LINE USE INDICATOR FOR TELEPHONE SETS SHARING A SINGLE LINE

INDICATEUR D'UTILISATION DE LIGNE POUR TELEPHONES PARTAGEANT UNE MEME LIGNE

English Abstract

LINE USE INDICATOR FOR TELEPHONE SETS SHARING A SINGLE LINE
Abstract of the Disclosure
Where several telephone sets are connected to a single
telephone line, it would be convenient to have a visual indication
that the line is in use, to avoid lifting a handset to see if there
is a conversation, for example. A circuit is installed across the
telephone line, the circuit having a voltage divider connected
between Tip and Ring, a first transistor with its base connected to
the center of the voltage divider, a second transistor with its base
connected to the collector of the first transistor and a third
transistor having its base connected to the emitter of the second
transistor, with a light emitting device connected between either Tip
or Ring conductor and the collectors of the second and third
transistors. The light emitting device can be a light emitting
diode. An optical coupler cooperative with the light emitting device
can be used to turn on power to light emitting devices in all the
telephones, from a separate power source.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A line use indicator for telephone sets sharing a
single telephone line, comprising a circuit installed across the line,
between Tip and Ring conductors of the line, the circuit including a
voltage divider connected between said Tip and Ring conductors, a
first transistor having its base connected to the center of the
voltage divider, a second transistor having its base connected to the
collector of the first transistor, a third transistor having its base
connected to the emitter of the second transistor, and a light
emitting device connected between one of said Tip and Ring conductors
and the collectors of said second and third transistors, the emitters
of said first and third transistors being connected to the other of
said Tip and Ring conductors.
2. A line use indicator as claimed in claim 1,
including an optical coupler, said optical coupler including said
light emitting device, a detector and a switch, said switch switching
on a power supply on actuation of said detector by said light emitting
device.
3. A line use indicator as claimed in claim 2, in
which said light emitting device is a light emitting diode.
4. A line use indicator as claimed in claim 2, said
switch switching power to a light emitting device in each telephone
set sharing said single telephone line.
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5. A line use indicator as claimed in claim 4, the
light emitting device in each telephone set being a light emitting
diode.
6. A line use indicator as claimed in claim 5, said
switch switching an AC power supply, and including a diode and a
capacitor connected to said switch to provide a DC power supply for
said light emitting devices.
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Description

This invention relates to a line use indicator, and in
particular to a circuit which will give an indication when a
telephone line is in use, without listening. Listening to see if a
line is in use interrupts a conversation.
The situation arises that a single telephone line is
shared by two or more telephones - usually single line sets. A
typical situation is one where a telephone system in an office, store
on other premises becomes overloaded, with insufficient lines for the
number of telephones required. It then happens that, for example,
two people each have a telephone, plus also for example~ one or more
secretaries each with a telephone, are all connected to a common
single telephone line. A similar situation can be deliberately
produced if individual use of telephones is very limited, to provide
a more economic system.
However, it is desirable that it be possible to tell if
the common line is being used by any of th~ telephone sets connected
to it. The present inven~ion provides for a visual indicator, such
as a lamp, to be lit when the line is in use. When the line is not
being used, the lamp is not lit.
Broadly, the invention comprises a circuit, installed
across the line, the circuit having a voltage divider, a first
transistor having its base connected to the center of the voltage
divider, a second transistor having its base connected to the
collector of the first transistor, a third transistor having its base
connected to the emitter of the second transistor and a light
emitting device connected between one of the telephone conductors and
the collectors of the second and third transistors. The light
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emitting device can be a light emitting diode. The liyht emitting
device can be optically coupled to a detector which can be caused to
turn on power to light emitting devices powered from a separate power
s~lpply .
The invention will be readily understood by the
following description in conjunction with the accompanying drawing of
a typical circuit.
As illustrated in the drawing, the Tip and Ring
conductors of a telephone line are indicated at 10 and 11
respectively. A polarity guard is provided, consisting of the
diodes, D1, D2, D3 and D4. A capacitor C1 acts to prevent the
circuit from operating during line ringing and resistor Rl and diode
D6 act as a protective circuit in case of excessive voltage, for
example a lightening strike.
The line voltage appears across the two resistors R2
and R3, which act as a voltage divider, the center point being
connected to the base of transistor Q1. When the line voltage is
high, as when the telephone line is not in use, the voltage at the
base of Q1 is sufficient to cause Q1 to conduct. With Ql conducting
the voltage at the base of Q2 is low, preventing Q2 and Q3 from
conducting. The emitter of Q2 is connected to the base of Q3 and the
collectors of Q2 and Q3 are connected. A light emitting device~
indicated at D5, is connected between the Tip conductor 10 and the
collectors of Q2 and Q3. The light emitting device D5 can be a
discrete device operating from a circuit located in each telephone
set. This has the disadvantage of requiring sufficient series
resistance with the device to prevent excessive current drain on the
line. The device will not be very bright, especially on long lines.

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An alternative is to use the light emitting device,
which could be a light emitting diode, in an optical coupler. This
is illustrated in the drawing. The device D5 is coupled to a
detector Q4 ~hich controls a switch Q5 to turn on power from a
separate power supply indicated at 13. With such an arrangement, it
is possible to use one circuit to control lighted indicators in more
than one telephone set. The connections to further telephone sets
are indicated by conductors 14a to 14f.
With the device D5 being a light emitting diode, the
switch Q5 can be a triac. As the power source at 13 is 10 volts AC,
diode D7 and capacitor C2 are provided to give a DC power supply for
light emitting diodes in the further telephone sets.
The arrangement is extremely simple. As stated, with
no telephone sets in use, the voltage at the base of transistor Q1 is
sufficient for it to be conducting, with transistors Q2 and Q3
non-conducting. If any telephone set connected to a line goes "off
hook", the voltage in the line drops, and the voltage at the base of
the transistor Q1 is lnsufficient to maintain the conductive state.
With Q1 non-conduckive, the voltage at the base of Q2 becomes
sufficient to make Q2 conductive and hence also Q3. This causes the
device D5 to become emitting. Via the optical coupling, detector Q4
and the remaining part of the circuit, lighted indicators will be
powered in the other sets connected to the szme line.
The circuit presents a high impedance to the line when
khe line is not in use. It meets the requirement of a maximum of
10 A with up to a 100 V applied to the line. By using optical
coupling, the current drain is lower when the line is in use because

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the current requirements are lower, and by usiny the coupler to turn
on power to the light emitting devices from a separate po~ler source,
the devices can be operated at a good visual level. Also devices
requiring higher current supply, such as incandescent lamps, can be
used. The coupler isolates the line from the auxiliary power supply.
A buzzer 14, actuated by a switch 15, can be provided.
Only one circuit is required per line, although two or
three, or possibly even more telephone sets are connected to the same
line.