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Salvage It!
BY JULIAN EDGAR
Building a human-powered LED torch
for next to nothing
Would you like to have a torch where you
wind a knob just a few times and a white
LED stays on brightly for two minutes and
then remains visible for hours? Well you
can and the only parts that you’ll have to
buy new are the LED and the box to mount
everything in.
Y
OU’LL NEED A VARIETY of components from different salvaged
goods to make this design, so it’s one
to keep in mind as you collect bits and
pieces over a period.
First, you need the turntable motor
from a microwave oven. This is an AC
synchronous motor that’s about 20mm
high and 50mm in diameter. In addition to the motor, inside the package
is a system of plastic reduction gears
that normally gives an output shaft
speed of just 5 RPM (or thereabouts).
By turning this shaft with a knob, it’s
possible to easily generate up to several hundred volts AC output! There’s
our power source.
WARNING! Exercise extreme caution when salvaging parts from a
microwave oven. The large capacitors in the EHT (extra high tension)
power supply can retain a lethal
charge, even after the power has been
switched off.
Although these capacitors should be
discharged by bleeder resistors when
power is removed, don’t take it for
granted. Older microwave ovens may
not be fitted with bleeder resistors,
or the resistors may have gone open
circuit. For this reason, always make
sure that the capacitors in the EHT
supply have been discharged before
removing parts from a microwave
oven.
Plugpack bits
Next, a small transformer is need to
step that voltage down to something
that can be rectified (ie, converted to
DC) and used to drive a LED. This can
be done using one or more diodes or
a bridge rectifier. And guess what –
inside any older plugpack you’ll find
just those components, already wired
up and ready to go!
Don’t pick a recent lightweight
plugpack, though – these use switchmode circuits that don’t work in this
application. You can also recognise a
switchmode design by the large number of internal components.
We tried a variety of older 240V
plugpacks with transformers and those
with nominal outputs in the range of
6-12V DC all worked well.
Energy storage
The primary components needed are the turntable motor from a microwave
oven, an old plugpack and some high-value, low-voltage capacitors. These
salvaged parts shouldn’t total more than a few dollars but be careful when
salvaging the turntable motor – the bite from the EHT circuitry in a microwave
oven can be lethal, even with the power off (see warning in article).
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To store the power you’ve generated,
you need lots of capacitors. In addition
to being small enough to fit inside your
chosen box, these should have as much
capacitance as possible, while having
a voltage rating of about 10-16V.
Several 10,000uF 10V capacitors
are ideal, for example, but it doesn’t
matter if you use 10,000uF 16V caps
instead. Electrolytic capacitors are
January 2006 109
While it looks as though the parts might cost a fortune, all you
need to buy are the high-brightness LED, the box and possibly
the trimpot. The alternator and gearbox come from a discarded
microwave oven, the transformer and rectifier diode from a
salvaged plugpack and the capacitors from a wide range of
junked electronic equipment. This “optioned-up” version also
includes a neon lamp (salvaged from a cook-top) and a lens
from a discarded video camera.
used in nearly every piece of discarded
electronics equipment – always keep
an eye out for large-value low-voltage
units to salvage.
A 10-100kW trimpot (preferably
multi-turn) will make it easy to set
the LED current. Again, these can be
salvaged from lots of gear but failing
that, are cheap to buy new.
Another essential item is a suitable
knob, so that the shaft of the motor/
gearbox can be turned by hand. This
knob will need to fit a D-shaped shaft
and if you can’t salvage one for nothing, you’re not really trying!
White LED
You also need a white LED and this
will probably have to be purchased. A
5mm high-brightness white LED works
well. However, if you want more light
output and are prepared to turn the
knob more often, higher-rated units
can be used.
For example, with this design, a
1W Luxeon LED can be strongly il-
luminated, although not
to full brightness.
However, the more powerful the LED, the shorter the time the
capacitor pack will keep it on after
you’ve stopped turning the knob.
With the 1W Luxeon, the capacitor
pack will drive the LED for less than
a second, so in many ways a lower
rated LED is more practical. (Note: if
you use a powerful LED, you should
uprate the power rating of the current
limiting resistor.)
Finally, if you want to build the
“optioned-up” model, you’ll need a
lens (one salvaged from an old video
camera is perfect) and a neon indicator
from an old stove or cook-top.
Building it
To allow testing, roughly assemble
the electronic circuitry for the torch
on the bench before building the final
version into a box.
The first step is fit the knob to the
shaft. In my case, I used a knob taken
from the dashboard heater controls of an old car.
That done, connect a multimeter
set to “AC Volts” to the output of the
motor (now working as an alternator!).
Now turn the knob but make sure
that you’re not touching the output
terminals. You should get a no-load
output of 100–200V, depending on
how fast you turn the knob. If you
come in contact with the output when
you are turning the knob, you will give
yourself a shock, so be careful!
Note that you should never try to
wind the knob flat out – you’ll strip the
gears inside if you do. Instead, just turn
the knob progressively and evenly at
a slow speed (the optioned-up model
has this aspect covered)!
Before pulling the plugpack apart,
follow this simple procedure to check
that it is suitable. First, connect the
plugpack’s mains input terminals to
the alternator outputs (polarity doesn’t
matter). That done, connect the plugpack’s output to the capacitor pack,
Fig.1: a salvaged microwave turntable motor is used as an alternator to generate high-voltage AC. This is then fed to
a plugpack transformer and rectified to produce 6-12V DC by the innards of a plugpack. The capacitors are used for
energy storage, while the trimpot allows the current through the LED to be adjusted to its rated value (or less).
110 Silicon Chip
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Here’s the non-optioned version. It’s pretty simple to
look at but by turning the knob a few times, you can
have the LED shining brightly for about two minutes
and then remaining visible for hours.
taking care to connect negative output
lead (usually black or non-striped) to
the negative side of the pack.
Now connect your multimeter (set
to Volts DC) across the capacitor pack
and turn the knob. You should be able
to read a voltage that gradually rises
as you keep on turning. Any voltage
from about 6-12V is fine.
The trimpot (wired as a variable
resistor) and the LED can now be
added to the circuit. The circuit will
look like Fig.1, except you have to add
your multimeter to measure the LED
current. To do this, simply wire the
multimeter (set to milliamps DC) in
series with the LED.
Next, set the trimpot to its highest
resistance and turn the alternator knob
10 times. It’s then just a matter of
slowly adjusting the trimpot until the
maximum current rating of the LED is
reached. For example, if the maximum
current rating of the LED is 100mA,
set the trimpot to provide this current flow. Check that further turning
the alternator knob doesn’t cause the
required value to be exceeded.
Alternatively, you may want to set
the trimpot so that the LED operates
at less than full brightness, so that it
stays on longer after you stop turning
the knob.
Final assembly
The plugpack can be opened to retrieve the parts by crushing the case
slowly in a bench vice until it cracks.
That done, you can build the unit into
an off-the-shelf jiffy box. Seal the box
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(eg, with silicone sealant) if the torch
is to be used in the rain or in wet areas.
And the optioned-up model? Well,
it includes a neon lamp wired straight
across the alternator. This lights at
any voltage over 70-100V (the voltage
depends on the neon lamp) and so it’s
a good guide as to when the knob is being turned quickly enough to generate
sufficient power. In practice, it should
only just light.
Neon indicators salvaged from
stoves and cook-tops already have a
series resistor built into their bodies,
in which case you can just wire it
straight in.
The second option is to add a lens.
Our prototype used a video camera
lens, supported by a cut-down section
from a Portaflood light. This is ideal
if you want a long narrow beam – the
prototype has a beam range of at least
SC
100 metres!
This is the “optioned” version. Again,
it’s just a matter of turning the knob a
few times to get the LED shining.
Other Versions
In the February 2004 issue of
SILICON CHIP we covered a different design of human-powered
torch. That approach used a
direct-drive stepper motor as the
power source.
So what are the advantages of
taking the approach shown here?
Because of the built-in gearing of
the microwave oven motor, you
can generate much more power in
a shorter time – just a few turns of
the knob will keep the LED brightly
lit for a reasonable period.
However, the use of a gearbox
also has downsides – when being
wound, the torch is noisier than
a stepper motor design and the
plastic gears have a finite life.
Rat It Before You Chuck It!
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 have a use for the highquality 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!
January 2006 111
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