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GENERAL
INTEREST
49
11/2002
Elektor Electronics
COMPONENTS LIST
Resistors:
R1,R4 = 10k
Ω
R2 = 8k
Ω
2
R3 = 2k
Ω
2
R5 = 100
Ω
R6 = 220k
Ω
R7,R24 = 47
Ω
R8,R10 = 1k
Ω
5
R9,R13,R14 = 100k
Ω
R11 = 15k
Ω
R12,R22 = 33k
Ω
R15 = 1k
Ω
R16 = 560
Ω
R17,R18 = 220
Ω
R19,R20 = 4k
Ω
7
R21 = 47k
Ω
R23 = 180k
Ω
P1 = 10k
Ω
preset H
P2 = 22k
Ω
potentiometer, linear,
mono, miniature version
Capacitors:
C1 = 680nF
C2 = 68pF
C3 = 47
µ
F 25V radial
C4 = 220
µ
F 10V radial
C5,C6,C10 = 1
µ
F 63V radial
C7,C14,C15,C16,C18 = 100nF
ceramic
C8 = 470pF
C9,C17 = 10
µ
F 63V radial
C11 = 470nF
C12,C13 = 22pF
C19 = 470
µ
F 25V radial
Semiconductors:
D1 = LED, red, 3mm
D2 = LED, green, 3mm
D3 = 1N4148
D4 = zener diode 12V 1.3W
D5 = 1N4002
T1 = BC547B
T2 = BC557B
IC1 = AT90S2313-10PC,
programmed, order code
012016-
41
IC2 = TS922IN (Farnell)
IC3 = 7805
Miscellaneous:
K1 = 4-ay SIL header
K2 = 8-way SIL header
S1,S2 = pushbutton, 6x6 mm
Re1 = subminiature relay
16x119x11.5 mm with SPDT
contact, e.g., Maluska FRS1B-S DC
5V, (5 V, 56
Ω
, Conrad Electronics #
505188)
X1 = 10MHz quartz crystal (C
load
=
32pF, parallel resonance)
Tr1 = line transformer, Bourns LM-
NP-1001 B)
MIC1 = electret Microphone element
(e.g., Monacor/Monarch MCE2000)
PCB, order code
012016-1
Disk, C source code and hex files,
order code
012016-11
(see Readers Service page)
DTMF
Signal Generation
Simple DTMF generator IC’s (like the
TP5088 from NS) are practically no longer
available. These days DTMF generators
come with all sorts of bells and whistles
that we do not need in this application.
They are generally difficult to find and have
lots of interface signals to take care of (The
MT80444 needs the
Φ
2-Signal from the
obsolete 6502 processor). A good solution
to this problem is to generate these tones
in software.
DTMF stands for Dual Tone Multi Fre-
quency signalling (tone dialling). Eight fre-
quencies are used comprising a group of 4
low tones and a group of 4 high tones. Each
of the 16 possible keys on a telephone key-
pad is represented by a different combina-
tion of one high tone and one low tone.
The reasoning behind this was that it is
unlikely that these sounds would occur in
normal speech patterns. The frequency of
all the eight tones needs to be accurate to
within 1.5 % otherwise the exchange
equipment may not recognise the DTMF
signal as a key press.
A D/A converter would be an ideal com-
ponent to generate the tones necessary for
DTMF signalling. Unfortunately the Atmel
microcontroller used in this project does
not have such a device on-board. The con-
troller is however equipped with a Pulse
Width Modulated (PWM) output and this
can do the job just as well. This output has
a resolution of 8 bits with a clock frequency
of 19.6 kHz (fCK/510). The dual sine waves
are produced by loading the PWM counter
with values from a look-up table to control
the width of the pulse. The spectrum of the
resultant waveform shows peaks at the two
sine frequency fundamentals and a peak by
f = 0 (this dc component is removed by
capacitors C7 and C11). Other frequency
components of this signal are at the PWM
switching frequency. The low-pass filter
formed by R15 and C11 ensures that these
square wave components on the output
signal are removed to leave just the two
sine waves. The corresponding amplitude
components for the high and low tone
groups are stored in a sine wave table. If
the stored period does not result in the
necessary frequency accuracy then two or
three different values are used. Altogether
the sine wave table contains approximately
200 bytes of data.
The microcontroller’s clock fre-
quency of 10 MHz may seem high
but it is necessary to achieve the
accuracy for DTMF tone generation.
The microcontroller can be pro-
grammed via connector K2. The
pin-out of this connector corre-
sponds to the standard used by Lat-
tice to program their ispCPLDs. The
same cable can be used to download
the program to Atmel as well as Lat-
tice chips. If you plan to order a pre-
programmed controller from the Pub-
lishers (and have no need to re-pro-
gram it) then you can omit connector
K2.
Assembling and adjusting
The compact single-sided PCB is
shown in
Figure 3.
Fit all the com-
ponents to the board starting with
the small items and finishing with
the largest. There are no wire links
necessary with this layout. For the
pushbuttons S1 and S2 choose types
with long buttons so that they pro-
trude through the plastic case for
easier access.
The microphone insert should be
mounted under the case for protec-
tion but don’t forget to make an
opening in the case so that sound
can reach the microphone. The tele-
phone cable outlet from the case
should be fitted with a strain relief
exit grommet.
A final check of the PCB is always
a worthwhile activity before the cir-
cuit is powered up. Make sure all
components are correctly positioned
and that there are no solder bridges
between the PCB tracks. At power-
up the green LED indicates that the
unit is operational. If a sound is
detected, this LED will start to flash.
Pressing pushbutton S1 will turn off
the unit and the green LED will go
out. Preset P2 is used to adjust the
sound level so that the cry from the
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