The circuit is designed to connect in parallel with the telephone line,
to monitor and detect if any telephone in the same line is busy, with
the indication of the LED and which is self-powered so that it does not
provide any load on a telephone line.
Light Emitting Diode (LED) – a semiconductor diode that is commonly a
source of light when electric current pass through it Metal Oxide
Semiconductor Field Effect Transistor (MOSFET) – a device utilized for
switching and amplification of signals BS108 – a 250 mA and 200 Volts
small signal MOSFET designed for high voltage, high speed switching
applications such as relay drivers, CMOS logic, line drivers, TTL or
microprocessor to high voltage interface and high voltage display
drivers Diode Bridge – also known as bridge rectifier which has four
diodes arranged in a bridge configuration where the output voltage has
the same polarity with either polarity of the input voltage 1N4007 – a
general purpose plastic rectifier with reverse voltage from 50 Volts to
1000 Volts and forward current of 1.0 Ampere
When none of the telephone lines is in use or on-hook, the voltage
across the line is around 48V. In this state, the gate of transistor Q2
is shorted to its source during the conduction of Q1. This causes the
LED to be disabled while Q2 is turned OFF. When one telephone extension
along the telephone line changes to off-hook or in use condition, a
voltage drop from 5V to 15V is detected. This will in turn cause Q1 to
turn OFF because of the very low voltage across the gate of Q1 which is
equal to 6% of the line voltage. Transistor Q2 then will be biased at
around half of the line voltage.
Off-Hook Telephone Line Indicator Circuit Diagram
The sudden line drop of voltage triggers Q2 to light up the LED that
will give a sign that the line is in use. Using the same line, the
circuit is unseen with other telephone devices. A current-limiting
resistor is used to maintain the low current of LED1 while the local
telephone line parameters dictate the variation of other component’s
values. The power of the circuit is provided by the telephone line.
Other voltage protection may be used with some reliable design in
addition to the current-limiting resistor. This is important to avoid
any grounding effects from conducting surfaces within the circuit.
To ensure that transistor Q1 is fully biased while the line is free or
not in use where LED1 is OFF, a 500K ohm MOSFET trimmer is used for the
desired adjustment. A MOSFET is a three-terminal semiconductor component
with a conducting channel in its output and a built-in capacitor at its
input. To increase the values of any of the two resistors connected to
the gate of Q2, a 200V MOSFET can be used in the place of Q2 if BS108 is
not available. However, plain transistors like the bipolar junction can
be used but with lower values to allow greater currents to pass through
the line that is not in use. The bridge rectifier comprising of four
1N4007 diodes are performing the conversion of AC input into DC output.
MOSFET can function in two ways. The first is known as depletion mode
wherein the channel shows its maximum conductance in the absence of a
voltage on the gate. The second way that the MOSFET can function is
known as enhancement mode wherein the device is not conducting even in
the absence of a voltage on the gate because no channel is produced. A
channel is being created with the application of a voltage to the gate.
To generate better conductivity, greater voltage to the gate is
required.
MOSFET drivers are applied in electronic motor control for different
types of motors. Also, they are specifically used with long duty cycles,
high operating frequency above 200 KHZ, lower output power, and wide
load variations. The largest application of MOSFETs are the switched
mode power supplies and in battery charging applications. In transducer
drivers for high power devices such as light bulbs and motors, large
current output with a small input is provided by MOSFETs. Since they are
more non-linear than BJTs while producing less distortion, they can be
utilized with Hi-Fi amplifiers. In constructing integrated circuits,
MOSFETs are very useful since they can be made very compact. Although
MOSFETs can get damaged by static electricity at higher voltages, they
still provide several advantages as compared to other transistors which
include faster switching time than BJT, lower losses than BJT, very
small switching current, and least effects of temperature.
This telephone line indicator does not only tell when a telephone line
is in use if a plurality of telephones are all setup to the same
telephone line, but also prevents interruptions during personal calls.
Additionally, it can also help to prevent costly and unwanted disruption
of modem calls and fax, it alerts a person when a call is done and the
phone is free to use, and the LED light indicates the line is in use.
To avoid any injury, it is a prerequisite to take extra precautionary
measures when connecting any circuit to the telephone lines, which can
produce life-threatening voltages during normal operation. During a
lightning storm, it is better to keep distance from telephone lines..
Legal aspects are imposed in different countries for connecting things
to telephone lines. The circuit should be better built with a plug-in
cord for easy removal in case of fault occurrences. Otherwise, it would
be best to consult a licensed telephone operator.
