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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates generally to systems for accessing various
remote instrumentalities, and in particular, to a circuit for selectively
activating a unit capable of interfacing with the instrumentality
responsive to detected tones.
Recently, there has been significant interest in the development of systems
for interrogating various remote instrumentalities to obtain desired
information without requiring personnel to travel to the remote location
at which these instrumentalities are stationed. One general class of
remote interrogation involves the detection of alarms indictating that the
instrumentality in question has undergone a certain change in state. This
change in state may signify any of a number of alarm conditions such as a
fire, an unauthorized entry, the departure from specified norms, the
measurement of a specified parameter, etc. Another general class of remote
interrogation involves the accessing of meters and other information
gathering devices. This may include the accessing of electric meters, gas
meters and others, as well as devices for monitoring any of a number of
parameters including flow rates, temperatures, pressures, etc.
Generally, such interrogations are accomplished by transmitting a tone or
group of tones to the instrumentality to be accessed, so as to cause the
instrumentality to read the parameter or parameters which it monitors, and
to transmit this information to the source of the interrogation signal.
Ordinarily, these operations occur over telephone lines, since this
eliminates the need to run separate lines between the source of the
interrogation signal and the remote instrumentality. However, this has the
disadvantage of subjecting such operations to the regulations imposed
upon, and imposed by, the various telephone companies involved. One such
regulation, with which the present invention primarily concerns itself,
relates to the use of current from telephone lines to operate associated
equipment.
Various regulations place specific limits on the amount of power which can
be drawn from a telephone line, so as to avoid an unacceptable
interruption in telephone service, and so as to enable the fault detection
circuitry which is conventionally used by many telephone companies to
operate correctly. When a telephone line is placed in service (the
so-called "off-hook" condition), sufficient amounts of current may
generally be drawn from the telephone line to operate most available
remote monitoring devices. However, this requires continued occupation of
the accessed telephone line during these periods, which is often
unacceptable, and at times impossible, since the transmission of speech
and data communications must not be affected by such ancillary equipment.
When the telephone line is not in service (the "on-hook" condition),
prevailing regulations generally severely limit the amount of current
which may be drawn from the telephone line, often to only several
microamps, which is generally insufficient to operate most presently
available remote monitoring devices.
In many cases, the remote monitoring device forms part of a requested
service, or a service to which a customer subscribes. In providing
services of this type, the customer receives the benefit of the service
and expects the service to be performed. Accordingly, the company which
provides the service is generally able to rather freely access the
customer's power lines, which avoids the need to deal with the
above-discussed constraints.
However, in connection with certain types of meter reading equipment, the
remote monitoring device is operated for the convenience of a utility or
some other third party, and not for the convenience of the customer in
possession of the premises at which the remote instrumentality is
stationed. Since these operations are for the convenience of a third
party, and not the customer, federal regulation prohibits such systems
from drawing power from the customer's premises. Accordingly, the only
available source of power for operating the remote monitoring device is
often the telephone lines which enable communication between the
interrogation system and the remote instrumentality being interrogated,
subjecting the third party to the above-discussed constraints. Moreover,
since access to the customer's telephone lines is then rather limited
(generally to off-peak hours), and since ringing of the customer's
telephone is to be avoided, the normally on-hook monitoring device will
generally not receive sufficient current to operate as required.
It therefore has become desirable to develop a means for operating a system
for the remote interrogation of an instrumentality which is capable of
drawing necessary operating current from the telephone line over which
such communications take place, eliminating the need to draw current from
the premises at which the remote instrumentality is stationed, yet which
is capable of doing so without drawing more current than is permitted by
present regulations in this regard.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide an
improved system for remotely interrogating an instrumentality by means of
telephone lines.
It is also an object of the present invention to provide a system for
remotely interrogating an instrumentality by means of telephone lines,
which does not require power to be drawn from the premises at which the
remote instrumentality is stationed.
It is also an object of the present invention to provide a system for
remotely interrogating an instrumentality wherein all power necessary to
the interrogation is drawn from the telephone lines which service the
system.
It is also an object of the present invention to provide a system for
remotely interrogating an instrumentality which is capable of drawing its
operating power from the telephone lines servicing the system, without
exceeding maximum current limits as prescribed by prevailing regulations.
It is also an object of the present invention to provide a system for
remotely interrogating an instrumentality which is capable of providing
these advantages, yet which is simple and reliable in operation.
These and other objects are achieved in accordance with the present
invention by providing a tone-responsive circuit which operates in
accordance with an established duty cycle wherein, for a major portion of
each cycle, current is drawn from the telephone lines servicing the system
at a rate which is within the range permitted by prevailing telephone
regulations, to charge a capacitor over time, and wherein, for a minor
portion of each cycle, the charge stored by the capacitor is used to
briefly operate a tone detection circuit which is capable of determining
whether or not the remote instrumentality is being interrogated. If no
such tones are detected, the circuit continues its periodic operation as
previously described. If tones indicating a desired interrogation are
detected, the tone-responsive circuit activates the monitoring unit which
is used to poll the instrumentality. In activating the monitoring unit,
the telephone line which services the system is briefly caused to assume
its off-hook condition, to permit the monitoring unit to draw sufficient
power from the telephone lines to briefly (and presumably unobtrusively)
operate in its desired fashion, and to prevent incoming calls from being
placed on the line so as to permit a brief, yet uninterrupted transaction.
After transmitting the desired information to the source of the
interrogation signals, the remote monitoring unit is deactivated and the
tone-responsive circuit is returned to its initial condition, to again
assume its intermittent function as previously described.
For further details regarding a tone-responsive circuit in accordance with
the present invention, reference is made to the following detailed
description, taken in conjunction with the following illustrations.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating a tone-responsive circuit in
accordance with the present invention.
FIG. 2 is a timing diagram illustrating waveforms representative of
operation of the circuit shown in FIG. 1.
FIGS. 3-5 are schematic diagrams illustrating circuitry for embodying the
block diagram shown in FIG. 1.
FIGS. 6-8 are schematic diagrams illustrating preferred circuitry for
embodying the block diagram of FIG. 1.
In the several views provided, like reference numerals denote similar
structure.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Although specific forms of the invention have been selected for
illustration in the drawings, and the following description is drawn in
specific terms for the purpose of describing these forms of the invention,
this description is not intended to limit the scope of the invention which
is defined in the appended claims.
FIG. 1 illustrates a tone-responsive circuit 1 in accordance with the
present invention. Tone-responsive circuit 1 receives its input from an
available telephone line 2, incorporating tip 3 and ring 4 terminals which
are applied across the telephone line 2 in accordance with recognized
practices. Resistors 5, 6 define a load across the telephone line 2, and
operate in combination with a full wave rectification circuit 7, which
serves as a means for polarity correction, to supply appropriate currents
at terminal 8. A lightning arrestor 9 bridges resistors 5, 6 to protect
the circuit 1 from external conditions involving the telephone line 2.
Capacitor 10 is provided to further isolate the circuit 1 from transients.
The current developed at terminal 8 is applied to a current source
sub-circuit 11 which serves to provide a closely regulated and constant
current at 12 irrespective of variations in the voltage developed at the
terminal 8. Current source sub-circuit 11 is configured to limit the
current drawn from the terminal 8 to a value which will satisfy presently
existing telephone company regulations. Specifics regarding such limits
will be indicated below. A capacitor 13 couples the output 12 of the
current source sub-circuit 11 to ground, and accordingly serves as a
storage element which exhibits a specified voltage excursion, at 14, which
proceeds in accordance with the regulated circuit which is supplied at the
output 12 of the current source sub-circuit 11, and the capacitance of the
capacitor 13.
Terminal 14 is further coupled to ground by means of a charge voltage
comparator sub-circuit 15. Charge voltage comparator sub-circuit 15 is
configured to monitor the voltage excursion exhibited at terminal 14, and
to determine when the voltage developed across the capacitor 13 has
reached a specified triggering voltage. Upon sensing this triggering
voltage, charge voltage comparator sub-circuit 15 provides an output, at
16, which serves to activate (close) a normally open switch 17. In this
manner, the voltage developed at terminal 14 is coupled through switch 17
to a tone detect sub-circuit 18 responsive to the voltage excursion
exhibited at the terminal 14. This enables tone detect sub-circuit 18 to
momentarily search for tones indicative of a desired interrogation. To
this end, a capacitor 19 serves to couple the input of the circuit 1
through to the input 20 of the tone detect sub-circuit 18.
In the event that the appropriate tones are detected, tone detect
sub-circuit 18 provides an output at 21 which is used to initiate an
interrogation process which will be more fully described below. In the
event that the appropriate tones are not detected, deterioration of the
voltage exhibited at terminal 14 will eventually cause the output 16 of
the charge voltage comparator sub-circuit 15 to be removed from the switch
17, allowing the switch 17 to return to its normally open state and
returning the circuit 1 to its original condition. In such case, the
capacitor 13 will once again be charged until the triggering voltage is
reached, which will serve to periodically repeat the process as previously
described, until such time as the desired tones are detected.
Accordingly, the circuit 1 serves to periodically check the telephone line
2 with which the circuit 1 is associated to determine whether or not tones
indicative of a desired interrogation have been received. This periodic
check proceeds according to the charging characteristics of the capacitor
13, the current provided at the output 12 of the current source
sub-circuit 11, as well as the triggering voltage (e.g. 4.5 to 5.5 V)
which is to cause a change in state of the charge voltage comparator
sub-circuit 15, such that the capacitor 13 is periodically charged and
discharged in accordance with the curve 25 illustrated in FIG. 2. Applying
this periodic waveform to the charge voltage comparator sub-circuit 15
causes the tone detect sub-circuit 18 to operate in accordance with the
curve 26 illustrated in FIG. 2, defining a window 27 within which the tone
detect sub-circuit 18 operates to determine whether or not an
interrogation has been called for. This window is preferably on the order
of 60 to 200 milliseconds. Longer samplings are generally to be avoided in
order to maximize to number of instrumentalities which can be interrogated
within a given period of time.
It will be noted that, to this point, the circuit 1 operates without
requiring an external source of current. Rather, the capacitor 13
periodically serves as a source of current which is sufficient to achieve
the desired tone detection. To this end, it is preferred that the circuit
components which develop the above-described functions be micro-power
elements, to assure that the capacitor 13 is capable of providing
sufficient current to operate the circuit 1.
In the event that the requisite tones are not detected, the circuit 1
continues its searching function. During this searching, an acceptably low
current is drawn from the associated telephone line 2. In the event that
the requisite tones are detected, tone detect sub-circuit 18 serves to
activate circuitry, as will be described more fully below, which is to
accomplish the desired interrogation. In so doing, steps are taken to
place the accessed telephone line 2 in its off-hook condition, which
accomplishes two tasks. First, this places the instrumentality to be
interrogated in communication with the interrogation unit, via the
telephone system, enabling the instrumentality to be accessed in
appropriate fashion. Second, once placed in the off-hook condition,
pertinent telephone company regulations no longer constrain the current
which can be drawn from the telephone line 2 to its nominal on-hook value,
enabling much greater currents to be drawn from the accessed telephone
line. This increased current supply is withdrawn at 29, and is sufficient
to supply the monitoring unit which is used to interrogate the
instrumentality. After completion of the interrogation, this increased
current draw is discontinued and the circuit 1 is returned to its
quiescent state, periodically searching for a renewed interrogation. At
this time, the monitoring unit is also returned to its quiescent state, so
as to keep occupation of the customer's telephone line to an acceptable
minimum.
FIGS. 3-5 illustrate the circuitry which is generally described in FIG. 1
in greater detail, in connection with an application wherein the amount of
current which may be drawn from the telephone line in its on-hook
condition is limited to a value on the order of 100 microamps.
Accordingly, FIG. 3 again shows a telephone line 2, across which has been
placed a pair of loading resistors 5, 6, in series combination with a
bridge circuit 7 as previously described, to develop a current at terminal
8. The current developed at terminal 8 is applied to the current source
sub-circuit 11 as previously described. It will be noted that the current
source sub-circuit 11 comprises light emitting diodes 30, which serve as a
convenient means for developing a controlled and constant reference
current useful in charging the capacitor 13 as previously described.
The voltage developed across the capacitor 13 is in turn applied to the
charge voltage comparator sub-circuit 15, to monitor the voltage excursion
developed at terminal 14; shown at 25 in FIG. 2 of the drawings. In
response, the charge voltage comparator sub-circuit 15 develops an output
(shown at 26 in FIG. 2) which is capable of operating the switch 17, here
a switching transistor, on a periodic basis.
Activation of the switch 17 in turn serves to activate the tone detect
sub-circuit 18, which is illustrated in FIG. 4. As previously described,
the input of the circuit 1 is capacitively coupled to the input 20 of the
tone detect sub-circuit 18. Input 20 is preferably provided with a 60 Hz
filter 31, to eliminate noise, and an automatic gain control 32, to
accommodate variations in level. The output of automatic gain control 32
is applied to each of a pair of two-stage tone filters 33, 34, which are
configured to selectively detect a desired pair of tones which are to
indicate a requested interrogation. The outputs of tone filters 33, 34 are
respectively coupled to a pair of amplitude comparator circuits 35, 36,
which compare the outputs of the tone filters 33, 34 against a reference
voltage 37 which is developed by means of the voltage divider network 38
which is shown in FIG. 3 of the drawings. It will be noted that all of the
foregoing circuitry operates from the current supplied by the capacitor
13, at 39, and does not require the use of any external power supply. It
is for this reason that the use of micro-power circuit elements is
preferred.
With reference to FIG. 5, the outputs of amplitude comparator circuits 35,
36 are respectively coupled to switching transistors 40, 41, which serve
the function of an AND circuit. Accordingly, upon the application of each
of two appropriate tones to the tone detect sub-circuit 18, a signal will
be developed at 21 which serves to turn on a transistor 42, activating a
circuit 45 for monitoring the instrumentality in question. Pertinent
portions of such a monitoring circuit 45 are shown in FIG. 5.
To this end, a hook relay current circuit 46 is activated, to draw
sufficient current from the telephone line 2 to assure that the telephone
line assumes its off-hook condition; and appropriate voltages are
developed by means of a voltage reference circuit 47, to initiate
operation of the monitoring circuit 45. This initiates the interrogation
process, which then proceeds in accordance with conventionally available,
and otherwise known techniques.
In activating the tone-responsive circuit 1, care must be taken to assure
that the selected interrogation-signaling tones are detected by the
circuit 1 despite its periodic operation. Accordingly, it is preferred
that the selected tones be presented to the circuit 1 for a period of time
which is at least equal to the length of one complete charge/discharge
cycle of the capacitor 13 (up to four seconds).
In activating the monitoring circuit 45, care must also be taken to
maintain operation of the monitoring circuit 45 for a sufficient period of
time to perform its functions. Since this period of time will normally
exceed the period of time during which the interrogation tones are applied
to the tone-responsive circuit 1, a maintenance transistor 48 is placed in
parallel across the series combination of transistors 40, 41 to maintain
the transistor 42 in its activated state throughout the interrogation
process. Maintenance transistor 48 receives its input from a multivibrator
circuit 49 which operates responsive to the activation of the monitoring
circuit 45. A second multivibrator circuit 50 is provided to enable
servicing of the system, in the absence of a detected interrogation
sequence.
The above-described circuitry is intended for use in applications wherein
prevailing telephone company regulations limit the current which may be
drawn from the telephone line servicing the system, in the on-hook
condition, to a value on the order of 100 microamps. However, often,
prevailing telephone company regulations limit the current which may be
drawn from the telephone line (on-hook) to a value on the order of 10
microamps. In such cases, the above-described circuitry will not be
effective in achieving desired operation.
FIGS. 6-8 illustrate circuitry which is capable of operating in accordance
with the present invention, even when prevailing telephone company
regulations limit the current which may be drawn from the telephone line
in its on-hook condition to a value less than 10 microamps. Since such
circuitry is also capable of operating when increased currents are
available, with improved efficiency, the following circuitry is preferred
in accordance with the present invention.
As in the circuit 1 illustrated in FIGS. 3-5, the current source
sub-circuit 11 again serves to limit the current which is drawn from the
terminal 8. However, in the circuit 1' of FIGS. 6-8, this current is
limited to 9.6 microamps. Charging of the capacitor 13 again causes a
varying voltage to be developed at the terminal 14, in accordance with the
characteristic curves illustrated in FIG. 2.
However, in the present embodiment, this change in voltage is monitored by
a charge voltage comparator sub-circuit 15' which is comprised of field
effect transistors, in place of the bipolar transistors and Zener diodes
of the charge voltage comparator sub-circuit 15, to enable the sub-circuit
15' to operate at the lower current levels which are available.
Further as a result of the lower current levels which are available, there
is now insufficient current to support a two-tone detection scheme as
previously described. Accordingly, only one tone is used, and tone detect
sub-circuit 18' operates to detect the single tone selected only if the
telephone line 2 displays an on-hook status. This is sensed by a hook
status detection sub-circuit 60 (see FIG. 6).
Tone detect sub-circuit 18' is a single channel, single tone filter 61,
with an input squarer circuit 62 instead of an automatic gain control
circuit. The squarer circuit 62 assures a uniform signal output amplitude
over the entire range of specified input signal levels and power supply
voltages. An output integrator 63 serves to detect signal level and
duration at the selected frequency.
The output of integrator 63 serves to control (turn "on") a MOSFET
switching transistor 41. If the telephone is on-hook, hook status
detection sub-circuit 60 will serve to activate (turn "on") the MOSFET
switching transistor 40, in turn activating the monitoring circuit 45 as
previously described by means of the transistor 42.
It will therefore be seen that the foregoing serves well to achieve each of
the objectives previously set forth. It will be understood that the
foregoing is capable of variation without departing from the spirit and
scope of the present invention. For example, as is apparent from the
foregoing description, a variety of circuit implementations may be used to
provide the functions of the current source sub-circuit 11, the charge
voltage comparator sub-circuit 15 and the tone detect sub-circuit 18.
Moreover, the switch 17 may take various forms, as desired. It will also
be understood that the timing of the above-described circuit
implementation is capable of variation, as needed. Lastly, it will be
understood that any of a number of tone combinations (single or plural)
may be used to activate the tone-responsive circuit 1 of the present
invention, depending upon telephone company regulations, and depending
upon the other, unrelated circuit elements which may be associated with
the accessed telephone line.
For example, it will be understood that the tones selected to activate the
tone-responsive circuit 1 must be selected so as not to correspond to
those tones which are used in connection with the touch-tone dialing
system of the telephone company. Also to be avoided are tones which
correspond to those tones which are used in connection with other
peripheral equipment, including Modem's, telephone answering devices, and
the like. The following tone pairs have been identified by the various
telephone companies as being appropriate for transmission over telephone
lines without interfering with the operation of telephone equipment.
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Combination Tone No. 1
Tone No. 2
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1 578.4 HZ 485.0 HZ
2 578.4 HZ 375.2 HZ
3 520.6 HZ 375.2 HZ
4 520.6 HZ 330.5 HZ
5 468.0 HZ 330.5 HZ
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These tones have been selected by the telephone company for private use.
For single tone applications, a tone may be selected from these lists as
desired. For plural tone applications, tone pairs may be selected and
intermixed in various combinations as desired. It is expected that
additional tones and tone pairs will also become allocated by the
telephone companies, which may be substituted for the above values. Such
tones are freely selectable so long as they do not interfere with, or
result in the activation of, other telephone or peripheral equipment
associated with the system.
It will therefore be understood that various changes in the details,
materials and arrangement of parts which have been herein described and
illustrated in order to explain the nature of this invention may be made
by those skilled in the art within the principle and scope of the
invention as expressed in the following claims.
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