Note: Descriptions are shown in the official language in which they were submitted.
'" ' 2 ~ 2 ~
OFF-HOOK DETECTOR FOR AUTOMATIC
METER READI~G EQUIPMENT
The invention relates to off-hook detector circuits
for equipment used in conjunction with telephone
equipment.
Damoci et al., U.S. Pat. No. 4l958,371, discloses a
circuit for sensing the off-hook condition of a
telephone, and generating a pulse to a microprocessor.
The micro-processor then responds to disable other
circuitry which might utilize the telephone line.
Hashimoto, No. 4,833,704, Fig. 1, shows an off-hook
detection circuit utilizing optically coupled circuits
and a microcomputer.
Szlam, No. 4,742,538, Fig. 5, shows an off-hook
detector circuit utilizing a comparator to generate an
off-hook signal to a control unit.
There are many other off-hook detection circuits
known in the art for various different purposes, such as
operating telephone extensions, telephone answering
machines or other telephone-related equipment~
One type of equipment used with telephones is
automatic meter reading equipment of the type disclosed
in Thornborough et al., U.S. Patent No. 5,031,209. Such
equipment is designed to communicate over phone lines
with a utility data collection center for the purpose of
reporting meter data from individual utility consumers.
Usually, such calls are scheduled for off-peak times, so
as not disturb the customer's use of the telephone for
voice cornmunications. In the event, however, that a
customer call is initiated during a meter data reporting
communication, it is desirable to have an off-hook
detection circuit which will prevent a conflict between
these activities.
.
2124~
The invention will enable one to provide an off-
hook detection circuit with low power requirements for
automatic meter reading equipment.
The invention will also enable one to provide an
off-hook detection circuit for automatic meter reading
equipment that is usable with telephone e~uipment made
to different specifications by various manufacturers.
The invention relates to an off-hook detection
circuit for an automatic meter reading (AMR) device to
be located at a customer site. The invention detects
the off-hook state of a phone during the period the AMR
is operating on the telephone line. The circuit
functions by sensing a change in loop current in the
telephone equipment and generating an appropriate signal
pulse to a microelectronic processor in the AMR unit.
The circuit includes inputs for connection to the
telephone circuit and an open-collector output for -
transmitting a logic-level DC signal recognizable by a
microelectronic processor. An optical isolation circuit
is connected to receive the loop current from the
telephone circuit. The optical isolation circuit has a
detecting portion for producing a corresponding signal
on an opposite side of an isolation interface from the
telephone circuit. A solid state switch is connected
across the detecting portion of the optical isolation
circuit, the solid state switch producing a voltage in
response to an increase in the loop current. A ;
comparator is responsive to the voltage to control the
means for transmitting the logic-level DC signal to the
microelectronic processor in response to a change in the
loop current in the telephone circuit.
The invention is advantageous in providing a
circuit with low power requirements, because the circuit
is operated only when the phone receiver is off its
2 ~
hook. The invention provides a circuit that is operable
for telephone loop currents over a rather substantial
range from 20 milliamps to 90 milliamps.
The invention is also operable across an isolation
interface which is desirable for isolating the phone
equipment from the control electronics of the AMR unit.
In drawings which illustrae a preferred embodiment
of the invention:
Fig. 1 is a schematic diagram of a preferred
embodiment for practicing the present invention; and
~ ig. 2 is a detail sketch of voltage vs. time
waveform at junction V2 in Fig. 1.
Referring to Fig. 1, the present invention is
embodied n an off-hook detection circuit 10. The
circuit 10 may be included in an automatic meter reading
(AMR) unit of the type disclosed in Thornborough et al.,
U.S. Patent No. 5,031,209. The circuit 10 is connected
to a telephone (not shown) in series with an off-hook
~elay (not shown) thro~lgh input terminals 11 and 12.
20 This establishes a loop current IL flowing from terminal ~-
11 to terminal 12 through resistor Rl. With a circuit
10 as described herein, this loop current may range from
20 milliamps to 90 milliamps. The circuit is also
designed to operate with house phone impedances of less
than 1000 ohms.
The ~low of loop current IL causes current to flow
to bi-directional optical isolator circuit ISO 1, which
may be a PC733H circuit available from Sharp. The
circuit ISO 1 has light emitting diodes Dl, D2,
connected in opposite directions and in parallel, for
handling current ~low in either direction. Optic
emitters Dl, D2 are also connected in series with a 62-
ohm resistor R2, and this combination is connected in
parallel with a 47-ohm resistor Rl. This provides for
--" 212~10~
bi-directional operation (independent of the polarity of
"tip" and "ring" connections to terminals 11 and 12).
The circuit ISO 1 also has an optically sensitive NPN
transistor Ql which produces current in response to
light represented by the arrows in Fig. 1. The
collector of transistor Ql is pulled up to a logic high
voltage of +3.6 DC volts. The optical isolator circuit
provides physical and electrical isolation between the
loop current IL of the ring circuit, and signal Vout to
the microcomputer (not shown).
The emitter of transistor Ql is connected to a gate
(source) of a field effect transistor FRT 1, which in
P/N 2N7002, available from Siliconix. A first drain of
F~T 1 is connected to the base of transistor Ql and a
second drain of FETl is connected to ground through a
resistor R3 of 15 kilo-ohms.
The emitter of transistor Ql is further connected
at junction Vl to an inverting (-) input of an
operational amplifier OA 1 configured to operate as a
comparator. Operational amplifier OA 1 is preferably an
LM2902 op amp available from Motorola. Resistor R5 of
2.2 meg-ohms is connected from the output back to the
inverting (-) input of op amp OA 1. The emitter of
transistor Ql is also connected through junction Vl to
the noninverting (+) input of the op amp OA 1, however
this branch of the circuit has a 0.10 ~if capacitor C2
and a 4.7 meg-ohm resistor R4 connected in parallel
between the noninverting input (+) and ground. This
produces a delay at the noninverting input (+) to
changes in voltage at junction Vl.
The output of op amp OA 1 is connected through an
RC filter consisting of 10 kilo-ohm resistor R6 and 1.0
~f capacitor Cl to the base of NPN transistor Q2, which
is arranged in the open collector configuration. The
-` 2 1 2 ~
collector of Q2 is pulled up to the DC supply of ~3.6
volts through resistor R7. Logic level voltage VOut is
produced at the collector and applied to an interrupt
(INT) input of a microcomputer (not shown) in the AMR
unit.
The AMR unit operates in the following manner. The
AMR goes off-hook and listens for a dial tone for (2)
secon~s. During this time, the loop current IL is
established. Interrupt control of the AMR unit is
enabled after establishment of the loop reference level.
If no interrupt occurs, the AMR unit dials the utility
control center (UCC) and communicates through a modem in
the AMR unit.
When loop current IL is established (only when the
telephone is off-hook), a corresponding current (i) is
established through transistor Ql. The base circuit of
transistor Ql is controlled by FET 1, and initially Ql
is "on" and FET 1 iS l'off.'l An increase in loop
current I~ causes the FET to turn on and clamp the base
circuit of transistor Ql to ground which reduces the
base bias voltage. The dual input differential
amplifier OP AMP 1 monitors the FET gate voltage. One
input is connected directly to the gate. The other
input connected to an R-C delay circuit.
The establishment of loop current IL causes current
through Ql, which in turn produces voltage at gate
(source) of FET 1. This also charges capacitor C2
through resistor R8 of 47 kilo-ohms and places voltage
on noninverting (+) equal to the inverting (-) input,
which holds output V2 at zero volts. At some threshold
of increasing IL, FET 1 is turned on and current is
conducted through FET 1 to ground, which pulls base of
Ql low to ground. This turns off Ql. AS the loop
current decreases, during a phone off-hook condition, so
21241 0~
does the gate voltage and the level at the (-) input of
amplifier. The (~) input of amplifier decreases more
slowly due to its R-C network, therefore, with a low
level at the t-) input and high level at the (+) input,
the amplifier output will go to a high state until the
R-C voltage reaches the same level as the (-) input.
This produces a pulse as seen in Fig. 2. The
output V2 rises to a level of approximately 2.5 volts.
After a time as the R-C voltage decays, the op amp OA 1
will switch off again to create the trailing edge of the
pulse 15. This type of pulse switches Q2 "on." Q2 is
utilized to adjust signal level to a proper logic signal
level (VOut) for input to a microcomputer (not shown).
The circuit 10 described above is advantageous in
establishing loop current (i). The circuit 10 is
advantageous in responding to currents in the range of
20 milliamps to 90 milliamps. (State other advantages
here.)
This has been a description of an of how the ~ ~
20 invention can be carried out. Those of ordinary skill in ~;
the art will recognize that various details may be
modified in arriving at other detailed embodiments, and
these embodiments will come within the scope of the
invention.
The embodiments in which an exclusive property or
privilege is claimed are defined as follows:
~'1'''""'"' ;
