Electronic Building Blocks
By Julian Edgar
Quick and easy construction
Great results on a low budget
High-powered voice alarm
Fig.1. The high-powered voice alarm is built
into a plastic box. It is pictured with a single
15W horn speaker but can drive two, creating
a very loud voice warning. You can easily
add the sounds of a siren to the recorded
message, making it especially effective.
D
o you want an alarm that
literally shouts voice messages
when triggered? That’s just what
this project will do. It’s ideal as a fire alarm
(my application) or burglar alarm. You can
record any message you like, complete
with background sirens. No one could
sleep through it, and it would take a very
brave burglar to ignore it – especially if
the sound is projected through externally
mounted horn speakers.
Such a system would have once been
the stuff of science fiction, but now
it’s cheap and easy. Take a look at this
parts list:
ISD1820 Sound Voice Recording
Playback Module
LM317 DC-DC 1.5A Step Down Power Supply Module
TDA7297 Amplifier Board
15W 8Ω horn speakers ×2
These items are available from many
eBay sellers – simply search using the
above terms. Then just add a box, some
low-cost hardware and wiring.
So how do the different components
work – and how does the system come
together? Let’s take a look.
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Voice module
The sounds played back
through the speakers are
created by the voice module. To find this pre-built
board, do an eBay search
under ‘ISD1820 module’
(at the time of writing, eBay
item 223868846287, £2.38
incl p&p). These modules
are equipped with:
ISD1820 sound record/
playback IC
Three pushbuttons
Fig.2. Inside the box, left to right: cooling fan, voice
Configurable links
module power supply, voice module, amplifier module
Power connections via
and heatsink. The front terminal block is for the two
header pins (note: 3-5V
speakers, rear block is for the 12V power connection.
– see below for the power supply)
Connector for an external speaker
the message with a fire siren, pause the
(instead, we will feed this output YouTube playback as you add a voice
into the amplifier module)
warning – ‘Fire in the kitchen! Fire in the
On-board microphone.
kitchen. Evacuate! Evacuate! – and then
finish the message with the sound of the
The recorded message can be up to 10 siren. Played on a continuous loop, this
seconds long and the quality of the recording is quite good.
After you have applied power to the
module, record a message by pressing
the ‘REC’ button and speaking into the
microphone. Best quality is achieved
with your mouth close to the microphone. The message is retained even
if power is removed. Linking the onboard ‘P-E’ pins causes the message to
be played in a continuous loop whenever power is again connected to the
board – just what we want in this
‘alarm’ application.
If you want the sound of sirens, the Fig.3. The required message is recorded on
easiest route is to find an appropri- this ISD1820 module. It can be configured
ate YouTube video and play that back to play the message on a continuous loop
through your PC speakers as you are when power is applied – just what we want
recording. For example, you can start in this application.
Practical Electronics | March | 2020
Next, build (if required) the TDA7297
module and then make the following
connections to the board:
n 1
2V (+) and (−)
n S
peakers ×2 (marked as ‘out’)
Fig.5. The voice module requires 3-5V,
which is provided by this adjustable LM317
power supply module.
Fig.4. Audio power is provided by a
TDA7297 amplifier board. It is available in
kit or prebuilt forms. Maximum power is
15W ×2 – a lot in this application!
is very effective. And, if you’re not happy with the message, just record it again.
Voice module power supply
To provide the 3.3-5V required by the
voice module, we use a small power supply. (The rest of the system
works off a nominal 12V.) This lower
voltage is easy and cheap to provide
with an LM317 adjustable linear regulator board; for example, eBay item
254489419130 (at the time of writing
£1.86 incl p&p). Most of these pre-built
eBay modules come with a heatsink,
but in our application the current draw
of the voice module is so small that no
heatsink is needed.
TDA7297 Amplifier
Now let’s look at what makes this
system really audible – the amplifier
module. Based on the TDA7297 module, these amplifier chips can output
15W ×2 – in this application, that’s
plenty! The one I bought is a simple kit
– just eight components and takes literally only a few minutes to assemble; for
example, eBay item 153801136438 (at
Fig.6. End view of the enclosure showing the
grille over the fan. A similar grille is located at
the other end behind the heatsink, allowing
airflow through the enclosure. (These grilles
are sold for use in cupboard doors.)
Practical Electronics | March | 2020
the time of writing £1.61 incl p&p). The
board is well labelled: (+) and (−) 12V
power, signal inputs and two speaker
outputs. The kit has no volume control
– if you want a volume control, select
one of the many other TDA7297 boards
that come with this feature.
Note that a substantial heatsink (or
fan cooling) is needed when the module is being run at full power – as is the
case in this project.
Building the project
As with many electronic projects, it’s
best to first set the system up in ‘quick
and dirty’ form on the bench and see
if it all works.
Start off by feeding 12V to the LM317
module and adjust its output to about
3.5V, as measured with a multimeter. (Be
careful – these boards are not usually
reverse polarity or short-circuit proof!)
Connect the power supply’s output
to the power terminals on the ISD1820
module. (You will need to solder to the
on-board pins.) Temporarily connect a
speaker to the output leads of the voice
module – these are the flying leads provided with a plug to connect to the board.
Apply power and press the REC button. Speak into the microphone and
record a message. Disconnect power,
link the ‘P-E’ pins with the provided
link and re-apply power. Your message should now play back through the
speaker on a continuous loop.
Fig.7. The TDA7297 amplifier requires plenty
of heatsinking. Here, a small heatsink has
been used, but it is fan-cooled. If a fan isn’t
used, the heatsink must be much larger.
The amplifier signal inputs are not
quite as you might first think. In our application, the two speaker output wires
from the ISD1820 voice module connect to the IN1 and IN2 connections of
the amplifier board. The signal ground
on the amplifier board is not connected. That is, the positive and negative
voice module speaker outputs become
the left and right inputs to the amplifier board. If you want plenty of volume
(and why wouldn’t you?), no volume
control is needed on the amplifier input – the voice module output levels are
a good match for the amplifier inputs.
Apply power to the system and
your recorded message should boom
through the connected speakers. Run
the system for only a few moments in
this form – without adequate cooling,
the TDA7297 will get hot very quickly.
Final form
I built the components into a plastic
box and fitted a relatively small heatsink to the amplifier module but used a
small fan to draw air through the enclosure. If you choose not to use a fan, you
will need a much larger heatsink than
shown here. (Note that the TDA7297
chip is temperature protected and is
rated to run at up to 70°C.)
Speaker and power connections were
made via terminal strips attached to the
outside of the box.
To run the system, you will need a
plugpack power supply capable of 2A at
about 12V. Of course, you could also use
a battery supply (eg, a float-charged SLA
battery), and this will allow the system
to operate even if mains power is lost.
To trigger the warning, use a relay to
feed power to the system when the right
conditions occur. For example, smoke
detectors are available with additional relay outputs, as are burglar alarms.
I used two horn speakers but you
could use normal ceiling-mounted PA
speakers, or even old Hi-Fi speakers.
And there’s also no need to use two
speakers – in many applications, running just one will be sufficient.
Summary
In use, this is an extraordinarily effective alarm. Everyone who heard it in
action – sirens blaring, ominous voice
messages shouting – were utterly disconcerted. In fact, when I was testing
the system, I needed to muffle the
speakers so that neighbours didn’t start
running to my aid!
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