Note: Descriptions are shown in the official language in which they were submitted.
106~'793
,SPECIFICATION
This invention relates to an attachment for a
telephone and it particularly relates t~ an attachment which
selectively permits ringing of the telephone bell in accordance ''
with the dialing of a specific code.
It is often desirable to avoid answering undesirable
calls but to imm,ediately recognize a call which is desired to
be answered. An unlisted number is usually now used for this
purpose; however, not only may an unlisted number be dis-
covered in one manner or another but if a legitimate callerforgets the number, he may not be able to place the call even
if it is important to the person being called.
This problem was, heretofore, substantially solved
bv placing an attachment on the telephone whereby even though
,' the number was listed, the telephone bell at the receiving -'
, end would only ring when a certain identifying code ~was ,
additionallv dialed. However, this prior type of attachment
had certain inherent disadvantages. For example, a relay ,~ '
was used which was left in an open condition until such time
as the proper code was received. This was`not only un-
reliable because of stresses which might inadvertantly
close the relay, but consumed an inordinate amount of energy.
The prior device was also utilizable only in a
three-wire system used in party lines. However, the majority
of single-party phones used today utilize only two wires
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` 1066i793
instead of the three wires provided in the prior d~vice,
the ringing signal and the voice signal being both provided
acro-~s the same pair.
Another di~advantage of the prior device was its
use of pha~e-locked loops which not only required a con~id-
erable expenditure of enerqy but were not sufficiently stable
and also made the system unduly complex and expensive.
It is one object of the pre~ent invention to over-
come the afores~id disadvantages by providing a system wherein
the bell is prevented from ringing with a normally clo~ed
relay which i8 both more reliable and consumes lesq energy
than a normally opan relay.
Another object of the pre~ent invention is to pro-
vide a ~ystem of the aforesaid type which i8 capable of use --
in a two-wire sy~tem.
Another object of the present invention is to pro-
vide a system of the aforesaid type which accompli~he~ the
same function as one u~ing phase-locked loops but with a con-
sidorably amaller expenditure of enerby and with greater
stability.
Another object of the present invention i~ to pro-
vide a system of the afore~aid type which employs only two
deto¢tor~.
Other objects and many of the attenda~t advantage~
of thi~ invention will be readily appreciated as the same
becom~ batter undQrstood by reference to the following de-
scription when r~ad in conjunction with th~ accompanying
1066793
drawing~ wherein:
Fig. 1 is a schematic view of a bell ringer cir-
cuit embodying the present invention, said circuit being de-
activated when a specific code signal is detected.
Fig. 2 i~ a schematic view of a modified portion
of the circuit of Fig. 1.
Fig. 3 i8 a sch~matic view of a decoding circuit
embodying the pre~nt invention, this decoding circuit being
coupled to the ringer circuit of Fig. 1 for operation thereof.
Fig. 4 is a schematic view of a modified portion
of th6 circuit of Fig. 3.
Fig. 5 is a schematic view of the detector cir-
cuits u~ed in the systems of Fig-q. 3 and 4.
Fig. 6 is a schematic view of a specific embodi-
ment of the bQll-deactivating sy~tem shown in Fig. 1.
Fig. 7 is a schematic view of a specific embodi-
. . .
; ment of the oscillator and detector mean~ ~hown in Fig. 3.
Fig. 8 is a schematic view of a ~pecific embodi-
ment of th~ coding mean~ ~hown in Fig. 3.
Referring now in greater detail to the figure~ of
the drawings wherein similar reference characters refer to
similar parts, there is shown in Fig. 1 a circuit, generally
~; de~ignated 10, comprising a p~ir of tslephone wires re~pec-
-
tivoly designated 12 and 14, wire 12 being a red wire and
~ire 14 being a green wire, the~e wire~ being connected to
the belephone transmission line ~not shown~ Between the
wires 12 and 14 i~ a bridge rectifier con~i~ting of diodes
~ 3
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1066793
16, 18, 20 and 22, the diode~ 16 and 20 being connected by
line 24, the diodes 18 and 22 being connected by line 26,
the diodes 16 and 18 being connected by line 28 and the
diodes 20 and 22 being connected by line 30. The line 24
is coupled to wire 12 by a line 32 having a capacitor 34
interposed therein, and the line 26 is coupled to wire 14
by line 36.
Positioned across line~ 28 and 30 is a resistor
38 connected to a flip-flop (hereinafter de~cribed in Fig~.
3 and 4), thi~ flip-flop acting as a switch means in the
manner hereinafter described. The re~i~tor 38 is also
coupled to a tran~i~tor 42 in series with a normally closed
relay 44. The relay acts to open and close the circuit be-
tween the phone line and the t~lephone instrument. Also po-
~itioned between the lines 28 and 30 i9 a circuit comprising
a rectifier 46 in seri~s with a resi~tor 48 and a capacitor
50- An output is provided at 52~ A parallel circuit i8
provided between resistor 48 and line 30 comprising a c~pa-
citor 54 and a resistor 56 with an ou~put at 58 for acting
as a re~et means for the flip-flops hereinafter de~cribed.
The wire 12 is also capacitively coupled through
line 60 and capacitor 62 to a filter 64 which i9, itself,
coupled through line 66 to the wire 14. The filter 64 i9
al~o coupled to a compression amplifier 68. The contacts
70 of relay 44 are provided in ~he line 12 leading to the
voice receiver (not shown).
In operation, when a rin~ing signal appears be-
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4- -
1066793
tween the wire~ 12 and 14, the ~ignal is passed through
capaci~or 34 and the bridge rectifier con~isting of recti-
fiers 16, 18, 20 and 22, whereby a unidirectional pulsating
voltage (full wave rectified DC) appear~ between line~ 28
and 30. If the flip-flop, hereinafter de~cribed, which acts
a~ a ~witch, opens the circuit, the re-qistor 38 cau~es tran~-
i~tor 42 to 4aturate AO that relay 44 opQrates to di~connect
the phone in~trument from the phone line, thereby preventing
the bell in the in~trument from operating. If, conversely,
the circuit is clo~ed, then transistor 42 would be cut off,
the relay 44 wouad not operate and the phone in~trument would
be connected the the phone line, permitting the bell in the
instrument to operate normally. Similarly, if there i8 no
ringing signal across the line, then there i~ no voltage
available to operate the relay 44 regardle~s of the condition
of the transi~tor 42: therefore, the phone in~trument is nor-
mslly connected to the phone line for normal incoming and
~; outgoing calls in the ab~ence of a ringing signal on the line.
When there is a ringing ~ignal on the line causing
a voltage to appear between line~ 28 and 30, some current
,~ i8 conducted through the rectifier 46 and resistor 48 to
^~ charge capacitor 50. Normally, there is a four-3econd in-
.
~i t~rva~ between ringing signal~ and OhQ en~rgy stored in ca-
~ pacitor 50 is employed during the~e intervals to provide op- -
-~ erating power for all the oth~r e1ements in the sy~t~m. I~
the relay 44 i~ sensitive, the total en~rgy taken from the
phone line during a ringing 8ign~1 can be made identical to
5.
1066793
the energy which would have been u~ed by the bell if it had
been ringing. Since the entire circuit i~ capacitively
coupled by means of capacitor 34 to the phone line, the cir-
cuit is not polarity sensitive and, therefore, the lines 12
and 1~ may be int~rchanged at will.
When a tone i9 sent through the phone line, it i8
coupled via capacitor 62 into the filter 64 which is coupled
to tho co~pression amplifier 68. The filter removes all
fregu-ncie~ except the prede~ermined desired frequencies.
The unde~irable fr-guencie~ which are remov~d would ba all
tho~e other frequencies which are typically sent through the
line by telephone companie~.
Since different phone lines have differing at-
tenuation characteristic~, signals coming down the lino ar-
rive with differing amplitudes. The compression ~mplifier
~-nses the actual amplitude of the signal tone and adjusts
the gain of the ~plifier portion 80 that when the to~e leaves
the compression amplifier, it will alway~ have the ~ame
amplitude regardle~s of th~ amplitude of the ~ignal ~nt~ring
the compres~ion a~plifier.
The filter 64 and compre~sion amplifier are not
~sential for th oparation of the ~ysbem but subatantially
increa~e it~ reliability.
Fig. 2 di3closes the same system a~ Fig. 1 except
that i~ i8 used in a three-wire system which include~ three
~ , .
; ~ire~, 72, 74 and 76, which are re~pectively, the yellow, -~
~` green and red wire~. The bridge, which is otherwise identical
6.
1(~66793
to that in Fig. 1, is capacitively coupled to line 72 by
capacitor 78 interposed in line 80. A line 82 connecte the
bridge to a switch 84 that may be manually operated. Thi~
switch 84 i8 movable between two contacts 86 and 88 to sel-
ectively couple the rectifier bridge either to the wire 74
or to the wire 76. All other parts are identical to those
in Fig. 1~
me system ~n Fig. 1 is operated by the ~ystem
sh~wn in Fig. 3. me system of Fig. 3, generally dQsigDated
90, comprises a high freguency o~ci~lator 92 which operates
at a frèquency which i~ very nearly the same as the frequency
to be detected, and its output i~ fed to one input of a de-
tector 94. The ~ignal received from the compression ampli-
fi r 68 i~ fed through line 96 into the other input of the
deOsctor 94 as well as into the input of detector 98.
If any freguency in the signal fed iato detector
94 is substantially the same as the freguency of the oscil-
lator ~2, an output is produced by the detector 94. A low
frequoncy oscillator 100 i8 coupled to detector 98 to feed
its output thereinto, and if any frequency in the sig~al fed
into detector 98 is ~ubstantially the same a8 the fr~quency
of the oscillator 100, the detector 98 produces an output.
The outputs of the two detectors 94 and 98 are fed
into an A~D ~te 102. This gate operate~, therefore,
only whe~ the prop~r high freguency and low frequency ~ig-
nal8 ar- ~imultaneou~ly present. At such time, an output i~
fed from the A~D gate 102 to flip_flop 104. In thi~ ~anner,
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7.
~06~793
to detect a single digit which (a~ in the case with touch-
tone~) consists of two frequencies, only two detectors are
required to determine if a particular number is present. All
other numbers are ignored since they have no relevance. In
this manner, if a correct first digit is detect~d, using an
output from the ~ND gate 102, the flip-flop iR caused to
toggle from its re~et ~tate to its ~et state. A matrix of
8tandard dQsign~ indicated at 106, i~ employed to determine
the freguenci~ at which the o~cillators operate.
When the flip-flop 104 toggles, the oscillators
begin to oscillate at a new pair of frequencies, which may
be the same as or different from th~se u~ed previously. A
second digit can then be detected in a similar manner to the
first, thereby again causing the flip-flop 104 to toggle.
When flip-flop 104 toggles a second time, the output is ap-
plied through line 108 to set flip-flop 110. This cu~ ~ff
the transistor 42, thereby preventing the relay 44 from op-
erating, which, as a resul~, permits the phone bell to ring.
Anothor input to flip-flops 104 and 110 indicated at 11~,
permits resetting the flip-flops, this input corresponding
to input 58 in Pig. 1.
Fig. 4 shows a portion of a system ~hich is the ~
same a9 in Fig. 3, except for the use of an ~dditional flip- --
flop to accommodate additional digits, the three flip-flops
in thi~ embodiment being de~ignated 112, 114 and 116 respec-
tively. This arrangement would accommodate four digits po-
tentially, but a-q many flip-flops as de~ired may ke u~ed in -~
8.
;: . . ,
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106f~7~;~
accordance with the number of digits in the code.
The original conditions of the fLip-flops are de-
termined by resetting them at the beginning of each call.
They are reset when a positive voltage is applied to their
respective inputs. In this manner, when there is no ringing
signal on the line in Fig. 1, capacitor 50 is in a discharged
state such that ~J+ equals Vo and the voltage across capacitor
54 is zero. ~hen a ringing signal appears and capacitor 50
begins charging, if the time constant of capacitor 54 and
resistor 56 is long compared to the time constant of resis-
tor 48 and capacitor 50, then the reset voltage on line 58
will be approximately V+ until capacitor 54 charges through
resistor 56, at which time the reset line becomes zero volts
and the flip-flops are free to operate.
Fig. 5 shows an illustrative embodiment of each of
the detectors shown in Fig. 3. Each of these detectors in-
cludes an exclusive OR gate 118 coupled to an R-C low-pass
filter comprising resistor 120 and capacitor 122 connected
to an unvarying reference voltage. The R-C filter is coupled
to a normally open relay 126.
In operation, the oscillator is made to produce a
rectangular waveform with a 50% duty cycle (square wave~ which
is applied to one input of the OR gate, as at 126. The signal
; is applied to the other input, as at 128. The output of the
OR gate passes through the` low-pass filter which acts as an
averaging device. As the output of the OR gate swings through
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106~ 93
the complete supply voltage applied to the g~te, then, in
the absence of a sign~l, the output of the low-pass filter
equal~ one-half the ~upply voltage because of the 50% duty
cycle.
When a signal i8 applied, beat notes (sum and dif-
ference frequencie~) appear at the output of the low-pass
filtor. Since the low-pa~s filter pa~es only low frequen-
Ci-8, the ~um frequency is very small and may be ignored.
The closer the frequencie~ of the osci~lator input and ~ignal
are to each other, the lower will be the difference freguency
beat note and becau-~e of tho presence of the low-pa~ ilter,
the larger will be the a~plitude of the keat noto issuing
from the filter.
me output of the low-pa88 filter i~ connected to a
dotector which de~ect~ any excur~ion of the output of the ;-
filter fro~ one-half the ~upply voltage. Thi~ i~ accompli-
~hed in the ~y~tQm -~hown in Fig. 5 by connecting on~ end of
the coil of a normally 4pen relay 126 to a fix~d one-half of
tho supply voltage and the other end of the coil to the ou~-
put of the filt~r. The output of the circuit i~ indicated
at 134, and thi~ output i8 caused to be po~itive by the con-
tact~ 124 of th~ relay 126. ~;~
Alternativ~ly, a bipolar exclusive OR gabe couldbe usod in place of relay 126.
Fig. 6 and the figures following ~how a more de- -
tailed o~bodiment of the pre~ent i~v~ntion. In thi8 embodi-
ment, tho upper portion of Fig. 6 correspond~ to the left
10.
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1066793
portion of Fig. 1. Terminals 200 and 202 are connected to
the phone line ~uch that if switch 288 is closed when a
ringing ~ignal appears on the line, it is pas~ed through a
capacitor 204 to a set of four diode~, resp ctively de~ignated
as 206, 208, 210 and 212, connected a~ a bridge rectifier.
~he signal is tapped off through a diode 214 in ~eries with
a resi~tor 216 to charge capacitor 218. A Zener diode 220
limit~ the voltage to which the capacitor 218 ~an charge. The
~eri~s combination of the capacitor 204 and resistor 216 de-
termin s the amount of current tak~n from the phone line.
Volt~g~ from the capacitor 218 is coupled both to a compres-
sion amplifier through line 222 and through a resistor 223
to a capacitor 225 which provides pawer indicated as V2 for
all t~e oth~r circuits. A capacitor 228 and a series resistor
230 are coupled off from V2 to establish a re~et voltage.
During the ri~ging ~ignal, the pulsating DC voltage
i8 also applied through relay 232 in serie~ with tran~istor3
234 and 236. A diode 238 is connected in parallel with relay
232 to prevent exce~ive voltage ~pike~ during collapse of ~-
the magn~tic field in the relay coil.
When the first ringing ~ignal appaars on the phone
lin~, if it i~ imcomplete (i.e. le~ than 2 ~econds), the
voltage V2 may not become sufficiently establi~h~d for the
logic circuit~ to have sufficient voltage to properly esta-
blish their states. Under ~udh condition3, the o~tput at 240
may bacome temporarily positive. Thi~ would cause transi~tor
1066793
242 to turn on and transistor 234 to turn off, whereby the
relay 232 would not operate to prevent the bell from ringing.
In order to prevent this from happening, the capacitor 22~,
which is coupled to V2 through resistor 226 and to the output
240 through diode 2~8 and resistor 250, is initially in a
di~charged state and acts through the diode 248 to prevent
the tran~i~tor 242 from turning on until V2 can become es-
tablished. In other words, the time con~tant of the capa-
citor 224 and th- re~i~tor 226 i8 longer than the time con-
stant of the resi~tor 216 and the capacitors 218 and 225.
me lower portion of Fig. 6 corresponds to the
filter and compre~sion amplifier position of Fig. 1 and in-
cludes a capacitor 252, a capacitor 254 and an inductor 256
in a standard Chebychev high-pas~ filter arrangement which
eliminates the standard 20-Hz ringing signal from the input ~-
to the compr~ssion amplifier. Considerable attenuation take~
place since the 20-Hz ringing signal is typically 250 volts
p~ak-to-peak. Tran~ormer 258, ~ollowed by a resi~tor 260
and capacitor 262, provides additional filtering.
The compre~sion amplifier includes a capacitor 264
through which the signal is applied to the base of ~ transi~tor
266 which is initially biased by resistors 26~ and 270 as a
st~ndard small-signal amplifier where ~he voltage gain is de- -
termined by the ratio of a collector resistor 272 to the
transi~tor emitter re~istance. The emitter resi~tance i8 in-
ver~ely proportional to the base voltage. In thi~ manner, any
decrease in base voltage will cause an increa~e in emitter
106~'793
resistance and a decrease in voltage gain.
The transistor 266 is coupled to a transistor 274
which serves as a standard emitter follower to permit impe-
dence amplification that allows the use of a large collector
resistor 272 for transistor 266, thereby reducing the current
taken from the capacitor 218. The output of the emitter
follower is capacitively coupled to a standard voltage doubler
circuit consisting of transistor 278 connected as a diode
plus a standard series diode 280. The capacitor 264 acts as
a filter capacitor for the voltage doubler in addition to its
other input coupling duties.
The voltage doubler circuit is arranged to produce
a negative voltage so that when the signal on the emitter of
transistor 274 increases, a negative DC voltage is produced
across the capacitor`264 and on the base of transistor 266, `
thereby causing the gain of the transistor stage 266 to de-
crease. Since the action is logarithmic, a change in voltage
at the input of the amplifier by a factor of 200 causes a
change of less than 25% at the amplifier's output. A trans-
istor 282 and its associated resistors 284 and 286 form a
standard audio-freguency amplifier.
The NC contacts 233 of relay 232 are connected in series
with the phone instrument which is connected between
terminals 292 and 290. When Sw 288 is open, any ringing sig-
nal on terminal 200 is disconnected from the bridge networks
200, 208, 210 and 212 so that no signal is applied across the
coil of relay 232. The contacts of relay 232 then remain
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' 106f~'7~3
closed connecting terminals 200 and 290 together so that
the user of the phone observes no difference from when the
unit is not connected to the telephone instrument since
electrically there is no difference.
The upper and lower portions of Fig. 7, the upper
portion serving as the high frequency group detection and
the lower portion serving as the low frequency group detec-
tion, are schematically identical so that a description of
the upper portion serves as a description of the lower por-
tion as well. As shown, the transistor 300, transformer302, and their associated components, comprise a standard
emitter-coupled feedback oscillator with its frequency being
determined by the inductance of a tertiary winding on the
transformer 302 and by a capacitor 304. The diac 306 be-
tween leads 3~8 and 3I0 determines the amplitude of oscill-
ation. Transistor 312 and its associated components such
as capacitor 3I3 and resistor 314 operate as a standard ~ -
buffer amplifier. The collector of transistor 312 is coupled
to the input of NAND gate 315 connected to line 316 through
a biasing resistor 318, the output of the NAND gate 315
being coupled to the input of NAND gate 320. The inverter
gates operate as limiters to provide a fast rise time os- -
cillation from the oscillator for presentation to a toggling
flip-flop 322.
The oscillator operates at double the desired fre-
quency so that the output of the toggling flip~flop will be
at the desired frequency with a duty cycle that is extremely
- 14 -
1066753
close to 5~% (within the noise limitations of the toggling
flip-flop). The square wave from the flip-flop 322 and the
signal are applied to the two inputs o' an exclusive OR gate
326, the output of which is connected to a low-pass filter
consisting of two resistors 328 and 330, a capacitor 332 and
a capacitor 334. The output of the low-pass filter is con-
nected to a NAND gate 336 which is used as a threshold de-
tector and is biased by a resistor 338.
If no signal is present or no frequency in a signal
is present which produces a beat note within twice the
cutoff frequency of the low-pass filter, the output of the
threshold detector is a logic "O". If such a frequency is
present in the signal to produce a beat note of less than
twice the cutoff frequency of the low-pass filter, then the
output of the threshold detector, in synchronism with the
beat note, periodically presents a logic level "1". Optional-
ly, a bipolar detectOr may be used.
The gate 336 is coupled to a resistor 340, which,
when combined with a diode 342, a resistor 344 and a capa-
citor 346, constitutes a shaped low-pass filter which, when
monitored by a NAND gate 348 used as a threshold detector,
causes a continuous logic "O" level to be present at output
350 when a fre~uency component of the signal is within twice
the cutoff frequency of the low pass filter of the oscillator
freguency.
As seen in Fig. 8, the outputs of the high and low
frequency detectors are passèd through inverters 352 and 354
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-~`` 10667~3
to a NAND gate 356 followed by a resettable monostable con-
sisting of a resistor 358, a diode 360, a capacitor 362 and
a threshold detector 364, so that the output of the threshold
detector 364 is a logic "1" if and only if both frequencies
to be detected are simultaneously present for a period of
time at least as long as that set by the resettable
monostable.
The resettable monostable is followed by a non-
resettable monostable to eliminate any switching transients
by means of a positive feedback. This non-resettable mono-
stable consists of a capacitor 366, a resistor 366, a NAND
gate 370, a capacitor 372, a resistor 374, a threshold de-
tector 376 and a resistor 378. The inverters 364 and 376
are partially represented bv a broken line to indicate that
they are non-complimentary ~lOS employing a resistor in place
of the normal complimentary resistor. By thls means, each
monostable requires only 5 microamperes for operation as
compared to several milliamperes for standard complimentary
configurations.
The output of the non-resettable monostable is
passed through another NAND gate 386, connected as an in-
verter, which is also used as a limiter. In addition, the
output is passed to the flip-flop 382 corresponding to flip-
flop 104 in Fig. 3. This flip-flop is also used to set f~ip-
flop 384 corresponding to flip-flop 110 in Fig. 3.
The complimentary outputs of the toggling flip-
flop are also passed through NAND gates 386, 388 and 390,
- 16 -
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1066793
which are connected as inverters and used as ~uffers, the
outputs of which bias the reYpective transistors 392, 394,
396 and 398 into saturation or cutoff. The collectors of
these four transistor-~ are connected to a matrix of connectors
106, ~the connection heing of ~tandard conqtruction and not
shown). The remaining terminals of the matrix are connected
to the appropriate lQads on the high and low frequency trans-
fo~R~rs shown in Fig. 7. ~herefore, when transi_tors 392
and 394 are in saturation, one pair of oscillator frequen-
cies i8 cho_e~, while ~hen tr~nsi_tors 396 and 398 are in
saturation, with transistors 392 and 394 in cutoff, a second
pair i-~ chosQn.
Although the above-doscribed system has been di-q-
closed in use with telephone-bell blocking arrangement, it
i_ not nece~sarily limited to Yuch use but can be used in
any ~ystem to det~rmine whether or ~ot a predetermined fre-
quency component is present in a signal.
17.
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