This is only a preview of the November 2004 issue of Silicon Chip. You can view 23 of the 112 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "USB-Controlled Power Switch":
Items relevant to "A Charger For Deep-Cycle 12V Batteries, Pt.1":
Items relevant to "The Driveway Sentry":
Items relevant to "SMS Controller, Pt.2":
Items relevant to "Picaxe Infrared Remote Control":
Articles in this series:
Purchase a printed copy of this issue for $10.00. |
Is this one of Stan’s wind-ups?
Emergency power,
when all else fails...
B
ack in the 20th century, it used to
be said that you really knew you
were a parent when battery costs
for your kid’s toys exceeded monthly
power bills.
Fortunately, recent spectacular improvements in rechargeable technology now offer cost-effective secondary
batteries and chargers – and at keen
prices.
NiMH “AA” cell energy capacities
have near tripled from 750mAh to well
over 2000mAh since the year 2000.
It’s assumed you’ll have a nearby
mains outlet for recharging – but many
occasions arise (commuting in a peakhour train maybe) when you’re away
from such facilities but with hi tech
toys crying for a top up.
Such an occasion arose recently,
when bad weather meant a mate on
a weekend walkabout, hunting in
a nearby but isolated NZ mountain
region became hut-bound for nearly
a week – trapped by a flooded river.
Although warm and dry indoors,
the batteries in his mobile phone,
radio and torch all progressively ran
flat. Even his digital camera, intended
for the 12-pointer deer he’d hoped to
shoot, gave up!
Perhaps a solar panel would have
helped, although the scudding winter rain clouds, which caused the
flood in the first place, rather hinted
otherwise.
In despair at his “Joule less” plight,
he even took to the old trick of warming batteries in the oven and chewing
their outer casings to persuade a few
more electrons to flow…
At least he had an ultra bright white
LED torch! Let’s face it – these efficient
lighting devices have been one of the
most benevolent developments in
94 Silicon Chip
by Stan Swan
Above: the Benex Dynamo
Torch, from Jaycar, which forms
the basis of our emergency power plant.
Below, winding the handle produces
the power to charge the batteries.
siliconchip.com.au
The opened-up
torch, showing
where we “tapped
in” to the rectified
dynamo supply
(blue arrows).
It’s not difficult
to do and the full
functions of the
torch are retained.
Of course any neat output socket
could be used but simple red and
black banana types were found most
versatile – space inside the torch just
allows for these – and likely to be the
most useful with cold hands or broken
connectors.
The torch’s existing lighting circuitry remains quite unchanged, so
naturally it still operates as a very
bright and ergonomic LED torch.
Performance?
decades. But even they typically only
offer 80 hours on “lite” mode – perhaps
a week of evening use if half charged.
Yes, all very inconvenient – but
fortunately just as Search and Rescue
were being briefed the lost hunter
reappeared.
Improvise . . . or starve!
Back at civilisation, the challenge
was put to me: “Fix me up a reliable
charger, or no venison next trip”. A
past SILICON CHIP article on modified
disk drive generators sprang to mind
but their output looked limited to LED
lighting only.
Additionally, bush tramping demands near-bullet-proof devices,
otherwise rain and mud may rule over
fine engineering. Marine emergencies
further throw their own brand of cruelty and inconvenience…
Short of carrying in a generator, PV
panel and SLA gel cell, rigging a thermocouple to the fireplace or poking a
wind turbine above the bush line, just
what other ready approaches exist to
generate a few crucial watts? Digital
smoke signals?
output of a good half amp.
Impressive, although perhaps rather
optimistic unless the LEDs are pulsed
and actually draw less current!
Disassembly of the torch revealed a
sturdy brushed geared electric motor,
measured as providing about 9V DC
at some 200-300mA to a simple 12V
lamp dummy load. While not 0.5A, this
output (say 6V <at> 0.25A) hence satisfies the magical 1W energy budget our
emergency quest demands.
A 4-diode rectifier bridge on the
lamp’s PC board ensures unidirectional
generator output (along with two 0.6V
voltage drops), and soldering across two
diodes readily allows external connections to be made.
For even more efficiency you could replace the four diodes with suitably rated
Schottky diodes, with their much lower
(~0.1V) forward voltage drop. However,
these devices are fairly expensive.
Many mobile phones and UHF
transceivers now idle at just a few
milliamps on squelched receive, so
almost a 50:1 energy benefit could
result.
Typically, a minute’s winding (about
all you’d get away with on a peakhour train before dirty looks develop!)
should extend reception by about half
an hour.
Outgoing transmissions will be
power hogs, so text messages may be
the best energy investment (although a
“minutes winding for a minutes talk”
may be tolerable in an emergency).
As a bonus the 5V or so delivered
by this modified “1 Watt Wonder” can
also charge up to four normal AA or
AAA NiMH/NiCds.
You need to appreciate charging
maths – dead flat cells, of say 1200mAh
capacity, will need hours of unrealistic
winding to bring to full charge.
But even five minutes may be enough
to persuade your digital camera to snap
that trophy shot, find your dropped car
keys, phone your mates or – phew – call
the rescue chopper.
SC
I see the light!
Here’s where I got lucky: a call
to Jaycar Electronics revealed a
Swiss-designed BENEX Dynamo
LED torch – Cat ST3337 – amongst
their superb lighting range, priced
around $AU30.
Aside from the torch’s normal multiple 15,000mCd lighting options, a
sturdy handle folds out to hand charge
the unit’s internal 3.6V Li-Ion battery.
Claims that “1 minute wind-up = 30
minutes lighting” imply a 30:1 charge/
discharge ratio, so that a 20-30mA
drain LED may indicate a dynamo
siliconchip.com.au
We found a pair of banana sockets (polarised of course) with matching banana plugs
gave the best result. There’s room inside the torch for a variety of small sockets.
November 2004 95
|