Wireless Communication via Mobile Phone Using DTMF

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Wireless Control via Mobile Communication
Using of DTMF signals

What is DTMF signal ?
 The 12 keys on a cell phone ( 0,1,...,8,9,*,#)
has unique signal associated with itself. This
is DTMF signal.
 When the call is on, the pressing of any
numerical key leads to generation of DTMF
signal which is audible on the other side.

DTMF TONE !!!
o Take any Nokia cell phone (others I haven't tested), turn on
keypad tones, press any key, the tone your hear is DTMF
tone.
Note: Don't confuse keypad tones with the tone that is heard on other side
while call is on. The tone heard on other side(while call is on) isDTMF
tone. Keypad tones can be different than DTMF tones (as in Samsung
that use Sa Re Ga Ma ... tone). It is just Nokia mobile manufacturer that
uses DTMF tones for keypad tones also.

How are DTMF tone generated?
The DTMF tone for each key is sum of two sinusoidal waves of frequencies
as given in following table. Thus each key has unique frequency pair and
thus unique DTMF tone. For example, DTMF tone for key 6 is sum of two
sunusodial waves of frequency 1477 Hz and 770 Hz.
The extra keys A,B,C and D are not present on cell phone. They are actually specially
used for special purposes. For example, public payphones that accept credit cards use
these additional codes to send the information from the magnetic strip.
Don't worry if you don't understand this part
!!!
1209H
z
1336H
z
1477H
z
1633H
z
697Hz 1 2 3 A
770Hz 4 5 6 B
852Hz 7 8 9 C
941Hz * 0 # D

Taking Input and Output from mobile…
Output can be taken from speaker ( that is near
to your ear while calling) and using microphone
to convert sound waves to electrical signals......
Complex Work….
The other way is through earphone jack.

3.5 mm Earphone Jack
Ground wire
One wire comes from the ground
terminal of the jack that is connected to
common ground of the complete circuit.
The other wire can be attached to any
one of two signal terminals.

DTMF Signal through cellular
NETWORK
The DTMF signals from source to
destination follow the same path
as that normal voice would have as
you normally talk on mobile
passing through many base stations
and even satellites in case of large
distances.

What do really mean when you say
WORD "SIGNAL" ?
It may confuse you or make your concepts more clear. You can
omit this topic.
 Most people interpret signal as varying voltage. But this is not only the
case. Signal should be seen as piece of information and it may be
present in any form. It can be varying voltage ( as most of the cases
are ), varying currents, the pressure wave or sound wave ( audible
voice signals ), electromagnetic signals ( optical fibres ), etc
 These forms are interchangeable to each other by use of some device.
For
example, microphone converts sound wave to voltage wave and
speaker the vice versa. LED converts voltage signal to
electromagnetic form. Photodiode converts light(electromagnetic
signals) to varying current signals.
 DTMF signal doesn't means it is electrical signal or audible
voice signal( as intuition may guess ). It is combination of
two sinusoidal waves and sinusoidal waves may be present
in any form. Infact the DTMF signal starts with electrical
form on transmitter mobile, then encoded on
electromagnetic wave, then again converted to electrical
wave by receiver mobile and then to sound signal which is
audible.

8870 DTMF DECODER
It is an IC that takes DTMF signal as input and
decodes it and shows the corresponding key as
four bit output.

8870 Circuit Diagram

8870 Functional Table
TOW( or TOE ) pin is enable pin
which make the IC active when
it is high. Note that in the circuit
diagram T0E(pin10) is connected
to 5 V (high). FLOW and FHIGH
represent the unique frequency
pair for each key.
You only need to know the
output 4 bit binary code for each
key. Note that output for key '0'
is not '0000'.

Connecting things together. ..
NOTE: Only lines carrying signal shown. Power supply, ground, clock, etc.

Points to note !!!
o You can use any mobile as transmitter.
o For receiver mobile, use mobile which has auto
answer capability ( most cell phones have ). I suggest
to get mobile with 3.5 mm jack( that one that is
available on Nokia Music Express series and on
desktop computer or laptop). The reason being the
easy availability of its connector.T

Points to note !!!
o For other type of earphone jack use converters.

Points to note !!!
o For initial debugging and testing, you may connect four
bit output of 8870 IC to 7-Segment LED display through
7447 Decoder.

Conclusion
o One mobile is in your hand (cell 1) and other on the bot (cell
2). Cell 2 should be set to auto answering mode so that cell
2 automatically picks up the call. The earphone plug, 8870
DTMF decoder is connected to cell 2. You make a call from
cell 1 to cell 2. Cell 2 being in auto- answering mode picks up
the call. Now keypad of cell 1 acts as remote. As any key is
pressed on cell 1, the corresponding 4-bit output appears on
output pins of 8870 which can be further used.

IVRS
Now you have basic idea about how mobile can be manipulated
as remote. Here signals were sent only from one mobile to other.
Can you think of such things like other mobile responding by
sending back voice signal when some key is pressed. I mean
developing IVRS. Of course it requires some more knowledge and
thinking. You have learnt taking output from mobile. Find a way to
give it an input. And think from where input to mobile will come.
All others things, explore !!!

Thank you

Wireless Communication via Mobile Phone Using DTMF

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