The other day I was in a major capital city, the
cold winter dusk bleeding light from the sky. It was the most dangerous of times
to be on the roads, with visibility poor and a thronging rush hour spilling cars
and people onto the streets. And despite the chill, plenty of cyclists were
hitting the road for the pedal home.
Sitting in a warm taxi, it was easy to pick the
cycling survivors from those, that er, soon might not be. The survivors were the ones
wearing bright, reflective clothing – and using decent lights. As I was stuck in
the traffic jam, dozens of cyclist rode by. Some had front and back lights so
brilliant that the flashing could be seen reflected in roadside signs and even
the bitumen itself. Others were using feeble lights that were a defence against
a police ticket, not against myopic drivers...
Bike lights have improved immeasurably over the
years, the use of high intensity LEDs revolutionising the technology. But as my
roadside observations showed, not all lights are of the same quality. In fact,
pick up a bike magazine or check a few websites and you’ll soon see that bike
lights vary enormously in price and claimed effectiveness.
So what do you do if you want good lights, but
don’t want to shell out big bucks? As we showed in the series Building a High Performance LED Lighting System, great things can be
done with high power LEDs like those from Cree and Luxeon. But such an approach
requires quite a lot of fabrication and the production of custom bike
attachments.
So is there a simpler approach? One that modifies
existing, relatively low cost, lights? We think there is. It’s still not a cheap
system but it’ll provide lights that are excellent for urban use. (For open-road
country riding, we’d stick with the much more powerful system described in that
previous series.)
Starting Points
The starting point for our lighting system
comprised two Smart 1W LED headlights. These are available from various dealers
(disclaimer: including my wife’s business Speed Pedal), and cost about AUD$45
each.
While there are plenty of criteria than can be
used when selecting lights, the reason I picked these was because of their 1W
LEDs. That’s a powerful LED and since it comes complete with reflector and
optical diffuser (a “lens”), I thought it likely that the light output would be
good. And so it proved, with the design having a flat, relatively wide and
bright beam. However, the light had only one mode – ‘on’. That is, no flashing
mode was provided.
An internal battery pack comprised four AA cells,
wired in series to provide 6 volts from conventional cells, and 4.8V from
rechargeables.
This story uses the Smart headlights, but similar
results could be obtained from other high power LED designs.
Testing
After assessing the quality of the beam and making
sure that it was worthwhile proceeding with the project, the next step was to
measure the current draw. This was easily done by feeding power to the light and
measuring the current flow with a multimeter.
The Smart torch proved to have a current draw of
500mA at 6V. 500 milliamps is a helluva lot of juice to be drawing from a small
battery pack. And, since amps x volts = watts, the power being drawn from the
battery pack is 3 watts.
But hold on! The LED is rated at only 1 watt! What
is happening to the other 2 watts? The answer is that an unfair amount of it is
being wasted in a dropping resistor. The LED requires less than 6V, with the
voltage is decreased by a simple resistor. And, as you’d expect, that resistor
gets quite warm. So a heap of power is being used up in the resistor (arrowed), rather
than being utilised to produce light!
A dropping resistor is a low cost way of
regulating LED current, and so most cheap LED lights will use this approach
(rather than a DC:DC converter).
So what can be done to both reduce the current
draw and also, as a bonus, massively improve the visibility of the light? The
answer is deceptively simple - flash the light.
eLabtronics Pulser
Making the technical decision easy was the
presence on my desk of an eLabtronics Pulser. This module, that we’ve covered in
the series starting at eLabtronics Pulser Part 1, is a compact electronic board
designed to pulse (or flash) loads like lights, solenoids, horns and so on. It
uses a high current output transistor (something very uncommon in cheap
flashers) and can be adjusted both for the flash rate and also flash pulse
length (and that’s also very uncommon!).
At AUD$59 prebuilt, it is one of the cheapest
off-the-shelf ways of flashing powerful LEDs while still having plenty of user
adjustment. It is available from the AutoSpeed Shop.
So what’s this about ‘user adjustment’?
If you think of anything that pulses on and off,
there are two different factors that can be varied. The first is frequency – how
many times per second it turns on. For example, you might decide to have the LED
flash 8 times a second – that’s very fast. Or, you might slow the rate down to 3
times a second.
By turning the frequency pot on the module, the
pulsing rate can be varied from 10 times a second to once per hour. In this
application, once per hour might be a tad slow!
The other aspect that can be varied is duty cycle.
‘Duty cycle’ refers to the length of time that the LED is on, as a percentage of
the time available for it to be on.
Confused? Don’t be. If for example the LED is set
to flash once per second, and the pulse length is set at half a second, the LED
is on for half the time. This called a 50 per cent duty cycle. If the LED is set
to be on for one third of a second, the duty cycle would be 33 percent. And so
on.
By turning the duty cycle pot on the module, duty
cycle can be adjusted from 0 per cent (ie the output is never on) right through
to 100 per cent (ie output always on). Normally, of course, you wouldn’t have
this pot set to either extreme.
Now while this might all sound kinda technical for
the sake of being technical, it’s really, really important. Why?
Well, the Pulser module itself takes very little
power to run – at 6V, about 5 milliamps. So we can ignore the current
consumption of the module and look just at the consumption of the light. And
with the Pulser, we can change that dramatically.
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In
its specs the Pulser is rated for an operating voltage of 10-40V. So how can we
run it off 6V? The answer is that the 10V minimum is for the worst combination
of switch and power draw combinations. Wired as shown in this story, the Pulser
works fine on 6V.
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Dropping Current Consumption
We talked about duty cycle above. Now let’s look
at how we can make that work for us. If we set the Pulser to flash 5 times a
second, and set the duty cycle at 10 per cent, we’ve reduced the power
consumption of the LED light by a staggering 90 per cent! So instead of
drawing 500 milliamps, we’re now drawing only 50 milliamps!
That makes the same battery pack last ten times
long! And that’s with no changes to the dropping resistor or anything
else...
But surely having the light off for 9/10ths of the
time makes it completely useless? Not on your (literal) life!
If you get someone to watch the light as you
adjust the Pulser, you can soon come up with a frequency and duty cycle that’s
incredibly visible but consumes very little power. Set the duty cycle to a fast
flash rate and then gradually increase the duty cycle from zero. At the point
where the watching person says ‘Hold it!’, stop your adjustments. To actually
see the values you’ve ended up with, you’ll need an oscilloscope, but just doing
it by eye resulted in a measured 5 times a second flash rate at 12 per cent duty
cycle. The measured average current draw at 6V was only 60 milliamps.
OK, now you’re saying to yourself, hold on! That’s
all very well but how many dollars was that Pulser module? Couldn’t I just do
all this more cheaply with some off-the-shelf bike lights? Maybe – but here’s
something else to consider. A single Pulser can run effectively as many LED
lamps as you want, so you can add at will side-facing lights, more headlights or
more tail-lights. You can be a (very safe) flashing Christmas tree!
Doing It
Let’s take a step back. We’ve got our generic,
cheap but power-hungry 1W LED lights. We’ve also now got a way of reducing their
energy consumption by up to 90 per cent while actually making them far more
visible. In fact, the energy consumption is so much less that we may as well run
both the headlight and tail-light from just one of the original battery packs,
saving quite a lot of weight.
But how do we achieve all this?
The first step is to modify one of the lights so
that:
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power can be fed independently to the LED
-
power can be drawn independently from the battery
pack
To achieve the first of these points, the wires to
the LED were cut...
...and then new extension wires soldered to them.
Note the retained LED dropping resistor, which was relocated as shown.
The wires to the battery connections were soldered
into place.
The joins were insulated with tape and the headlight reassembled, with the two sets
of wires fed out through a hole in the body. As it contains the batteries, this
modified light can be thought of as the ‘master’.
The other light could then be modified next, but
this was much simpler – all that needed to be done was to solder some wires to
the existing spring terminals that connect to the battery holder. The springs
themselves were removed and the holder re-inserted minus batteries. (Removing
the spring contacts means that even if the holder is inserted with batteries, no
electrical contact will be made. This allows, if wished, a spare set of fresh
batteries to be stored in the tail-light.)
Oh yes, and the clear lens was swapped for a red
one salvaged from another tail-light.
Wiring
The wiring can be divided into two sections. The
battery pack in the ‘master’ light feeds power to the Pulser. So that the Pulser
switches on whenever power is applied, positive power is fed to the ‘in’
terminal as well as the ‘+’ terminal.
The ‘out’ terminal of the Pulser connects to the
positive side of both LED lights while their negative terminals connect back to
the negative side of the system. An on/off switch goes in the positive supply
line.
Note that the LED dropping resistors are not shown
on this diagram but must be retained.
No heatsink is needed on the Pulser’s output
transistor (MOSFET).
Final System
The Pulser was mounted in a box and a switch
placed on its underside. (The box is available here). The headlights and tail-lights
were mounted, and the wiring completed.
The result is far more impressive than I expected
– the headlight, especially, isn’t far behind much more expensive designs using
even higher powered LEDs. The tail-light? Not as good – perhaps the lens that
was salvaged from another tail light was a little thick. I might try just
painting the inside of the clear lens red and see how that works...
But the key is the control that you have over the
flash rate and duty cycle. Twiddling these pots makes a radical difference to
the power consumption of the system and yet it’s easy to come up with settings
that work extremely well, both for visibility and watts consumed!
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The
Pulser is available fully built and tested from the AutoSpeed Shop
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Pulser
Specifications
Input
voltage: 10 – 40 V DC (less than 6V OK in this application)
Output
power: up to 10 amps continuous with appropriate heatsink, up to 15 amps
short pulsed with appropriate heatsink, up to 100 amps with appropriately
heatsinked external solid state relay
Wiring
connections: power, ground, input, output
Frequency: adjustable from10 times a second to once per hour
Duty
Cycle: adjustable from 0 – 100 per cent
Fuse:
15 amps
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The
eLabtronics modules are engineered and manufactured by eLabtronics. The modules
are based on concepts and specifications developed by Julian Edgar, with the aim
being to provide cost-effective and useful modules for car modification (and
also industrial and educational and bike uses).
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