telephone jack 18 and a cable 12 to a designated telephone 10
In a similar manner, a signal may be transmitted from
telephone 10 through the same transmission path to another
designated telephone.
Situations may arise when a person having plural
extension telephones all coupled to the same local telephone
line does not wish telephone signals to be transmitted to or
from one or more of his or her telephones. For example, it
may be desirable not to have telephone signals transmitted to
or from an extension telephone located in a child's room after
a certain time. It may also be desirable not to have
telephone signals transmitted to an extension telephone
located in the same room as someone who is ill, so as not to
disturb the person. Further, it may be desirable to disengage
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extension telephones in an office after business hours so as
to prevent the unauthorized use thereof.
In the prior art, signal transmi~sion to or from a
particular kelephone is typically prevented by disconnecting
the telephone from the telephone jack connected thereto, which
may be accomplished by disconnecting connectors 14 or lS,
shown in Fig. 1, ~rom mating connectors 16 or 1~,
respectively. In a similar manner, the telephone is made
operative by re-connecting the disconnected connector. As is
to be appreciated, a telephone jack may have limited
accessibility, thereby making the connections and
disconnections thereto difficult. As a result, controlling
the signal transmission to one or more telephones is
inconvenient and time consuming. Further, numerous
connections and disconnections from the telephone jack may
damage connector 14 and/or mating connector 16 which, in turn,
deteriorates the quality of the transmitted signal. Thus, the
prior art has failed to provide a means for controlling signal
transmission to or from one or more telephones which is
convenient to use and of relatively low cost.
OBJEC~8 AND ~UMMARY OF T~ INVENTION
An object of the present invention is to provide
telephone controller apparatus which overcomes the foregoing
problems associated with the prior art.
More specifically, it is an object of the present
invention to provide telephone controller apparatus ~or
enabling or disabling a selected telephone by utilizing a
control signal having a frequency which corresponds to the
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desired state of the selectad telephone and which is supplied
through the telephone lines.
It is ~et another object of the present invention to
provide telephone controller apparatus as aforementioned which
prevents a telephone from being disabled while the telephone
is being operated.
An additional object of the present invention is to
provide telephone controller apparatus as aforementioned which
operates correctly even if the apparatus is ~ired in reverse,
thereby permitting the installation thereof by individuals
having no technical skill or ability.
Other objects, features and advantages according to
the present invention will become apparent from the following
detailed description of the illustrated embodiment when read
in conjunction with the accompanying drawings in which
corresponding components are identified by the same reference
numerals.
In accordance with an aspect of this invention,
telephone controll.er apparatus is provided for selectively
controlling the operating state of a telephone, the apparatus
comprising a switch device for selecting a desired state of
one or more selected telephones connected to a local telephone
line, a transmitter device for transmitting a control signal
throuyh the local telephone line having a frequency within a
predetermined frequency range which corresponds to the desired
state of the selected telephone in response to an output
signal from the switch device and a receiver device connected
to the local telephone line and coupled to the selected
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telephone for receiving the transmitted control signal from
the transmitter device and in response thereto for connecting
or disconnecting the selected telephone to or from the
telephone line, respectively, thereby controlllng khe state o~
the selected telephone.
BRIEF DEE~CRIPTION OF THE DRAWI~
Fig. 1 illustrates a telephone system according to
the prior art;
Fig. 2 illustrates a telephone system utilizing a
telephone controller apparatus according to an embodiment of
the present invention;
Figs. 3A and 3B are diagrams illustrating a
transmitter and a receiver~ respectively, according to an
embodiment of the present invention;
Eig. 4 is a schematic diagram of the transmitter in
Fig. 3A;
Fig. 5 is a schematic diagram of a transmitter in
accordance with a second embodiment of the present invention;
and
Fig. 6 is a schematic diagram of the receiver in
Fig. 3B.
DETAILED DE~CRIPTION OF ~HE PREFE~RED EMBODIMENT8
Fig. 2 illustrates a telephone system utilizing telephone
controller apparatus according to an embodiment of the present
invention. As shown in Fig. 2, the telephone system generally
comprises telephones 30 and 50, telephone jacks 38 and 58,
receivers 42 and 62, and a transmitter 66. Telephone signals
are transmitted through a telephone line 68 to a junction 71,
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whereupon the transmitted signal is supplied in parallel
through local telephone lines 72 to telephone jacks 38 and 58.
As used herein, a local telephone line is a communication link
within a home or office (or other establishment) to which one
or more extension telephone instruments may be connected for
communication over telephone line 68. Transmitter 66, which
receives power from a power supply 70, includes one or more
switches which enable a user to select which telephone or
telephones are to be connected to or disconnected from the
local telephone lines. As an example, the switch settings
illustrated in Fig. 2 indicate that telephone A (telephone 30)
is connected to the telephone lines whereas, telephone B
(telephone 50) is not. In response to each switch activation,
transmitter 66 is adapted to transmit a control signal having
a predetermined frequency unique to that switch. The control
signal from transmitter 66 is supplied through local telephone
lines 72 and telephone jacks 38 and 58 to receivers 42 and 62,
respectively Each of receivers 42 and 62 includes a
plurality of resonant circuits, which are each tuned to a
separate predetermined frequency. Receivers 42 and 62 are
adapted to receive the control signal from transmitter 66 and
when the frequency of the control signal matches that of a
resonant circuit therein, a signal from the respective
resonant circuit causes the selected telephone to be
appropriately connected or disconnected from the telephone
lines. In a preferred embodiment, each of receivers 42 and 62
includes two uniquely tuned resonant circuits, that is, one
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for connecting and one for disconnecting a selected telephone
to or from the local telephone lines~
Telephones 30 and 50 are coupled to receivers 42 and
62, respectively, and, depending upon tha switch selections of
transmitter 66 as previously described, are adapted to receive
telephone signals from telephone line 68 or to transmit
telephone signals thereto.
It is to be appreciated that while two telephones
are illustrated in Fiy. 2, the present invention is not so
limited and can be applied to a system employing any number o~
telephones. The transmitter and receiver for use with the
present invention will now be described in more detail with
reference to Figs. 3-5.
Transmitter 66, as illustrated in Fig. 3A, generally
comprises switches 80A, 80B, 80C and 80D (hsreinafter,
switches 80A-D); a voltage controlled oscillator 82; an
amplifier 84; a transmit indicator 85 and a voltage regulator
9o. Transmitter 66, as shown in Fig. 3A, is configured for
use with a telephone system employing two telephones
designated as "A" and "B", although as previously mentioned
the transmitter is not so limited. In one embodiment, in
response to a switch closure, switches 80A-D, which in a
preferred embodiment are momentary contact switches, are
adapted to supply an output signal therefrom. The output
signal from the closed switch is supplied to voltage control
oscillator 82, which is adapted to produce a signal having a
frequency within a predetermined frequency bandwidth and
unique to the selected switch setting. As is to be
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appreciated, four unique ~requencies ~for example, 305 kHz,
400 kHz, 475 kHz and 575 kHz) are used to control two
telephones, that is, one for enabling and one for disabling
each telephone. Thus, when switch 80A is selected, so as to
turn "on" telephone A, voltage controlled osc:illator 82
produces a signal with a frequenay of 305 kHz. In a similar
manner, closing switches 80B, 80C and 80V produce signals with
frequencies of 400, 475 and 575 kHz, respectivelyO A fine
tune adjuster 83 is coupled to oscillator 82 ~or adjusting the
signal generated by VCO 82 as may be needed.
The signal from oscillator 82 is supplied to
amplifier 84 which amplifies the received signal and outputs
the same. An enable switch 81, which is ganged to switches
80A-D as indicated by the dashed line, enables the output
signal from amplifier 84 to be supplied through terminals 94
to the local telephone line whenever one of switches 80A-D is
closed. On the other hand, whenever switches 80A-D are all
open, transmitter 66 is disposed in a standby mode and, enable
switch 81 is also open thereby disconnecting transmitter 66
from the telephone lines.
Power for driving oscillator 82 and amplifier 84 is
supplied from voltage regulator 90, which is adapted to
receive a 12 volt D.C. signal from a standard 12 volt wall
adapter (not shown) through an input terminal 88 and r~gulate
the same to 9 volts D.C. Voltage regulator 90 al50 supplies
power to transmit indicator 86 which, in turn, is coupled to
switches 80A-D and is adapted to provide an indication signal
to the user. More specifically, since switches 80A-D are
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preEerably momentary contact switches, it is preferred that
they be held for a sufficient time to insure that the
receiver, as hereinafter described, has ample time to respond
accordingly. Therefore, when a switch 80 is held closed for a
time which is sufficient for the receiver to xespond, transmit
indicator 86 provides an indication of such to the user. In a
preferred embodiment, transmit indicator 86 is an LED which is
visible to the user.
A schematic diagram of transmitter 66 is shown in
Fig. 4. As previously described, a 12 volt DoC~ signal is
supplied through input terminal 88 to voltage regulator 90
which produces a 9 volt D.C. output signal. The 9 volt D~Co
signal is coupled to switches 80A-D, voltage controlled
oscillator (VC0) 82, transmit indicator 86 and amplifier 84.
Fine tune adjuster 83 is coupled to VC0 82 and i6 adapted to
adjust the frequency of the signal generated thereby as
previously mentioned.
Switch 80A is coupled to the base of a transistor
94, in which the emitter thereof is connected to ground and
the collector is coupled through capacitor 100 to VC0 82.
Switches 80B and 80C are similarly coupled through respective
transistors 96 and 98, and capacitors 102 and 104 to VC0 82.
A~capacitor 101 is coupled to the input of VCO 82 and, thus,
to the capacitors connected to the collectors of transistors
94, 96 and 98. This capacitor is included in the fre~uency
determining circuit of the VCo. VCO 82, in turn, is coupled
through a resistor network 130 and a shaping network 122 to
amplifier 84, which includes transistors 126 and 128.
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Amplifier 84 is coupled to the local telephone lines v~a
switches 8lA-D.
Switch 80A is further coupled through a diode 106 to
a one-shot circuit 114, which includes a diode 115, a resistor
116, a capacitor 117 and Darlington configured transistors
118, 120. Switches 80 B-D are also coupled through respective
diodes 108, 110 and 112 to circuit 114. Upon receipt of a
signal from one of switches 80 A-D, circuit 114 is adapted to
generate a signal, after a predetermined time period, which
turns on transistors 118 and 120. More specifically,
capacitor 117 is adapted to store energy received from the
selected switch at a rate which is determined by resistor 116
and to thereafter supply the same to the base of transistor
118 so as to turn on transistors 118 and 120. Transistors 118
and 120 are coupled to transmit indicator 86.
The operation of the transmitter shown in Fig. 4
will now be described below.
Upon closing switch 80A, a signal from regulator 90
is supplied to the base of transistor 94 so as to render the
transistor conductive, thereby placing capacitors 100 and 101
in parallel with each other. The values of capacitors 100 and
101 are selected such that a signal having a predetermined
frequency of, for example, 305 kHz is produced by VCO 82. As
such, the capacitance of capacitors 100 and 101 may be 120 and
56 picofarads, respectively. VCO 82 generates a siynal having
a preferably triangular waveform, which is limited in
amplitude by resistor network 130 to a predetermined level,
which in a preferred embodiment does not exceed -15 dB
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volts/135 ohms which is a current FCC re~uirement. Due to
harmonics, the triangular output signal from network 130 is
reshaped to a sine wave by shaping network 122. The output
signal from shaping network 122 is ampli~ied by amplifier 8~
and supplied as a control signal to th~ loaal telephone lines
through switch 81A. That is, closing switch ~OA turns
transistor 94 "ON" so as to initiate signal processing as
previously described and connects the output signal from
amplifier 84 to the telephone lines. ~.
In a similar manner, upon closing switch 80B, a
signal from regulator 90 is supplied to the base of transistor
96, rendering it conductive and placing capacitors 101 and 102
in parallel, which are selected so that a signal having a
predetermined frequency of, for example, 400 kHz is produced
by VCO 82. Likewise, closing switch 80C activates transistor
98 and places capacitors 101 and 104 in parallel, thus
enabling VCO 82 to produce a signal havin~ a predetermined
frequency of, for example, 475 kHz. Closing switch 80D
produces a signal from VcO 82 having a predetermined frequency
of, for example, 575 kHz, even though no capacitor is placed
in parallel with capacitor 101. Further, switches 81B-D are
ganged with switches 80B-D such that closing switches 80B-D
connects the output signal from amplifier 84 to the local
telephone lines.
Closing switches 80A-D further supplies a signal
from regulator 90 through respective diodes 106, 108, 110 and
112 to one-shot network 114 which, as previously described,
supplies an output signal after a predetermined time delay
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which drives transis-tors 118 and 120 -to activate transmit
indicator 86. The predetermined time delay, that is, the time
delay which exists between the closure o~ the selected switch
and activation of transmit indicator 86, corresponds to the
switch closura time which is sufficient to ln~;ure that the
receiver had time to respond. Thus, upon activation of
transmit indicator 86, a user is made aware that the
respective switch was held closed for a time sufficient to
insure that the receiver had time to respond.
The individual circuit components without reference
designations depicted in Fig. 4 are connected as shown and
will not be discussed further, since the connections and
values are apparent to those skilled in the art and are not
necessary for an understanding of the present invention.
Fig. 5 shows an alternate embodiment of a
transmitter in accordance with the present invention which is
configured for use with a system using three telephones and
comprises similar circuitry and operates in a similar manner
to the transmitter shown in Fig. 4 except as described below.
It is to be appreciated that elements similar t~ those in Fig.
4 are identified by the same reference numerals.
As shown in Fig. 5, transmitter 66' generally
comprises a regulator 31; switches 80A, 80B, 80C, 80D, 80E and
80F; oscillators 340, 341, 342, 343, 344 and 345; a one shot
circuit 114; a transmit indicator 86; a resistor 330; a
shaping network 322 and an amplifier 8~. Regulator 91 is
coupled to switches 80 A-F and inverting buffer amplifiers 360
and 370, and is adapted to produce a regulated voltage (for
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example, 5 volts) Erom a voltage applied to an input terminal
88 (such as 12 volts). Switches 80 A-F, which are preferably
momentary contact switches, are respectively couplecl to
oscillators 340 to 345, which are each adapted to supply a
square wave signal having a unique predetermined frequency.
Oscillator 340, enclosed by the dashed line, generally
comprises inverting buffer amplifiers 350 and 351 and a
ceramic resonator 352. As shown, inverting buffer amplifiers
350 and 351 are connected in series to switch 80A~ Resonator
352, which is adapted to supply a signal having a fixed
predetermined frequency, is coupled to a resistor network 355
across amplifier 350. Resonator 352 is further connected
through capacitors 353 and 354 to ground. As is to be
appreciated, oscillators 341 to 345 are similarly connected
and, as such, will not be further described. Further, in a
preferred embodiment, the invPrting buffer amplifiers included
in oscillators 340, 341 and 342 are fabricated on a single
integrated circuit chip and, as such, share common power and
ground terminals, for example, as shown connected to amplifier
360. In a similar manner, oscillators 343, 344 and 345 also
share common power and ground terminals as shown connected to
amplifier 370~ These connection thus provide operating
potential for all of the oscillators.
Switches 80 A-F are connected to shaping network 322
which includes inductor 323 and, depending upon which switch
is closed, a respective one of capacitors 324-329 and is
adapted to receive the square wave signal from a respective
one of oscillators 340-345 and to re-shape the signal to that
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of a sine wave. More specifically, the shaping network
connected to switch 80A includes inductor 323 and capacitor
324, the shaping network connected to switch 80B includes
inductor 323 and capacitor 325, and so forth. The output of
shaping network 322 is coupled to resistor 330, which is
adapted to limit the amplitude o~ the suppliedl sine signal to
a predetermined value. For example, in a preferred
embodiment, the predetermined amplitude is equivalent to -15
db volts/135 ohms which, as previously mentioned, is the
maximum amplitude currently allowed by the FCC. The output
from resistor 330 is coupled through amplifier 84, which
includes transistors 126 and 128, to the local telephone
lines.
Switches 80 A-F are further coupled through diodes
106, 108, 110, 112, 314 and 316, respectively, to one shot
circuit 114, which includes a diode 115, a resistor 116 and a
capacitor 117. The output from circuit 114 is connected
through Darlington transistors 118 and 120 to transmit
indicator 86 which, in turn, is coupled to input terminal 88
to receive operating voltage therefrom. Terminal 88 is
further coupled to amplifier 84.
As previously mentioned, the transmitter shown in
Fig. 5 operates similarly to the transmitter in Fig. 4 except
as noted below:
A 12 volt D.C. signal is applied from a standard 12
V D.C. wall adapter (not shown) through input terminal 88 to
regulator 91, whexeupon the signal is regulated to 5 volts
D.C. and supplied to the inverting buffer amplifiers included
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in oscillators 340-345. This 5 volt operating potenti~l
activates oscillators 340-345, whereupon each oscillator
generates a square wave signal having a unique predekermined
frequency. For example, the fre~uencies of the signals
generated by oscillators 340-345 may be 375 kHz, 450 kHz, 525
kHz, 630 kHz, 705 kHz and 780 kHz, respectively.
Upon closing one of switches 80A-F, the square wave
signal from the selected oscillator is supplied through the
respective switch 80 A-F to shaping network 322, whereupon the
signal is converted to a sine wave signal having the same
frequency as the square wave signal. The sine wave signal
from network 322 is supplied to resistor 330, wherein tha
amplitude of the signal is limited to a predetermined value
which, as previously mentioned, is preferably equivalent to -
15dbv/135 ohms. The output signal from resistor 330 is
supplied through amplifier 84 to the local telephone lines,
whereupon the signal is transmitted to all receivers which are
connected to the local telephone lines. The square wave
signal ~rom the selected oscillator supplied through the
respective switch 80 A-F is further supplied through
respective diodes 106, 108, 110, 112, 314 and 316, one shot
network 114 and transistors 118 and 120 to transmit indicator
86 in a manner similar to that described with reference to the
transmitter of Fig. 4.
Fig. 3~ illustrates in block form a receiver in
accordance with an embodiment o~` the present invention. ~s
receivers 42 and 62 (Fig. 2) are similarly constructed, only a
description of receiver 42 will be provided. As shown,
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receiver 42 is generally comprised of tuned circuits 204 and
218, rectifier circuits 206, 216 and 220, trigger circuits 208
and 222, silicon control ractifiers (5CR) 210 and 224, and a
double pole double throw relay 211~ An input signal from the
local telephone lines is received by input terminals 201 and
202 and simultaneously supplied to tuned circuits 204 and ~18,
which are each adapted to detect a signal having a
predetermined and unique frequency. As the output signals
from circuits 204 and 218 are processed in a substantially
similar manner, only one processing path will be described.
An output signal from tuned circuit 204 is supplied
to rectifier circuit 206, which supplies a rectified signal to
trigger circuit 208. In rssponse to the rectified signal
supplied thereto, trigger 208 is adapted to supply a trigger
signal to SCR 210, which is coupled to a set coil 212
contained within double pole double throw (DPDT~ relay 211.
The input signal from terminals 201 and 202 is
further coupled to rectifier circuit 216 which supplies a
rectified output to a capacitor 214. Capacitor 214 has one
terminal connected to ground and is adapted to store energy
received from circuit 216 and supply the same to relay 211.
As previously mentioned, rectifier circuit 220,
trigger circuit 222, SCR 224 and reset coil 226 are similarly
connected and, as such, will not be further described.
The operation of receiver 42 described above with
reference to Fig. 3B is as follows
An input control signal transmitted by transmitter
66 (Fig. 3A), through the local telephone lines is received at
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input terminals 201 and 202. The received controL signal is
simultaneously supplied to tuned circuits 204 and 218, each of
which is tuned to a respective predatermined and un.ique
frequency which matches one of the ~requencies produced by VCO
82 (Fig. 3A). For example, tuned circuits 204 and 218 may be
tunad to frequencies of 305 XHz and 400 kHz, and are thereby
adapted to turn telephone A on and off, respectively. Thus,
if the input control signal has a frequency of, for example,
305 kHz, the signal is detected by tuned circuit 204
whereupon, the signal is passed therefrom, rectified and
supplied to trigger circuit 208. Upon receipt of the
rectified signal, tri~ger circuit 208 is activated to supply a
trigger signal to SCR 210. As a result, SCR 210 is rendered
conductive to activate set coil 212, thereby connectiny
telephone A to the telephone lines.
On the other hand, if the input control signal has a
frequency of 400 kHz, the signal is detected by tuned circuit
218 and passed to rectifier circuit 220 which activates
trigger circuit 222 to turn on SCR 22~, in a manner similar to
that previously described, so as to activate reset coil 226,
thereby disconnecting telephone A from the local telephone
line
As is to be appreciated, if the fr~quency of the
input control signal is neither 305 kHz nor 400 kHz, the
control signal is not detected by either tuned circuit 204 or
tuned circuit 218, whereupon telephone A remains in its
current state (i.e. either connected to or disconnected from
the local telephone lines).
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As hereinafter d~scribed, rectifier circuit 216 is
adapted to ensure that receiver 42 will operate properly even `
if the connection to the local telephone lines is wired .in
reverse. Capacitor 214, which receives an out:put signal from
rectifier 216, charges to a predetermined voltage level only
when the telephone attached to the output of receiver 42 is in
an on hook condition, that is, only when its respective switch
hook is closed. As a result, the voltage stored on capacitor
214 maintains set coil 212 and reset coil ~26 in their present
state when the attached telephone is on hook, thereby
eliminating the need for a separate power supply for receiver
42. On the other hand, when the telephone attached to the
output of receiver 42 is off hook, the voltage across
capacitor 214 is relatively low which prevents the telephone
from being turned off while in use (i.e. while off hook).
A schematic diagram of the receiver illustrated in
Fig. 3B is shown in Fig. 6. As a general description of
receiver 42 and the operation thereof was discussed above, the
following discussion is primarily directed to the individual
components which were not depicted in Fig. 3B and, as such,
were not discussed. Further, as previously described,
receiver 42 includes two similar processing portions and, for
brevity, only one portion, that is, the portion which
activates set coil 212 will be described.
I~n input control si~nal is received at terminals 201
and 202 and supplied to the primary winding of a tuned
transformer 250 which is included in tuned circuit 204. Tuned
transformer 250 is tuned to a uni~ue predetermined frequency
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which is substantially the same as one of the control signal
frequencies supplied from transmitter 66, for example, 305
kHz. Upon receipt of the control signal, tuned tranaformer
250 is adapted to determine iE the frequency of the control
signal matches the tuned frequency. If a matoh is detected,
further processin~ continues. On the other hand, if a match
is not detected, the control signal is not supplied through
transformer 250 and further processing is not initiated.
Transformer 250 has an internal impedance matched to the
impedance of the local telephone lines connected to terminals
201 and 202. A signal from the secondary windings of
transformer 250 is supplied through rectifier circuit 206,
which includes diodes 254 and 256, to trigger circuit 208.
Trigger circuit 208 includes a Zener diode 258 (or other
avalanche breakdown device) which is adapted to sense the
voltage level supplied from rectifier circuit 206 and, when
the voltage level reaches a predetermined value, to supply the
same to the base of transistor 260 s~ as to render it
conductive. As a result, a signal is supplied from transistor
260 to SCR 210 which activates set coil 212, thereby
connecting telephone A to the telephone lines.
The input signal from terminals 201 and 202 is
further supplied to rectifier circuit 216 which, in a
preferred embodiment, includes a full wave diode bridge to
ensure that an acceptable D.C. signal is provided therefrom
regardless of the orientation of the telephone line
connections to receiver 42. An output signal from rectifier
circuit ?16 iS connected to capacitor 214 and a Zener diode
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262 (or other avalanche breakdown device) which l:imits the
voltage level across capacitor 214 to a predetermined value,
which in a pre~erred embodiment is approximately 27 volts.
This voltage is applied to set coil 212. As
previously stated, the portion o~ receiver 42 ~or activating
reset coil 226 is similarly connected. That ls, if the
frequency of the input control signal supplied to terminals
201 and 202 matches that of circuit 218, the control signal is
received through circuit 218 and processed through rectifier
circuit 220, trigger circuit 222 and SCR 224 so as to activate
reset coil 226, thereby disconnecting telephone A from the
telephone line.
The individual circuit components without reference
designations depicted in Fig. 6 are connected as shown and
will not be discussed further, since the connections and
values are apparent to those skilled in the art and are not
necessary for an understanding o~ the present invention~
While receiver 42 illustrated in Figs. 3B and 6 was
described for controlling telephone A, wherein as shown in
Fig. 3A the "on" and "off" frequencies were selected as 305
kHz and 400 kHz, respectively, it is to be appreciated that by
selecting the frequencies of tuned circuits 204 and 218 to be
475 and 575 kHz, respectively, the receiver can be utilized as
receiver 62 (Fig. 2) so as to control telephone B. Similarly,
in a three telephone system, that is, when using transmitter
66' (Fig. 5), three of the receivers shown in Figs. 3B and 6
are used in which the tuned circuits 204 and 218 have
frequencies of 375 kHz and 450 kHz, 525 kHz and 630 kHz, and
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705 kHz and 780 kHz, respectively. Thus, it is to be
appreciated that while ambodiments of the present invention
were described Eor use with two or three telephones, any
number of telephones can be utilized by merely selecting other
frequencies.
Although a praferred embodiment o the present
invention has been described in detail herein, it is to be
understood that this invention is not limited to that precise
embodiment, and that many modifications and variations may be
effected by one skilled in the art without departing from the
spirit and scope of the invention as defined by the appended
claims.
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