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AMATEUR RADIO
By GARRY CRATT, VK2YBX
Build this simple DTMF
for custom control circuits
Dual Tone Multi-Frequency signalling, or DTMF as
it is commonly known, is being adopted by
amateurs to control a variety of station operations.
You can build your own DTMF encoder using just
one integrated circuit and a numeric keypad.
Dual tone multi-frequency signalling has been with us since the
development of the "Touch Tone"
dialling system, introduced in the
late 1940s by Bell Laboratories in
the USA, to replace decadic dialling
in telephone systems. Since that
time amateurs have adopted the
signalling system on an ever increasing basis to control a variety
of station operations.
While this system is used by
many dedicated groups to control
amateur repeater operations,
remote antenna switching and
beacon control, the DTMF system is
now gaining momentum in the PSTN
(Public Switched Telephone Network), as older decadic telephone
·e xchanges are being withdrawn
from service and replaced with
" tone" exchanges. Even the latest
telephone answering machines use
DTMF signalling for remote operation.
As an aside, the chances are that
your local telephone exchange has
tone dialling available. If so, and
you have a phone which is capable
of tone dialling, then you can have
this convenience. It's just a matter
of phoning your local Telecom
C1
7
i
47 ~16VW
voo
osc
+
+
..:c..
4.5V ..:C..
IN
R1
1M
C2
33pfl
IC1
5089
OUTPUT
8
CJ
33pfl
osc
OUT
vss
6
COL COL
3
2
4
5
ROW ROW ROW ROW
4
3
2
1
11 12 13 14
-:-
•
0
C4
10
+ -
16
+
Table 1
R2
1k
.,
#
DTMF SENDER
Fig.1: the circuit uses a standard TCM5089 DTMF encoder chip (IC1), together
with a numeric keypad. When a key is pressed, IC1 synthesises and mixes two
sinusoidal tones to produce a single dual tone output.
72
SILICON CHIP
business office and asking for "tone
dialling". They can change it over
very quickly and the resulting dialling is much quicker.
Decadic dial phones still work so
you can have the best of both
worlds. Anyway, back to the subject in hand.
Many amateur transceivers
available on the commercial market
incorporate a DTMF "pad" on the
microphone, allowing the transmission of these tones for control purposes. However, there are many
older transceivers in use that do not
have this facility. Would you like it
for your transceiver? You can have
it.
This article describes the construction of a self contained DTMF
generator which can be acoustically coupled to any microphone on an
amateur transceiver or telephone
handset.
A standard 12-key DTMF encoder selects one tone from a high
group of sinusoidal tones and
another from a group of low
sinusoidal tones and mixes the two
Tone
Frequency
Row 1
Row 2
Row 3
Row 4
Column
Column
Column
Column
697Hz
77GHz
852Hz
941Hz
1209Hz
1336Hz
1477Hz
1633Hz
1
2
3
4
PARTS LIST
1 TCM5089 DTMF encoder
(IC1)
1 3.579545MHz crystal,
Jaycar Cat. RQ-5272
1 plastic zippy box, 130 x 67
x 43mm, DSE Cat. H-2853
1 4 AA or AAA cell holder
1 numeric keypad, Altronics
Cat. S-5380 or equivalent
1 miniature 80 loudspeaker
Capacitors
1 4 7 µF 1 6VW electrolytic
1 1 0µF 16VW electrolytic
2 33pF ceramic
Resistors (¼W, 5%)
1 1MO
1 1 kO
We simply lashed the circuit up to prove the concept but it could just as easily
be built onto a small piece of Veroboard and housed in a plastic zippy box.
Fig.2: when a DTMF key is
pressed, the selected row and
column inputs are pulled low.
together to produce a single dual
tone output.
Table 1 lists the standard tones
for 12-key and 16-key DTMF
keyboards.
How it works
Our circuit (Fig.1) uses a standard DTMF encoder chip from
Texas Instruments, the TCM5089.
This is a CMOS chip designed
specifically to operate from a standard NTSC colour burst crystal
(3.579545 MHz]. The crystal ensures good frequency stability and
highly accurate tones. It also
eliminates the need for any
frequency adjustment.
The TCM5089 synthesises the
sine waves and their resulting
harmonic distortion is less than
- 30dB (3%).
When generating a dual tone
signal, the encoder generates one
column tone and one row tone and
adds them for its output. For
example, if you press button 8, the
two tones generated will be 852Hz
(row 3) and 1336Hz (column 2).
Table 2 shows the expected row
and column frequencies when using
a 3.579545MHz crystal, and the
variations from the "standard" set
of tones. For all practical purposes,
these tones are sufficiently ac-
1 680
Parts availability: The TCM5089 is
available from VSI Electronics
(Aust) Pty Ltd. Phone (02) 439
4655. VSI has offices in every
state. DTMF keypads are available
from Altronics and Jaycar, as are
the 3.579545MHz crystals.
Readers may also be interested to
know that Motorola have available a
DTMF receiver/decoder, the
MC145436, which can be used
for DTMF control. Details from VSI.
curate for our control applications.
The tolerance of the crystal should
be better than 0.02 % .
Unlike the dynamic or scanned
inputs commonly encountered in
computers, the static row and column inputs of the TCM5089 ensure
that no noise is generated. Fig.2
shows how a single key selects two
tones.
Several of the other inputs to the
Table 2
Tone
Frequency
Deviation from Standard
Row 1
Row 2
Row 3
Row 4
Column
Column
Column
Column
701.3Hz
771.4Hz
857.2Hz
935.1 Hz
1215.9Hz
1331.?Hz
1471.9Hz
1645.0Hz
+0.62%
+0.18%
+0.61 %
-0.63%
+0.57%
-0.32%
-0.35%
+0.73%
1
2
3
4
KEYBOARD
REAR VIEW
0
0
Fig.3: this diagram shows the pinouts
for the Altronics Cat. S-5380 numeric
keypad.
SEPTEMBER1989
73
TCM5089 need explanation, as
follows:
• Single tone enable input: this inhibits the generation of single tones
when taken low or left open. If this
pin is held high, single tone
operation is enabled.
• Tone enable output: when held
low, this pin causes the output of
the encoder to be dis a bled.
• Keyboard active output: this
output provides for switching of an
external receiver, transmitter or
other functions. The output is low
whenever one or more column inputs are active, and at a high impedance when all column inputs
are inactive.
The supply voltage may be anywhere between 3V and 10V. As the
current drawn during signalling is
around 2mA, the device lends itself
to battery power and portable
operation.
Construction
Our photo shows the circuit in
an experimental lash-up but it
would be easy to build it onto a
THE VHF FM MONITOR RECEIVER described in the March 1989 issue is now
available in kit form from Dick Smith Electronics. The kit is priced at $59.50,
comes with full instructions and features a re-designed PCB for improved
parts layout. It's available from your nearest Dick Smith Electronics store.
piece of Veroboard or matrix
board. This could then be housed in
a zippy box or mounted inside existing amateur equipment.
Our prototype used the standard
12-keypad layout which is most
commonly available. Depending on
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the chosen supply voltage, the
on/off switch is optional. At 10V
DC, the standby current from the
unit is less than 200µA. But at 6V or
under, the standby current is less
than 10µA so no on/off switch is
necessary.
~
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SEPTEMBER 11-15, 1989
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74
•
The Institution of Radio and Electronics Engineers Australia
\ Commercial Unit 3, 2 New McLean St (PO Box 79) EdgeclitfNSW 2027
(02)327.4822 - Fax (02)327.6770 - Publx AA21822 (Quote User No. SYl35)
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f • MONDAY, SEPTEMBER 11 SILICON CHIP
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