-= H A C K E R S =-

                           Issue #8,  File #4 of 9

***************************************************************************
                      D T M F   S C A N N E R
***************************************************************************

                                  By:  Jojo

This article gives descriptions and plans for building a small electronic
circuit that reads digits dialed on a phone line. I made it all up myself
so u won't find anything about it in books; it works fine, I sold a lot
of these devices to friends.

If u already have experience in building electronic circuits then u will
find this circuit very easy to build: two graphic files (DTMF.BMP and
DTMFPCB.BMP) give circuit diagrams *and* printed circuit board layouts :-)

If u r a beginner and want to learn about building an electronic circuit
using the printed circuit board layouts, just print the graphic file
DTMFPCB.BMP and take it to an electronic components shop. They will
tell u how easy it is to use those layouts: they can make the board
for u for less than $10, then all u'll have to do is soldier the
components on it.

If u just want to know more about how a digit is dialed on a phone line
then read the "how does it work?" part.




W H A T ' S   A   D T M F   S C A N N E R   ? :
-----------------------------------------------

It's an electronic device that connects to a phone line and reads the
digits dialed on it. DTMF stands for: Dialing Tones using Multiple (or
Modulated) Frequencies. The scanner described here can be used in 2
different ways:

1.      PHONE LINE --> DTMF SCANNER --> YOUR EYES
        The scanner has LEDs (Light Emitting Diodes) on it; when a
        digit is dialed on the phone line, some of these LEDs will
        light, others won't, depending on which digit is dialed. So
        you can "read" the digits just by looking at the LEDs :)))

2.      PHONE LINE --> DTMF SCANNER --> COMPUTER
        The scanner can also be connected to the Centronics interface
        of a computer: the centronics is the 25-pins connector at the
        back of your computer that is usually used for connecting a
        printer.

        The computer then uses a little program to read the signals
        received on the Centronics interface and display the digits
        dialed on the phone line. These digits can be saved in a file.

        NOTE: this scanner uses special electronic components called
        opto-isolators that ELECTRICALLY SEPARATE the signals on the
        computer side from the signals produced on the scanner side.
        That means that your computer CAN *NOT* be damaged even if
        the scanner goes electrically "mad". You could even connect
        1000 Volts to the scanner's input, the output on the computer
        side would still be held at a standard low voltage!



W H A T ' S   T H E   G O O D   T H I N G   A B O U T   T H I S
D T M F   S C A N N E R ?
---------------------------------------------------------------

Some people use sound cards (like SoundBlaster) to scan digits
dialed on a phone line. The DTMF scanner can be more usefull
cuz it's a *small* device that u can carry in ur pocket :) :) :)

It uses low-cost components: the whole thing costs less than $20.




T H E   C I R C U I T   D I A G R A M :
---------------------------------------

UUencoded graphic file (DTMF.BMP):

[ Section: 1/1  File: dtmf.zip  Encoder: Wincode v1.4 ]
Original Input File Size: 11647
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`
end
[ Section: 1/1  File: dtmf.zip  Encoder: Wincode v1.4 ]

How does it work?

A digit dialed on a phone line is a simple sound containing
two special frequencies:
        1 --->  697Hz & 1209Hz
        2       697     1336
        3       697     1477

        4       770     1209
        5       770     1336
        6       770     1477

        7       852     1209
        8       852     1336
        9       852     1477

        *       941     1209
        0       941     1336
        #       941     1477

The circuit used here uses 5 cells with a NE567 device and a
LED in each cell. Each cell will be adjusted for detecting one
of the special frequencies above in the sound carried on the phone
line. When the cell detects the frequency it LIGHTS THE LED.

The uA741 part: it's here to fix the level of the sound (like an
amplifier) that is sent to the 5 NE567 cells. At first just adjust
the 100K variable resistor at half its way.

Use the variable resistors (220K) to fix the frequency detected
on each of the 5 cells:
        cell 1 has to light ONLY IF digits 1 or 2 or 3 are dialed
        cell 2 has to light ONLY IF digits 4 or 5 or 6 are dialed
        cell 3                             7 or 8 or 9
        cell 4                             1 or 4 or 7 or *
        cell 5                             2 or 5 or 8 or 0

So cell 1 will detect frequency 697Hz
        2                       770
        3                       852
        4                       1209
        5                       1336

You can then know what digit is dialed simply by looking at the
LED or LEDs which are on:
        LED1 & LED4 ---> 697Hz & 1209Hz ---> digit=1
           1      5      697     1336              2
           1             697     1447              3
           2      4      770     1209              4
           2      5      770     1336              5
           2             770     1447              6
           3      4      852     1209              7
           3      5      852     1336              8
           3             852     1447              9
                  4      941     1209              *
                  5      941     1336              0

NOTE: if u find that too many LEDs are on then it means that the uA741
      part delivers too powerfull sounds. If u find that too few (or none)
      LEDs are on, then it means that the uA741 part delivers sounds at
      a too low level. All u have to do is fix the 100K variable resistor
      to get a good working sound level delivered to the 5 cells.

The outputs OUT1...5: these r used if u want to connect ur scanner to
a computer's centronic interface. When LED1 is on, OUT1 is at the low TTL
level (0V). When LED1 is off, OUT1 is at the high TTL level (+5V). LED2..5
and OUT2..4 work in the same way.

If u r using these outputs then u must have D0=+5V so that the "TIL" chips
(these r the opto-isolators) can work. D0 is the Data bit0 output of the
centronics interface, so ur program on the computer must output a "1" on
this pin to take it to +5V.




T H E   P R I N T E D   C I R C U I T   B O A R D :
---------------------------------------------------

The PCB construction plans (tracks, pads, components layout) used for
building it on a printed circuit board is contained in a file named
DTMFPCB.ZIP: it contains a high resolution graphic file that you can
print on a laser printer.




U S I N G   I T   W I T H   Y O U R   C O M P U T E R :
-------------------------------------------------------

If you want to use your computer for reading the digits, u have to
write a little program that reads the 5 Centronics interface inputs
(pins 10,11,12,13 and 15 connected to the scanner circuit).

The electrical state of the Centronics inputs are usually at port addr
379h: the byte read at this addr has bit7=NOT(pin11_of_the_Centronics),
bit6=pin10, bit5=pin12, bit4=pin13, bit3=pin15. All of these 5 pins are
connected to the OUT1..5 of the scanner circuit, so u just have to read
the byte at addr 379h to know which LEDs are on.

Reading the byte of the centronics inputs can be done using instructions
like "INPORTB" in C or "BYTE=INP(&H379)" in QBASIC.


***************************************************************************
       [ Jojo (Jojo on Brinta BBS / email an617527@anon.penet.fi)]
***************************************************************************
*   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *