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Here I am introducing such
a system which does not require any radiations, any laser beam
which has no limitation of range, I mean it can be used from
any distance from meters to thousand kilometers using a simple
telephone line or mobile phone.
Here I am using a
telephone as a media, which serves main part of this system.
By using home phone as a local phone and another phone -
either landline or mobile phone as a remote phone we are
controlling devices.
Features:-
1.
You can control up to 10 devices. It may be any
electric or electronic appliances or devices with simple to
heavy appliances. Each device is given a unique code.
2.
It makes accurate
switching, any false switching of device are not done.
3.
There is no risk for
false switching.
4.
Your local phone (i.e.,
home phone or office phone) can be used for normal use by
using a DPDT switch. So you need not use a separate telephone
line for this device controlling.
5.
To perform any operations
through remote phone line, the user needs to dial to the local
telephone (to which the interfacing circuit is connected) then
the respective code of the device is dialed.
6.
This circuit does not
require any complex IC, so any one with little knowledge of
electronics can construct this circuit, because it does not
need any programmable IC's or programming.
7.
This system detects the
ringing signal from your exchange with the help of ring
detector and automatically switches ON.
8.
This device saves your
money. This circuit switches OFF after a time of 60 seconds
(you can change this switch ON-Time which is discussed in
detail in coming section).
9.
Before changing the state
of the device we can confirm the present status of the device.
10.This circuit gives an
acknowledgement tone after switching ON the devices to confirm
the status of the device.
11
You can control devices
from local telephone. It can also be controlled by PCO.
Taking a tour of the project:
This system uses Dual
Tone Multi Frequency (DTMF) technology of our telephone set.
Every telephone set will have this facility. We have two type
of dialing facilities in our telephone system (i) Pulse
dialing mode (ii) Tone dialing mode. Here this system works on
tone dialing mode. The DTMF mode is shortly called as tone
dialing mode. (Check for availability of tone dialing mode in
your telephone set).
This system
is divided into two sections 1: Remote Section 2: Local
Control Section.
1: Remote
Section:
It is nothing but remote
telephone set which is present in the remote place. This may
be your workspace (office / school) phone or mobile phone or a
phone in PCO. Signals are sent through this telephone.
2: Local
Control Section:
This is a control
system through which you can control your appliances. This
contains one telephone line and a control unit. The appliances
to be controlled must be connected to telephone line through
control unit .Control unit is kept with a sufficient backup.
WHAT IS DTMF?
When you press a
button in the telephone set keypad, a connection is made that
generates a resultant signal of two tones at the same time.
These two tones are taken from a row frequency and a column
frequency. The resultant frequency signal is called "Dual
Tone Multiple Frequency". These tones are identical and
unique.
A
DTMF
signal is the algebraic sum of two different audio
frequencies, and can be expressed as follows:
f(t) =
A0sin(2*П*fa*t)
+ B0sin(2*П*fb*t)
+ ...........
------->(1)
Where fa
and fb are two different audio frequencies with A
and B as their peak amplitudes and f as the resultant DTMF
signal. fa
belongs to the low frequency group
and fb belongs to
the high frequency group.
Each of
the low and high frequency groups comprise four frequencies
from the various keys present on the telephone keypad; two
different frequencies, one from the high frequency group and
another from the low frequency group are used to produce a
DTMF signal to represent the pressed key.
The amplitudes of
the two sine waves should be such that
(0.7 < (A/B) < 0.9)V
-------->(2)
The
frequencies are chosen such that they are not the harmonics of
each other. The frequencies associated with various keys on
the keypad are shown in figure (A).
When you send these
DTMF signals to the telephone exchange through cables, the
servers in the telephone exchange identifies these signals and
makes the connection to the person you are calling.
The row and column frequencies
are given below:
Fig (A)
When you press the digit 5 in
the keypad it generates a resultant tone signal which is made
up of frequencies 770Hz and 1336Hz. Pressing digit 8 will
produce the tone taken from tones 852Hz and 1336Hz. In both
the cases, the column frequency 1336 Hz is the same. These
signals are digital signals which are symmetrical with the
sinusoidal wave.
A Typical frequency is shown in
the figure below:
Figure (B)
Along with these
DTMF generator in our
telephone set provides a set of special purpose groups of
tones, which is normally not used in our keypad. These tones
are identified as 'A', 'B', 'C', 'D'. These frequencies have
the same column frequency but uses row frequencies given in
the table in figure (A). These tones are used for
communication signaling.
The frequency table is as
follows:
Figure (C)
Due to its accuracy and
uniqueness, these DTMF signals are used in controlling systems
using telephones. By using some DTMF generating IC’s (UM91214,
UM91214, etc) we can generate DTMF tones without depending on
the telephone set.
CIRCUIT DESCRIPTION:
This system is divided into two sections, 1: Remote Section 2:
Local Control Section.
REMOTE SECTION:
This unit consists of
telephone set which is present in the remote place. This may
be your workspace (office / school) phone or mobile phone or a
phone in PCO. Signals are sent through this telephone. The
figure (E) shows the circuit diagram of the DTMF encoder which
resembles the telephone set. It uses DTMF encoder integrated
circuit, Chip UM 91214B. This IC
produces DTMF signals. It contains four row frequencies &
three column frequencies. The pins of IC 91214 B from 12 to
14 produces high frequency column group and pins from 15 to 18
produces the low frequency row group. By pressing any key in
the keyboard corresponding DTMF signal is available in its
output pin at pin no.7. For producing the appropriate signals
it is necessary that a crystal oscillator of 3.58MHz is
connected across its pins 3 & 4 so that it makes a part of its
internal oscillator.
Figure (E). Circuit diagram of
the DTMF encoder
This encoder IC requires a voltage of 3V. For that
IC is wired around 4.5V battery. And 3V backup Vcc for this IC
is supplied by using 3.2v zener diode.
The row and column frequency of
this IC is as on the fig. "B". By pressing the number 5 in the
key pad the output tone is produced which is the resultant of
addition of two frequencies, at pin no. 13 & pin no.16 of the
IC and respective tone which represents number '5' in key pad
is produced at pin no.7 of the IC . This signal is sent to the
local control system through telephone line via exchange.
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