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Salvage It!
BY JULIAN EDGAR
Making an adjustable loud screamer
This month we’re re-visiting our old friend, the
12V phone charger. As shown in April 2005, it’s
easy to give these chargers an adjustable output
voltage. However, it’s also easy to make them
perform a completely different function – and
here we use two to make a loud screamer.
I
N ADDITION TO a couple of 12V
car phone chargers, you’ll also need
a speaker for this project. You can
use any salvaged wide-range speaker
but in keeping with a mobile-phone
theme, we used a boxed speaker from
a hands-free car phone system. You
can often pick these up at the same
time as phone chargers.
Cost? Well at garage sales and the
like, expect to pay only a few dollars
for the lot. But what can you do with
these bits and pieces? One answer is
to make a very loud pulsing screamer.
The components
In this system, the two phone chargers perform different functions. The
first is modified to produce a pulsing
output voltage, which in turn powers the second charger. The second
charger is modified to produce the
audio output tone which is fed to the
speaker. And the speaker? Well, it
makes the loud noises!
The modifications to the charger
boards are very easy and it takes only
a few minutes to get the screamer up
and running.
In addition to the chargers and the
speaker, you’ll also need a selection of
capacitors. You’ll only end up using
two of them but having a range available makes it easy to get the sounds
you want.
Building it
The first step is to modify one of
the chargers to produce the pulsing
output. Begin by removing the PC
board from its cigarette lighter plug
enclosure, then remove the output
filter capacitor. This is the electrolytic
capacitor that’s usually located near
to the output leads (a typical value is
680mF). Just desolder it and place it in
your parts drawer – you never know
when it might come in handy for some
other project.
The next step is to replace the timing
capacitor. It’s dead easy to find – it’s
the smallest disc-shaped capacitor
on the PC board and typically has a
value of 100nF. Carefully desolder
Fig.1: the pulsing screamer uses two slightly modified 12V phone chargers
and a speaker. The first charger pulses the second charger which in turn
produces the audio frequency that’s reproduced by the speaker.
94 Silicon Chip
this capacitor and temporarily replace
it with a 100mF electrolytic capacitor
(this can be tacked to the track side of
the board).
Note that electrolytic capacitors
are polarised, so be sure to connect
the negative lead of this capacitor to
the ground track of the PC board. You
might have to do some track tracing to
make sure you get this right.
Charger 2
The next step is to modify the other
charger so that it will produce the
sound (ie, an audio tone). As before,
start by removing the output filter
capacitor and placing it in your parts
drawer. That done, remove the timing
capacitor and temporarily replace it
with a capacitor of around 1mF.
Next, connect the outputs of the
“Pulsing” charger to the power supply
inputs of the “Tone” charger, making
sure that the polarity of the connections is correct – see Fig.2. You can
then connect the speaker to the “Tone”
charger’s output terminals.
Testing and Tuning
Now for the smoke test – connect
12V power to the “Pulsing” charger and
listen. It’s likely that the sound will not
be quite as you want it – it may be too
low in pitch and pulsing too slowly,
for example (or vice versa).
That’s easily fixed. To speed up
the pulsing, decrease the value of
the timing capacitor in the “Pulsing”
charger. Similarly, to increase the pitch
(frequency) of the sound, decrease the
value of the capacitor in the “Tome”
charger.
By making some simple capacitor
changes, it’s possible to have anything
from a deep, slowly pulsing foghorn
to an ultra-piercing, frantically pulsing
screamer - and everything in between!
When you’re happy with the sound,
siliconchip.com.au
Rat It Before You
Chuck It!
The pulsing screamer is easily made from two modified car phone chargers and
a speaker. In this case, we used a (brand new) speaker from a hands-free kit
which we picked up at a garage sale but any wide-range speaker is suitable.
properly solder the selected capacitors
in place.
Now run the system for a while (you
might want to wrap the speaker in a
pillow!) and check the temperature of
the two ICs. They are likely to be warm
but they shouldn’t be too hot to touch.
If they are, install a 5W 5W resistor in
series with the 12V supply to the system. This will drop the audio output
but the ICs will run cooler.
Incidentally, when testing, always
power the system using the voltage
that will be used in the final application. This is because the pitch and
pulsing frequency will vary with supply voltage. Note that depending on
the value of the capacitors used, the
circuit will work down to about 4V.
Making it louder
If you want to increase the loudness
Whenever you throw away an old
TV (or VCR or washing machine or
dishwasher or printer) do you always
think that surely there must be some
good salvageable components inside?
Well, this column is for you! (And it’s
also for people without a lot of dough.)
Each month we’ll use bits and pieces
sourced from discards, sometimes in
mini-projects and other times as an
ideas smorgasbord.
And you can contribute as well. If you
have a use for specific parts which can
easily be salvaged from goods commonly being thrown away, we’d love
to hear from you. Perhaps you use the
pressure switch from a washing machine to control a pump. Or maybe you
salvage the high-quality bearings from
VCR heads. Or perhaps you’ve found
how the guts of a cassette player can
be easily turned into a metal detector.
(Well, we made the last one up but you
get the idea . . .)
If you have some practical ideas,
write in and tell us!
of the output, solder a bridging wire
across the inductor on each PC board
(the inductor is placed near to the
output and is simply a coil of wire).
A second bridging link should also be
installed across the output diode on
each board (see Fig.2).
The prototype was configured to
produce a very loud 200ms burst of
300Hz sound at 1-second intervals – so
it was configured more as a “growler”
than a “screamer”! This involved using a supply voltage of 12.0V, a 470mF
capacitor in the “Pulsing” charger and
a 47mF capacitor in the “Tone” charger.
In addition, the inductors and output
diodes were bridged on both chargers,
as described above.
Housing your screamer
Fig.2: here is a typical circuit for a 12V phone charger. The primary
modification is to alter the value of the timing capacitor to dramatically
lower the frequency at which the charger is operating. Shorting the output
inductor and output diode increases the output level.
siliconchip.com.au
Many hands-free speakers use boxes
that are held together with screws,
allowing the enclosure to be easily
opened. If that’s the case, the two
modified chargers can be insulated (eg,
by being wrapped in electrical tape)
and then placed inside the enclosure,
one each side of the speaker basket.
Alternatively, the chargers can be
SC
housed in a separate case.
February 2006 95
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