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Build this 90-second
message recorder
If the 16-Second Message Recorder published in
July 1993 wasn’t long enough for you, then try
this 90-second model. It runs from a 6V battery
& features more power output, a pause button,
90 seconds of continuous recording time & zeropower memory storage.
By DARREN YATES
There’s no doubt about it – solid
state audio recorders are the big noise
in electronics at the moment. This
was shown by the popularity of our
16-Second Message Recorder project
published in the July 1993 issue of
SILICON CHIP.
So popular was this project that it
spawned a couple of pre-built imported surface-mount modules and at least
one retailer is now stocking the device
16 Silicon Chip
as a regular catalog item.
But as the 286 PC was to the XT, so
is this new 90-second sound recorder to that original project. It’s based
on the sec
ond-generation of sound
recorder ICs just released by Information Storage Devices. Called the
ISD2500-series, there are four mem
bers each containing 480,000 EPROM
cells as opposed to the 128,000 in the
ISD1000-series.
Despite the popularity of the original design (or perhaps because of it),
there were quite a few calls asking
“how can you make it longer?” It seems
as though people these days leave lots
of messages on the fridge!
This 90-Second Message Recorder
uses the new ISD2590P voice storage
IC. It operates from a 6V battery and
includes a PAUSE/START key and a
separate power amplifier IC.
Looking at the IC briefly, instead
of using standard digital technology,
the ISD2590P uses a patented analog
method which allows analog voltages
to be stored directly into the EPROM
cells. It contains everything to make
a complete audio record\playback
system from microphone preamplifier
to AGC, 480K EPROM storage cells as
well as anti-aliasing filters and output
amplifier.
During recording, this device samples the incoming audio signal and
D1
1N4004
0.1
2.2k
0.22
ELECTRET
MIC
0.22
S4
+6V
10
10k
PARTS LIST
220
16VW
28 7 9 10 16
17 VCCD A6 A8 A9 VCCA 14 10k
MIC
SP+
18 MIC
REF
A OUT
21
6V
10
1k
VOLUME
10k
LOG
1
3
6
2
4
1k
+6V
RESET
REWIND
S2
23
24
100k
RECORD
R/P
AGC
EOM
4.7
A0
26
A2
XCLK
A3
A7 A5 A4
12 13 8
B
A
+6V
22k
C
B
E
22k
A1
E
C
VIEWED FROM
BELOW
27
6
Q1
BC548
B
25
1
2
8W
PLAY
S3
PD
0.1
10
20
CE
START
PAUSE
S1
19
470k
A IN
IC1
ISD2590P
100k
470
5
IC2
LM386
E
C
LED1
PLAY
LED2
RECORD
Q2
BC558
3
4
680
680
5
4.7k
K
B
C
E
Q3
BC548
90-SECOND MESSAGE RECORDER
Fig.1: the circuit is based on IC1, an ISD2590P 90-second voice storage IC. Its
output appears at pin 14 & is fed to an LM386 audio amplifier (IC2) which in
turn drives a small loudspeaker. Transistors Q1-Q3 drive the PLAY & RECORD
indicator LEDs (LED 1 & LED 2).
stores these samples as analog voltages
in the EEPROM. This technique is
eight times more efficient than current
digital technology and has the added
bonus of zero power for memory
retention. In fact, ISD guarantee that
it will hold a message for 100 years.
And since the writing cycle is much
more gentle than the usual digital
EPROM programming methods, you
can achieve up to 100,000 record cycles with the device.
For more details on this device, take
a look at the data article published
elsewhere in this issue.
Operation
OK, let’s now go through the operation of the Message Recorder. Initially,
when power is applied, nothing will
appear to happen. If you now set the
PLAY/RECORD switch S3 to RECORD,
the unit is ready to record.
Pressing the START/PAUSE button
S1 once will start the device recording
and LED 2 will light up. Recording will
continue until either the device runs
out of memory or you press either the
START/PAUSE button or the REWIND/
RESET button.
Pressing the START/PAUSE button
will stop recording but will keep the
address counter at its present position
– it works just like the PAUSE button
on your tape deck. Pressing the RESET/
REWIND button will also stop recording but will reset the address counter
back to zero. Pressing the START/
PAUSE button again will commence
recording from the beginning, erasing
any previous recording.
To play back what you have just
recorded, flick switch S3 into PLAY
mode and press the START/PAUSE
button. You will now hear the first
recording which will continue until
either 90 seconds has passed or until
the device comes up against an endof-message indicator.
At this point, the device goes into
an automatic ‘pause’ mode, and by
pressing the START/PAUSE button
again, you will hear the next recording. At any time, you can PAUSE the
1 PC board, code 01202941, 97
x 85mm
1 battery clip
1 6V battery holder
4 AA size cells
1 red snap-action pushbutton
switch
1 green snap-action pushbutton
switch
2 SPDT toggle switch
4 10mm tapped 3mm spacers
1 electret microphone insert
1 28-pin machined IC socket
4 PC stakes
1 8Ω 250mW loudspeaker
1 knob
Semiconductors
1 ISD2590P 90-second audio
recorder (IC1)
1 LM386 low-power audio
amplifier (IC2)
2 BC548 NPN transistors
(Q1,Q3)
1 BC558 PNP transistor (Q2)
1 5mm green LED (LED1)
1 5mm red LED (LED2)
1 1N4004 rectifier diode (D1)
Capacitors
1 470µF 16VW electrolytic
1 220µF 16VW electrolytic
2 10µF 16VW electrolytic
1 4.7µF 25VW electrolytic
1 1µF 50VW electrolytic
2 0.22µF 63VW MKT polyester
2 0.1µF 63VW MKT polyester
Resistors (0.25W, 1%)
1 470kΩ
1 2.2kΩ
2 100kΩ
2 1kΩ
2 22kΩ
2 680Ω
2 10kΩ
1 10Ω
1 4.7kΩ
Miscellaneous
Screws, washers, solder, tinned
copper wire.
playback by pressing the START/
PAUSE button or reset the device to
the beginning by pressing the RESET/
REWIND button.
Circuit details
Let’s take a look then at the circuit
diagram in Fig.1. As you can see, there
are just two ICs, the ISD2590P and an
LM386 audio amplifier IC. The latter
February 1994 17
10uF
10k
0.22
22k
22k
1k
0.1
1uF
Q1 Q2
680
0.22
470k
220uF
MIC
IC1
ISD2590P
680
4.7uF
10k
10uF
A
LED1
A
LED2
IC2
386
1
1
4.7k
2.2k
Q3
1k
0.1
D1
10
S1
470uF
S2
VR1
100k
BATT
SPKR
S3
100k
Fig.2 (above): install the parts on the PC board as
shown here. Use a socket for IC1 & note that Q2 is a
PNP transistor while Q1 & Q3 are both NPN types.
Note also that switches S1 & S2 are oriented with
their flat edges towards IC1. Fig.3 at right shows the
full-size etching pattern for the PC board.
IC is used to boost the 2590P’s output
signal.
Looking at the circuit, the input
signal is obtained from an on-board
electret microphone insert, which is
biased via the 2.2kΩ and 10kΩ resistors. The 10µF capacitor at the junction
of these two resistors provides supply
decoupling and prevents clock hash
from IC1 entering the audio stage.
As soon as power is applied,
the circuit is switched to a special
‘push-button’ mode by virtue of the
fact that address lines A6, A8 and A9
are tied high.
The CHIP ENABLE (CE) pin becomes the START/PAUSE control line
(pin 23) and the POWER DOWN (PD)
pin becomes the RESET/REWIND
control. Because these controls are
now edge-triggered, only pushbutton
switches are required.
The AGC (automatic gain control)
filter components are the 470kΩ resistor and the 4.7µF capacitor on pin 19.
Replay and record selection is made
via switch S3. By pulling the R/P input
at pin 27 low, the device is placed in
record mode and when it is high, it’s in
play mode. Switch S3 also controls the
two LEDs which display the operating
mode. With switch S3 low, transistor
Q1 is biased off but Q2 is turned on.
With switch S3 high, Q2 is biased off
but Q1 is turned on.
However, both LEDs are also controlled by transistor Q3, which is
driven by the EOM output at pin 25
via a 4.7kΩ resistor. When the START/
PAUSE button is pressed, the EOM
line is pulled high for the duration
of the first message. This is always
the message that begins at address
location 0 hex.
While the EOM line is high, either
LED 1 or LED 2 will light up depend-
ing upon the operating mode – LED
1 for PLAY and LED 2 for RECORD. If
the START/PAUSE or RESET/REWIND
buttons are pressed while the device
is either currently recording or playing
back, the current operation ceases and
the corresponding LED goes out.
Since we are using the internal
microphone preamplifier, the output
which appears at pin 21 must be recoupled back into the main preamplifier
stage whose input is at pin 20. This
is done via a 1µF capacitor and 1kΩ
resistor.
Output signal
The output at pin 14 is coupled via
a 10kΩ resistor and 10µF capacitor to
a 10kΩ volume control pot. The 10kΩ
series resistor is included to improve
RESISTOR COLOUR CODES
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
No.
1
2
2
2
1
1
2
2
1
18 Silicon Chip
Value
470kΩ
100kΩ
22kΩ
10kΩ
4.7kΩ
2.2kΩ
1kΩ
680Ω
10Ω
4-Band Code (1%)
yellow violet yellow brown
brown black yellow brown
red red orange brown
brown black orange brown
yellow violet red brown
red red red brown
brown black red brown
blue grey brown brown
brown black black brown
5-Band Code (1%)
yellow violet black orange brown
brown black black orange brown
red red black red brown
brown black black red brown
yellow violet black brown brown
red red black brown brown
brown black black brown brown
blue grey black black brown
brown black black gold brown
the loading of IC1’s output stage. The
volume control feeds IC2, an LM386
audio amplifier IC. This is connected
in its minimum-component mode
and has a gain of 20. The output from
the LM386 is approximately 300mW
into an 8Ω loudspeaker with the 6V
supply.
Power is provided from a 6V battery, with four AA cells being the
most appropriate. Diode D1 provides
reverse-polarity protection and the
220µF capacitor provides supply decoupling. Typically, current consumption should be about 6-8mA quiescent
and about 30-35mA when recording
or replaying.
Construction
All of the components for the
90-Second Message Recorder, except
for the battery and power switch, are
installed on a PC board measuring 90
x 97mm and coded 01202941.
Before you begin any soldering,
check the board thoroughly for any
shorts or breaks in the copper tracks.
These should be repaired with a small
artwork knife or a touch of the soldering iron where appropriate.
Next, you should make sure that
the components will fit into the holes
drilled. You will probably have to do
a little work for the mounting of the
volume control and the PLAY/RECORD
switch. You can use a 3mm drill for the
volume control hole and then enlarge
it with a tapered reamer or round file
to suit.
Once you’re happy that everything
is correct, start off by installing the
wire links. Use the overlay wiring diagram (Fig.2) to make sure that they go
into the correct locations, then install
the resistors, capacitors, diode and
transistors. Note that most of these
components are polarised and need
to be installed the correct way around
for the circuit to work. Again, use the
overlay wiring diagram to make sure
that everything is correct.
Because the ISD2590P is an expensive device to replace, we suggest that
you use a 28-pin machined IC socket
– not one of the cheaper variety. The
cheap ones have a habit of becoming
unreliable after a very short time.
Solder the IC socket in the same
way you would the IC. You’ll find that
the socket has a notch in one end, just
as the IC does. This makes it easy to
remember which way around the IC
must be plugged in if it ever needs to
be removed.
Next up, solder in the LM386 amplifier IC. Once that has been done,
you can install the switches. All of
these except for the power switch S4
are installed on the board. The two
snap-action switches should fit snugly
into position on the board. Make sure
that the flat section on these switches
if facing towards the top of the board
(ie, towards IC1).
Testing
Finally, insert the ISD2590P into the
28-pin socket. Make sure that it goes
in the right way around. This done,
connect a 6V battery in series with
an external power switch and your
multimeter.
When the power is switched on, you
should find that the current consumption is about 8-10mA. If it’s any more
than 15mA, switch off immediately
and check the board for possible solder shorts or component positioning
errors.
If everything appears to be in order,
follow the operating routine outlined
earlier to record and play back to your
SC
heart’s content.
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