CA2124104A1 - Off-hook detector for automatic meter reading equipment - Google Patents

Abstract

Abstract of the Disclosure An off-hook detection circuit (10) utilizes a low-level current loop when an automatic meter reading unit is operating over a telephone line to detect a phone receiver being taken off-hook. Lifting the phone receiver changes the loop current (IL) which is detected and converted to a suitable voltage input (Vout) to a microprocessor, which then disables the meter reading operation until the phone receiver is returned to its hook.

Description

'" ' 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'''""'"' ;

Claims (2)

1. A circuit for detecting an off-hook condition for a telephone and signaling such a condition to a microelectronic processor, the circuit comprising:
inputs for connection to a telephone circuit to receive a loop current;
means for transmitting a logic-level DC signal recognizable by a microelectronic processor;
an optical isolation circuit connected to receive the loop current and having a transistor output portion for producing a corresponding signal on an opposite side of an isolation interface from the inputs;
a solid state switch connected across a base-emitter junction of the transistor output portion of the optical isolation circuit, the solid state switch controlling the base-emitter junction and providing feedback to lower the base drive current in response to a change in the loop current in the telephone circuit caused by lifting of a receiver off-hook which in turn produces an output voltage from the optical isolation circuit; and a comparator responsive to the output 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.

2. The apparatus of claim 1, wherein the solid state switch is a field-effect transistor having a gate connected to the emitter of the transistor and a drain connected to the base of the transistor.