Telephone ringing module

A multiple phone ring boost circuit rings multiple subscriber telephones synchronized with a ring signal sent from a telephone company central office is connected to multiple subscriber telephones. A local generator creates a local ring signal, and a local generator sensing circuit determines if the local generator is functioning properly. A ring sensing circuit senses if a ring signal is being received. A switch connects the line leads to the subscriber leads in a first position, and connects the local generator to the subscriber leads in a second position. A control circuit varies the switch between the second and first positions. When the ring sensing circuit detects an incoming ring signal and the local generator sensing circuit detects a properly functioning local generator, the switch is placed in its second position. When the ring signal pauses, the control circuit places the switch in the first position. When a receiver of a subscriber telephone set is lifted off its switch hook, an increased load is placed on the local generator causing the control circuit to switch back to the first position.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to telephone communication circuitry, and 
more particularly to circuitry designed for boosting an incoming ringing 
signal to ring several telephones on the same line. 
2. Brief Description of the Prior Art 
Telephone signals are sent from the telephone company's central switching 
office over the central switching office lines to a subscriber's 
telephone. The subscriber may have one or more telephones connected to a 
single line. Each telephone introduces additional resistance to the 
subscriber line. As a telephone ringing signal is sent from the telephone 
company's central switching office over the central switching office lines 
to the subscriber lines, its amplitude is diminished as it passes through 
the ringing apparatus of each telephone on the subscriber line. As more 
telephones are added, a point is reached where the voltage from the 
central switching office is no longer powerful enough to trigger the 
ringing mechanism of the telephones. When this happens, the telephones 
fail to ring even though a ring signal is being sent from the central 
switching office. The subscriber is therefore not aware that he is being 
called. 
Several devices have been made to resolve this problem. There are those 
devices which process the incoming signal and send this processed signal 
to the subscriber line, and those devices which generate their own ringing 
signal locally and send this signal to the subscriber line. 
U.S. Pat. Nos. 3,746,795, 3,763,320, 3,781,480, and 4,500,844 disclose 
devices which process the incoming signal from the central switching 
office. Each employs a different type of modulation and amplification to 
boost the signal received. This boosted signal is output to the subscriber 
line. The object of each of those devices is to amplify the signal such 
that the ring signal is great enough to drive numerous telephones on the 
subscriber line. 
In the process of amplifying the signal sent from the central switching 
office, noise inherent in the signal is also amplified. The modulation 
schemes employed by each device are aimed at minimizing the noise in the 
signal passed onto the subscriber line. 
The second group of devices, such as disclosed in U.S. Pat. Nos. 4,276,448 
("Embree"), No. 4,723,275 ("Hirth"), and No. 4,827,503 ("Takado") create a 
locally generated, high amplitude, ring signal which is passed on to the 
subscriber line. 
Hirth shows a method of switching between speech and ring circuitry, a 
sensing means to sense when the ring signal from the central switching 
office is being received, and a local ring signal generating means. The 
Hirth device has a sensing means to determine if the local ring signal 
generator is functioning, but does not pass the ring signal from the 
central switching office on to the subscriber line in the event of a ring 
signal generator failure. The Hirth device is driven by a microprocessor. 
Embree discloses a device which senses a ring signal and generates its own 
local ring signal. This ring signal can then be applied to the subscriber 
line. Embree has the capability of driving multiple ringers on the same 
subscriber line and producing multiple tones as a ring signal. The device 
also has circuitry to differentiate between dialing pulses and voice 
signals, as well as circuitry to make the device insensitive to line 
voltage variations. Embree discloses a device which can only drive an 
electro-acoustic transducer. 
Takado discloses a ring signal generator which can be switched onto the 
subscriber line. Takado does not have a sensing circuit to determine if 
the central switching office is sending a ring signal. Takado discloses 
essentially a ring signal generator which is not responsive to an incoming 
ring signal from the central switching office, and must be manually 
switched onto the subscriber line. 
U.S. Pat. No. 4,174,467 ("Ferrieu") discloses a device that senses the 
incoming ring signal from the central switching office, and uses this 
signal to synchronize a local ring generator. This local ring generator is 
not switched on-line, but inductively creates a ring signal on the 
subscriber line. Ferrieu monitors the incoming ring signal and creates a 
control signal which drives switches in the local ring signal generator at 
roughly twice the frequency of the incoming signal, to synthesize size a 
ring signal. This control signal is produced by means of a delta modulator 
sampling the incoming ring signal. 
Since the central switching office line usually travels a great distance, 
it is prone to high power interference, such as a lightening strike. This 
central switching office line eventually ends at one of the telephones in 
the subscriber lines, introducing a possibility of passing the power 
through the telephone lines and injuring persons or equipment on the 
subscriber line. Electronic isolation means are provided in some devices 
to protect persons from injury and damage to equipment. U.S. Pat. No. 
4,741,031 ("Grandstaff") discloses one such device. These isolation 
devices are designed to transmit up to a maximum power defined by the 
design of the device. Any power above the maximum is simply dissipated 
through a ground. 
There is a need for a simple and inexpensive device which is capable of 
boosting the ring signal from the telephone company to a level which is 
high enough to ring several telephones on the same line. There is also a 
need for a device which boosts the ring signal on the subscriber line 
which can use a variety of differing types of local generators. This would 
allow the device to be adapted to ring differing types of telephones. 
SUMMARY OF THE INVENTION 
The present invention provides an improved multiple phone ring boost 
circuit. This boost circuit includes a ring signal sensing circuit which 
monitors at least one incoming telephone line for a ring signal being sent 
from a telephone company, and also includes a local generator for 
producing a signal for ringing at least one subscriber telephone on a 
subscriber line. Each subscriber telephone has a switch hook having a 
connect state and a disconnect state. The ring signal sensing circuit 
employs a pair of capacitors connected to the incoming telephone line for 
splitting off the AC component of the signal, and blocking the DC 
component. The AC component is rectified and then fed to an opto-isolator 
which isolates the signal, producing a DC voltage. This DC voltage is fed 
into a comparator and compared to a preset voltage. The comparator 
produces a two-state output, the first state being produced when a ring 
signal is received on the incoming telephone lines, and a second state 
when there is no signal being received. 
The boost circuit employs a local generator which can produce a first 
trigger signal similar to that sent by the telephone company, or a second 
trigger signal depending on the load placed on the local generator. A 
local generator sensing circuit similar to the ring signal sensing 
circuit, having a two-state output, monitors the output of the local 
generator producing a first state output when the local generator produces 
a first trigger signal, and a second state output when the local generator 
produces a second trigger signal. A switching means capable of being 
switched between two positions by an external circuit connects the 
incoming telephone line to the subscriber line in the first position, and 
connects the local generator to the subscriber line in the second 
position. 
When the local generator is producing the first trigger signal, and is 
functioning under a normal load, the local generator sensing circuit 
produces the first state output. When a ring signal is sent over the 
incoming telephone line, the ring signal sensing circuit produces the 
second state output. A control circuit monitors the output state of the 
ring signal sensing circuit and the local generator sensing circuit. When 
the ring signal sensing circuit and the local generator sensing circuit 
both produce a first state output, the control circuit triggers the 
switching means into the second position connecting the local generator to 
the subscriber line. In the event that either the ring signal sensing 
circuit or the local generator sensing circuit produce a second state 
output, the switching means is switched to the first position connecting 
the incoming telephone line to the subscriber line. The local generator is 
switched off-line when the ring signal pauses causing synchronization of 
the local generator trigger signal and the ring signal received on the 
incoming telephone line. 
In the event that a subscriber picks up one of the subscriber telephone 
receivers off its hook while it is ringing and the switch hook goes from 
disconnect state to the connect state, the resistance of the subscriber 
line drops, causing a load to be placed on the local generator, causing 
the second trigger signal to be produced by the local generator. The local 
generator sensing circuit detects the second trigger signal and produces a 
second state output. The control circuit switches the switching means to 
the first position. 
Both the ring sensing circuit and the local generator sensing circuit 
employ opto-isolators which limit the maximum voltage which can be sent 
through the circuit. This protects persons and equipment on the subscriber 
line.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings in detail, wherein like reference numerals 
indicate like elements in each of the several views, reference is first 
made to FIG. 1, wherein a multiple phone ring boost circuit 10 according 
to the present invention is illustrated in a simplified block diagram. 
A telephone signal on an incoming telephone line from a telephone company 
central office 12 having an AC and DC component, enters the boost circuit 
10 through "tip" and "ring" line leads 20. A ring signal sensing circuit 
22 is connected to the line leads 20 and monitors the incoming telephone 
signal for a ring signal. The ring signal sensing circuit 22 is capable of 
creating a two-state output, the first state pertaining to a `high` signal 
and the second state pertaining to a `low` signal. The line leads 20 also 
connect to a voice signal rectifier 24. The voice signal rectifier 24 
rectifies the incoming telephone signal blocking the ring signal and 
passing the remainder of the signal, called a rectified signal, to a 
switching means 26. The rectified signal is a varying intensity DC signal 
which is compatible with subscriber telephones 32, 34. A local generator 
30 produces a first trigger signal which is a continuous oscillating ring 
signal when the generator 30 is operating under a normal load, and a 
second trigger signal of reduced amplitude when operating under increased 
load, such as when the resistance across output leads 80, 81 is reduced 
significantly. The output leads 80, 81 are connected to the switching 
means 26. 
A local generator sensing circuit 40 monitors the local generator load. The 
amplitude of the local generator sensing circuit 40 monitors the local 
generator 30 to determine if it is functioning correctly. The local 
generator sensing circuit 40 is capable of creating a two-state output the 
first state being `high` and the second state being `low`. If the local 
generator 30 is producing a first trigger signal, the local generator 
sensing circuit 40 will output a `high` signal. 
The output of the ring sensing circuit 22 and the local generator sensing 
circuit 40 are both fed into a switch control circuit 42. The control 
circuit 42 monitors the output of the ring sensing circuit 22 and the 
local generator sensing circuit 40. When both of these outputs are `high`, 
indicating that a ring signal is being sent from the telephone company 12 
and that the local generator 30 is producing a first trigger signal, the 
control circuit 42 activates the switching means 26 to a second position 
connecting the local generator 30 to subscriber leads 44. This effectively 
puts the local generator 30 on-line ringing telephones 32, 34 which are 
attached to the subscriber leads 44. 
When either the output of the ring sensing circuit 22 or the local 
generator sensing circuit 40 is `low`, or both are `low`, the control 
circuit 42 allows the switching means 26 to remain in a first position 
connecting line leads 20 through the voice signal rectifier 24 to the 
subscriber leads 44. This effectively allows a voice signal to be passed 
from the incoming line leads 20, through the voice signal rectifier 24, 
through the switching means 26, to the subscriber leads 44, and the 
telephones 32, 34, connected to the subscriber leads 44. This situation 
occurs between rings sent from the telephone company 12, or when the local 
generator 30 is not producing a first trigger signal. When one of the 
subscriber telephone receivers 33 or 35 is lifted off its switch hook 37 
or 39, the switch hook 37 or 39 goes from an "on-hook" or disconnect state 
to an "off-hook" or connect state, and the resistance across subscriber 
leads 44 drops significantly. 
When local generator 30 is connected to subscriber leads 44, and a 
subscriber telephone receiver 33 or 35 is lifted off its switch hook 37 or 
39, a load is placed on local generator 30 causing it to produce the 
second trigger signal. This causes the control circuit 42 to switch 
switching means 26 to the first position. Only two telephones 32, 34 are 
shown in FIG. 1, but the number of telephones is only limited by the 
strength of the output of the local generator 30. The multiple phone ring 
boost circuit 10 therefore produces a trigger signal synchronized with the 
telephone company central office 12 ring signal that is responsive to 
lifting one of the subscriber telephone receivers 33 or 35 off their 
respective switch hooks 37 or 39 taking the local generator 30 off-line. 
FIG. 2 shows a more detailed schematic diagram of the multiple phone ring 
boost circuit 10 according to the present invention shown in FIG. 1. The 
incoming signal from the telephone company 12 enters through line leads 
20. After entering ring signal sensing circuit 22, the AC component of the 
signal is split off through a pair of capacitors 45 and 46. 
The incoming signal is rectified by a signal rectifier 49 and then fed to 
an optical transistor 50 which produces a differential signal being the 
difference in voltage of the signal across the outputs of signal rectifier 
49. The optical transistor 50 isolates the line leads 20 from the 
subscriber leads 44 thereby protecting persons and equipment connected to 
the subscriber leads 44. An output of the optical transistor 50 passes to 
a first input 70 of a comparator 56 capable of creating a two-state 
output, the first state being `high` and the second state being `low`. 
A second input to the comparator 56 is an output lead 58 of a variable 
resistor 59 which is held between 5 volts and ground. The purpose of the 
variable resistor 59 is to provide for manual adjustment of the 
sensitivity of the comparator 56. Variable resistor 59 acts as a trim 
resistor allowing for adjustment of the ring signal sensing circuit 22 to 
allow for detection of differing intensity ring signals from telephone 
company 12. Variable resistor 59 is typically adjusted when the multiple 
phone ring boost circuit 10 is installed, or when subscriber phones 32, 34 
are added or removed. 
The comparator 56 outputs a `high` signal on line 61 when the rectified DC 
voltage at the first input 70 of comparator 56 is greater than the 
reference voltage set by the variable resistor 59. This indicates that a 
ring signal has been received which is greater than the preset reference 
voltage determined by variable resistor 59. 
The local generator 30 is capable of creating a trigger signal similar to 
the ring signal sent by the telephone company 12 is powered from line 
current (not shown). The output of the local generator 30 is fed both into 
two input poles of a DPDT switch 67, and also through capacitors 47, 48, 
to the generator sensing circuit 40. The local generator 30 has a pair of 
output leads 80, 81, and is sensitive to the resistance placed across the 
output leads 80, 11. As the resistance drops across the output leads 80, 
81, an increased load is placed on the local generator 30 causing it to 
overdrive. When the local generator 30 is overdriven, it outputs the 
second trigger signal having a reduced amplitude and altered waveform. The 
subscriber telephones 32, 34, affect the resistance across the output 
leads 80, 81, when the local generator 30 is connected to the subscriber 
telephones 32, 34. When at least one of the receivers 33 or 35 are lifted 
off of their respective switch hook 37 or 39, the telephone 32 or 34 
exhibits a lessened resistance, causing local generator 30 to overdrive, 
and produce the second signal. 
The generator sensing circuit 40 monitors the output of the local generator 
30 in a similar manner as the ring signal sensing circuit 22 monitors the 
line leads 20, setting its output line 62 `high` when it senses a first 
trigger signal from the local generator 30, and `low` otherwise. 
In FIG. 2 is illustrated in more detail the preferred structure of the 
generator sensing circuit 40 according to the present invention. The 
generator sensing circuit 40 as shown includes the pair of capacitors 47 
and 48 in connection with a signal rectifier 110. The signal rectifier 110 
is connected to a filtering capacitor 112 which is connected to an optical 
transistor 114. The optical transistor 114 in turn is connected to a first 
input 116 of a comparator 118, preferably of the operational amplifier 
type. The comparator 118 includes a second input 120 which is in 
connection with a variable resistor 122. The output of the comparator 118 
is passed via the line 62. 
In operation of the generator sensing circuit 40, the capacitors 47 and 48 
receive the trigger signal from the local generator 30 and output the AC 
component of the trigger signals to the signal rectifier 110. The signal 
rectifier 110 rectifies this trigger signal to a DC value, and then 
outputs through the filtering capacitor 112 and to the optical transistor 
114. The optical transistor 114 isolates the ring generator 30 from the 
rest of the circuit and outputs to the first input 116 of the comparator 
118. The comparator 118 outputs a `high` signal over the line 62 when the 
rectified DC voltage at the first input 116 is greater than a reference 
voltage set by the variable resistor 122 at the second input 120; 
otherwise the comparator will output a "low" signal. The variable resistor 
122, similar to that described above in relation to the variable resistor 
59, is held between 5 volts and ground and operates as a trim resistor 
which allows adjustment of the generator sensing circuit 40 in order to 
enable detection of differing intensity trigger signals received from the 
local generator 30. The variable resistor 122 typically is adjusted when 
the telephone receivers 33, 35 are on the respective switch hooks 37, 39. 
The output line 61 of the ring signal sensing circuit 22 and the output 
line 62 of the local generator sensing circuit 40 are both fed into a 
logical "AND" gate 64. When the logical "AND" gate 64 receives a `high` 
signal on both lines 61, 62 it sets its own output line 55 `high` biasing 
the base of a switch powering transistor 65. Switch powering transistor 65 
then powers a DPDT relay 66 of the DPDT switch 67. When switched, local 
generator 30 is connected through the DPDT switch 67 to the subscriber 
leads 44, connecting the local generator 30 to the telephones 32, 34 
connected to the subscriber leads 44. 
When line 62 is `low` indicating that there is no ring signal being 
received on-line leads 20, or there is a pause between rings, the "AND" 
gate 64 sets output line 55 `low`. When the line 61 is `low` indicating 
that the local generator 30 is not functioning properly, or subscriber 
telephone receivers 33, 35 are lifted off their respective hooks 37, 39, 
the "AND" gate 64 sets line 55 `low`. In this condition, DPDT relay 66 is 
not activated and DPDT switch 67 remains in the first position, connecting 
line leads 20, through a voice signal rectifier 24, through DPDT switch 67 
to subscriber leads 44. In the first position the local generator 30 is 
not on-line with the subscriber line 44. 
The present invention may be adapted to function with multiple input lines 
from the telephone company 12, or a multiplex office line system connected 
to the subscriber leads 44 without departing from the spirit or essential 
attributes of the invention Alternative embodiments exist and reference 
should be made to the appended claims rather than to one embodiment of the 
invention.