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The Digital Voice
Board can store four
separate voice
messages, each up to
30 seconds long. The
message played back
depends on which of
four on-board
comparators has been
tripped.
A Digital Voice
Recorder Board
Here is a complete digital voice recorder
board. It can be connected to a computer,
used in a security system, or used to monitor
various functions in your car. When
triggered, it can deliver up to four separate
•
voice
messages.
By JOHN CLARKE
This project is not a voice synthesiser which puts out those cornysounding announcements from computers. No, this is a complete digital
audio storage system based on a
new chip from Texas Instruments.
It can be used to record and play
back any audio signal but it is mainly intended for voice messages.
You can record messages using
24
SILICON CHIP
your voice in exactly the same way
as you would with a telephone
answering machine. The resulting
recording will not sound like a voice
synthesiser; it will sound like you.
While we have suggested that the
Voice Board be used with cars,
security systems or computers, we
are sure that readers will use this
project in a wide variety of applica-
tions. For example, it could be very
effective on a model railway layout
whereby it could provide a variety
of trackside sound effects, station
voice announcements and so on.
If used in a car, the Voice Board
could warn about doors being ajar,
high oil or water temperature and
perhaps low fuel. In a security
system, the Voice Board could point
to sensors that have been breached, call for a periodic sensor check
or ask for a response from a remote
check point. The Voice Board could
also be used to give specific instructions to operators of machinery in
factories.
You could also have a lot of fun
with this device. You could connect
it to the front and back doors of
your home and have it announce or
query visitors. Anyway, we are
sure you will think up plenty of
other applications.
Technical features
The Voice Board can record up to
four messages and then play them
back in response to signals from
four on-board comparators. Any or
all of the messages can be played
back at any time, depending on the
sequence of signals from the four
comparators. Each voice message
can be up to 30 seconds long. That's
long enough for virtually any
message you could want - you can
almost tell your life story in 30
seconds.
The key integrated circuit on the
Voice Board is a new device from
Texas Instruments, the TMS3477.
Texas Instruments refer to it as a
"single chip voice recording/
playback controller using Continuously Variable Slope Delta
modulation" [CVSD).
In simple terms, the TMS3477
converts an analog [audio) signal into 8-bit digital data which is stored
in dynamic RAMs [random access
memory). Then on command, it will
convert the digital data back into
audio signals for playback via an
amplifier and loudspeaker. So the
TMS3477 provides both analog to
digital (A-to-D) conversion for the
recording process and digital to
analog conversion [D-to-A) for
playback.
Inside the TMS3477 is an
oscillator to control the sampling
process, the A-D and D-A converters, and a number of enabling
pins to initiate the various func-tions.
Like a lot of these special chips,
the TMS3477 requires a few support chips to make it do its job. As
well as a number of dynamic RAMs,
it needs an op amp to amplify a
signal from a microphone, an audio
amplifier for the output signal,
various comparators and gates to
initiate playback, plus a number of
transistors to tell the user what is
happening.
Sound quality
vs. duration
Essentially, there is a tradeoff
between recording quality and
recording length, just as there is in
any other recording medium. If you
PARTS LIST
1 PCB, code SC07111891,
220 x 138mm
1 PCB label
2 8-way PCB terminal blocks
6 PC-mounting click action
switches
1 4-AA cell holder
1 battery snap to suit
1 8-way DIP switch
1 electret microphone
1 80 loudspeaker
Semiconductors
1 TMS34 77NL voice recorder
(IC?)
4 TMS4256-15NL 1 x 256K
dynamic RAMs (IC1O-IC13)
1 LM339 quad comparator
(IC1)
1 4030 quad XOR gate (IC2)
1 4011 quad NANO gate (IC3)
1 4017 decade counter (IC5)
1 40106, 7 4C14 hex Schmitt
trigger (IC6)
1 7 4HC32 quad OR gate (IC4)
1 TL072 dual op amp (IC8)
1 LM386 audio amplifier (IC9)
2 7805 3-terminal 5V 1A
regulators (REG1 ,2)
4 BC558 PNP transistors
(Q1-Q4)
17 1N914, 1N4148 signal
diodes (01-01 7)
3 1N4002 1A rectifier diodes
(020,021 ,022)
want long recording times, you
have to sacrifice sound quality. If
you want good sound quality, you
get shorter recording times.
In a digital recording system like
the Voice Board presented here, the
recording quality is set by the
sampling rate. In the TMS3477, the
sampling rate is set by the internal
clock and by the programmable internal divider. The internal divider
can be set for a nominal sampling
rate [depending on the clock frequency) of 16kHz, 32kHz or 64kHz.
Depending on how you select the
clock frequency and the internal
divider, the recording duration will
be between 5 and 30 seconds.
The Digital Voice Recorder
Board measures 220 x 138mm and
includes all the circuitry necessary
to make it work apart from offboard components which are the
DC power supply, an electret
2 1 6V 1W zener diodes
(018,019)
5 5mm red LEDs (LED1-5)
Capacitors
2 4 70µ,F 16VW PC electrolytic
2 100µ,F 16VW PC electrolytic
2 22µ,F 16VW PC electrolytic
4 1 Oµ,F 16VW PC electrolytic
3 1µF 16VW PC electrolytic
4 0.4 7 µ,F 16VW pigtail
electrolytic
9 0.1 µ,F monolithic ceramic
1 .04 7 µ,F metallised polyester
(greencap)
1 .004 7µF metallised polyester
1 .0022µ,F metallised polyester
1 1 OOpF ceramic
1 47pF ceramic
Resistors (0.25W, 5%)
4 2 .2MO
1 39k0
1 1MO
1 27k0
1 680k0
1 22k0
1 270k0
7 1 OkO
9 220k0
5 4700
11 1 OOkO
1 1 000
2 47k0
3 100
5 200k0 miniature horizontal
trimpots
2 1 OkO miniature horizontal
trimpots
Miscellaneous
Tinned copper wire, solder, etc
microphone and a small loudspeaker [OK, you can use a large
loudspeaker if you like).
The DC power supply can be between 9 and 12 volts and there are
two separate power inputs on the
board. One is for standby power to
keep the recordings in memory and
the second is the main input to fully
power up the board.
Standby current is 5mA and
when fully powered up the circuit
draws a minimum of 15mA and considerably more on playback and
when the LEDs are lit. We have included an on-board battery for
memory backup when external
power is removed.
Four sensor inputs
There are four inputs which can
be used to trigger the various voice
messages. The four inputs are connected to four comparators and
DECEMBER 1989
25
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Fig.1: the circuit is based on the
TMS3477 voice recorder chip from
Texas Instruments.
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Changing sense
STOP
PAUSE
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3
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The four 256K RAMs store the digital data generated by the TMS3477 voice
recorder chip during recording. If you don't need four channels, you can save
money by leaving out the memory ICs for those channels you don't want. We
used sockets for the memory chips but these are optional.
each of these has a trimpot to set
the voltage threshold at which it is
triggered. There is also an 8-way
DIP switch to set the polarity for the
signal being sensed.
This facility allows the comparator inputs to operate with
signals which are either normally
on or normally off or alternatively,
normally high or normally low.
There are six pushbutton switches on the board and these are used for control of the recording and
playback functions, as follows:
Reset which clears the memory of a
recording, Record, Play, Pause,
Stop and Memory Select.
The Memory Select switch is only
used when recording. It selects the
memory into which each voice
message is stored. Five LEDs are
used to indicate the particular
memory being accessed and
whether record or playback is taking place.
The board also includes several
trimpots. These are used for adjusting the internal oscillator (to
change the sampling rate), and for
setting the microphone level and
the playback volume.
Circuit details
Now let's have a look at the com28
SILICON CHIP
plete circuit for the voice board
(Fig.1). This may appear complicated at first but most of it is
repetitive to provide for the four
separate voice messages. Hence,
there are four comparators, four
memory select gates and four
separate memories.
First, let's have a look at the four
sensor input stages, each of which
is identical so we only have to
discuss one stage.
ICla is part of an LM339 quad
comparator. VRl, a 200k0 trimpot,
is connected to the non-inverting input of IC la to provide an adjustable
threshold. A 2.2MO resistor between pins 1 and 7 provides a
degree of hysteresis and the lOOkO
resistor at the output (pin 1) provides an external load which is
necessary since the LM339 is an
"open collector" device.
The sensor signal is fed to the inverting input of ICla via a voltage
divider consisting of a 220k0 and a
100k0 resistor, with overload protection provided by diodes Dl and
D2. The divider allows the circuit to
function with logic signals of + 12V
or more, even though the board
logic runs from a + 5V supply.
The 0.47µF capacitor removes
any glitches which may be present
due to switch contact bounce.
Switch S1, together with exclusive OR (XOR) gate IC2d, is used to change the sense of the comparator output. For example, if S1
is open, IC2d's output will change
from low to high when the sensor input to ICla goes high. If S1 is closed, IC2d's output will change from
low to high when the sensor input to
ICla goes low. Thus when Sl is
closed, IC2d inverts the output of
ICla.
IC2d's output must change from
low to high for the voice message to
be played back. ICla and IC2d can
be disabled by DIP switch S2 which
simply removes the sensor input.
Comparators IClb, IClc and
ICld, and XOR gates IC2c, IC2a and
IC2b, function in the same way as
ICla and IC2d. They also have DIP
switches to disable them: S4, S6
and SB.
The outputs from the four XOR
gates are connected to separate inputs of each of the gates in IC3, a
4011 quad 2-input NAND gate
package. The second input of each
NAND gate is connected to the '1 ',
'3', '5' and '7' outputs ofIC5, a 4017
decade counter (pins 2, 7, 1 and 6,
respectively).
As IC5 counts, the four NAND
gates are sequentially enabled so
that they can pass signals through
from the XOR gates. Only the odd
outputs from IC5 are used so that
there is a time gap between each of
the NAND gates.
As each output of IC5 goes high
in sequence, the NAND gates will
deliver a low output signal only if
the respective XOR gate output is
high.
Diode OR gate
The four NAND gate outputs are
connected to an OR gate consisting
of diodes D13 to D16, together with
a 100k0 pullup resistor. If one of
the NAND gate outputs goes low, the
OR gate and its associated lµF
capacitor delivers a momentary
low signal to Schmitt inverters IC6a
and IC6b which then pull pin 7 of
IC7 low, via diode D12 . IC7 is the
TMS3477 voice chip and when its
pin 7 is pulled momentarily low, it
goes into playback mode.
The NAND gate outputs perform
CERAMIC & POLYESTER
CAPACITORS
No.
D 9
D 1
D 1
D 1
D 1
D 1
Value
0.1µF
.047µF
.0047µF
.0022µF
100pF
47pF
IEC
100n
47n
4n7
2n2
100p
47p
EIA
104K
473K
472K
222K
10 1K
47K
two other functions. Each output
drives an associated transistor (Ql,
Q2, Q3 or Q4) via a 10k0 resistor.
These transistors drive the memory
LEDs. Thus, the LEDs indicate
which comparator is tripped to play
its respective voice message.
Finally, the NAND outputs select
the requisite memory via the four
OR gates of IC4, a 74HC32. The chip
select output of IC7 (pin 12, CAS1)
connects to one input of each of the
IC4 OR gates. Each OR gate will pass
this chip select signal when it is
enabled via its r espective NAND
gate output.
Note that we have specified a
high speed CMOS OR gate for IC4
because it has to pass through the
fast rise times of the memory
refresh signals without degradation.
Memory select
At this point we should explain
some of the labelling on the
dynamic memory chips, which
otherwise will seem very obscure.
The la belling on the pins is as
follows:
address inputs
AO-A8
column address strobe
CAS
data in
DI
data out
DO
row address strobe
RAS
write enable
WE
You will see that there are corresponding pins on the TMS3477
chip (IC7) for the connecting lines to
the memory chips. The CAS and
RAS lines are used for refreshing
the data in the memories but as far
as we are concerned, the CAS input
on each memory can be regarded
as the chip select line.
When IC7 is in the record or
playback mode, its CAS line is ac-
Fig.2: here's how to install the parts on the PCB. Be sure to install switches
S11-S14 with their flat sides as shown. The DIP switch (S1-S8) is installed with
the open side nearest the edge of the PCB.
tive (ie, strobing at the nominal
sampling rate of 32kHz or 64kHz).
The CAS line from IC7 is fed via
four OR gates (IC4) to the CAS inputs of the four memories. The CAS
line also goes to Schmitt trigger
IC6d.
When the chip select output from
IC7 is active, Schmitt trigger IC6d
performs two functions. First, it lets
us know that record or playback is
in progress by lighting up LED 5.
This is done by charging up the
0.1µ.F capacitor at the pin 9 input of
IC6e via diode Dl 1. The resulting
low output of IC6e drives LED 5.
The output of IC6d also stops the
clock of IC6f via diode DlO. This
holds the lµF capacitor at pin 13 of
IC6f high, which disables IC6f and
IC5 for the duration of the record or
playback period.
The Memory Catch switch (S9)
stops the clock in a similar manner
by pulling the input (pin 13) of IC6f
high. S9 is used to select the reDECEMBER
1989
29
Switches 2, 4, 6 & 8 of the DIP switch select the inputs to be used for
triggering while switches 1, 3, 5 & 7 select the polarity of the input. For low to
high triggering, these odd numbered switches should be off (open). For high to
low triggering, the switches should be on.
quired memory for recording.
The final control over IC5 is the
reset at pin 15. Both IC5 and IC6
run from the + 5V standby supply
which is derived from either a car
battery or on-board battery. When
the circuit is operating only from
the standby supply, diode D9 pulls
the input of IC6c low. This causes
IC6c's output to go high and reset
IC5.
The 22µF capacitor and 220k0
resistor at the input to IC6c provide
a turn-on delay when the main supply is applied.
Memory
The memory chips from IC10 to
IC13 are wired in parallel and connect to the voice annunciator chip
IC7. Note that the AO-A8 address
lines of IC7 do not connect to the
corresponding address lines for the
memory ICs. This does not matter
and is done so that the printed circuit board layout is more convenient.
The 10k0 resistor connected to
the AP6 address line selects the
64kHz data sampling rate. If this
resistor is removed, the rate drops
to 32kHz, with a corresponding
drop in the sound quality.
The external oscillator components for IC7 are the 47pF
capacitor, the 10k0 resistor and
the 200k0 trimpot. These are at
pins 6 and 7 of IC7. The power up
reset is at pin 5 and uses a 47k0
30
SILICON CHIP
resistor and lµF capacitor. Reset
switch S10 provides manual
resetting.
The Play, Record, Pause and Stop
switches connect directly to the
respective inputs at pins 8 to 11.
When a switch is pressed, it pulls
its corresponding pin to ground via
a 1000 resistor.
The microphone input to IC7 (pin
2) is driven by op amp ICBb which
is fed by an off-board electret
microphone. The gain of IC8b is adjustable over a modest range by
VR5, a 10k0 trimpot. IC8b has a
high frequency cutoff of 2.6kHz due
to the 100pF capacitor across the
680k0 feedback resistor.
The reconstituted audio signal
from IC7 (ie, playback) is fed to
IC8a, a low pass filter with a high
frequency rolloff above 2kHz. This
attenuates any high frequency hash
which is a normal byproduct of any
digital to analog converter. Following IC8a, the signal is fed to volume
control VR6 and then to IC9, an
LM386 power amplifier.
DC bias for IC8a and ICBb is
derived from a voltage divider consisting of 39k0 and 47k0 resistors
and an associated 10µF bypass
capacitor (shown near to the electret on the circuit).
Power
Power for the Voice Board is provided from two separate 5V supplies. All of the RAMs and IC1, IC2,
IC3, IC4, IC5 and IC6 are powered
from the standby + 5V supply while
the analog circuitry, IC7, IC8 and
IC9 are powered from the main
+ 5V supply.
Both supplies are based on 7805
3-terminal regulators. The main
+ 5V supply is the simplest. Its
7805 regulator (REG1) is fed from
an external + 12V supply via a 100
resistor and protected against
reverse polarity and excessive input voltages by 16V zener diode
D18. A 470µF capacitor filters any
hash from the input line to the 7805.
The standby supply is more complicated but uses the same components for input protection and
filtering. However, the standby
regulator (REG2) has diode D20 in
series with the GND leg to jack up
the output voltage by 0.6V which is
then "dropped" by diode D21. This
gives an output of + 5V which is
bypassed with a 100µF capacitor.
Diodes D21 and D22 are included
so that a 4.5V backup battery can
be included. The diodes provide
isolation of the normal 5V regulator
output from the 4.5V battery. If the
standby regulator is powered down
due to disconnection of the offboard supply, the onboard battery
maintains the data stored in
memory. It feeds the + 5V standby
rail via D22 and provides about
+ 3.9V which is adequate to keep
the memories powered up.
Construction
The voice board is coded
SC07111891 and measµres 220 x
138mm. While we have not done so,
it could be housed in a large standard plastic instrument case, as
available from Dick Smith Electronics, Jaycar and Altronics.
Construction of the board is quite
straightforward. First, insert all the
links and resistors as shown on the
overlay diagram (Fig.2). Next, the
ICs can be inserted and soldered into place. Be careful with their
orientation and ensure that when
soldering there are no shorted pins.
The next step is to solder in all
the diodes. Most of the diodes are
1N914 types except those in the
power supply section which are
zeners and 1N4002 types. The
capacitors can also be installed at
this stage but take care with the
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Fig.3: here is a half-size reproduction of the PCB. The full-size board
measures 220 x 138mm.
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Fig.4: use this artwork to label the various switches and controls on the PCB.
The labels can be attached using double-sided tape.
polarity of the electrolytics.
Now install the transistors and
other semiconductors, taking care
to ensure that each device is correctly oriented. Finally, complete
construction by soldering in the
pushbutton switches, DIP switch
and other hardware. The flat side
of each pushbutton switch must be
oriented as shown in Fig.2. Install
the DIP switch so that the open side
is at the edge of the PCB.
Powering up
To test the circuit, a 12V supply
capable of providing up to 500mA is
required. Connect the power supply
and check that + 5V is present at
pin 3 of ICl; pin 14 of IC2, IC3, IC4
and IC6; pin 16 of IC5; pin 8 of ICB,
IClO, ICll, IC12 and IC13; pins 1
and 28 of IC7; and pin 6 of IC9.
If all are correct, the circuit can
be tested for recording.
Connect up an electret microphone and loudspeaker to the terminal strip. Now close Sl, S3, S5
and S7 of the DIP switch. All four
memory LEDs should now slowly
flash in sequence.
To record, press the Memory
Catch switch to stop the LED at the
required memory. For example, to
record in Memory 1, press the
Memory Catch button when LED 1
flashes on. Hold the Memory Catch
button down and press the Record
button. This starts the recording
process, as indicated by the
playback/record LED.
The Voice Board will then continue recording until either the stop
or pause switches are pressed or
the memory is filled, which takes 30
seconds or so, depending on the setting of VR7.
When the first recording is complete, the record/playback LED will
extinguish and the next memory
will be accessed. It will begin playing back what is in its memory.
Since this has yet to be recorded,
you will just get a loud buzz.
Now press the Stop switch. The
next memory will now be accessed
and this will also play back noise.
Each time you press Stop again, the
Voice Board will move on to the
next memory.
When the first memory is accessed again, it will play the message
just recorded. You can re-record at
a faster or slower clock speed by
adjusting VR7. Recording at a
faster clock speed will give better
sound quality but will reduce the
maximum possible length of the
recorded message. Normally this
will not be a problem because most
recordings you are likely to make
will usually be much less than the
nominal 30 seconds.
You should also adjust microphone level control VR5 for best
results.
The three remaining memories
can be tested by recording as
before. Note that while the Stop
pushbutton can be used to cut short
a recording, this will mean that the
recorded messages for the remaining memories will all be cut to the
same length.
The Pause pushbutton operates
in a similar manner to a tape
recorder. It halts the recording process until pressed again.
The 4.5V battery pack can now
be connected to test that it maintains the memory when all other
continued on page 109
DECEMBER
1989
31
3½-digit
capacitance meter
hazard to faxes , modems and computers from the double whammy of
transients on phone lines and mains
wiring. The price? Just $59.95 from
all Arista stockists.
For further information, contact
Arista Electronics Pty Ltd, PO Box
191, Lidcombe, NSW 2141. Phone
(02) 648 3488.
If you have trouble reading the
codes on capacitors or you have
caps with the labels rubbed off,
it is so convenient to be able to
measure them on a capacitance
meter. Many digital multimeters
now feature capacitance measurement but they can rarely handle the full range of values in
normal use.
This new capacitance meter
provides nine ranges between
200pF and 20,000µF and a zero
adjus t ment for the l ow
capacitance ranges (ie, 200pF,
2nF and 20nF). Accuracy is
± 0.5% of reading + 1 digit for
the 7 ranges from 200pF to
200µF, ± 1 % + 1 digit for the
2000µF range and ± 2% + 1
digit for the 20,000µF range.
Over-range and low-battery indicators are also given on the
display, which updates itself at
the rate of about twice a second.
The new capacitance meter is
priced at $109.95 complete with
instruction manual and test
leads and is available from all
Jaycar stores.
12V/2A power supply
for CBs and car radios
Ceiling loudspeaker
grill looks like
a downlight
This hew power supply is designed for use with CB's, car radios,
alarm systems and DC power tools
and provides a regulated 12 volts
DC at up to 2 amps output. The unit
is fitted with 4mm banana socket/
terminals and is double insulated.
Price is $69.95 from Altronics
Distributors Pty Ltd, 174 Roe
Street, Perth WA. Phone (09) 328
2199.
Digital voice board ctd from page 31
This loudspeaker grill is
designed to look like a downlight
and could find many uses in
homes and office buildings, shopping centres and restaurants, or
anywhere normal speaker grilles
are not wanted. They are only
$3.95 each and are available in
white or black. They are supplied by Altronics Distributors
Pty Ltd, 174 Roe Str eet, Perth
WA. Phone (09) 328 2199.
power is disconnected. The battery
pack can be secured using cable
ties around it and through the holes
on the board.
You can check the operation of
the comparators by turning the
VR1-VR4 trimpots two thirds anticlockwise (from their maximum
clockwise setting). Now switch S1,
S3, S5 and S7 off and S2, S4, S6 and
S8 on. Connect a jumper wire from
+ 5V to input 1 and check that
memory LED 1 lights up after a
brief pause (IC5 needs time to cycle
round).
Simila rly, check that memory
LEDs 2, 3 and 4 also light when input 2, input 3 and input 4 are connected to + 5V.
Once all functions have been
tested, the Voice Board can be installed in its final location. First
select the options for switches 1 to
8. Switches 2, 4, 6 and 8 select the
inputs while switches 1, 3, 5 and 7
select the polarity of the input. For
"low to high" signal triggerings,
these odd numbered switches
should be off while for "high to
low" signal triggering the switches
should be on.
Unused inputs can be left with
the switches off.
~
DECEMBER1989
109
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