Last week in
High Performance
LED Light, Part 1 we took you through the production of an amazingly bright
handheld spotlight or bike headlight. This week, we cover the building of two
types of incredibly bright flashing red tail-lights. They’re perfect
for bike tail-lights or as emergency roadside breakdown beacons.
We’ll cover two different designs. The first is a
single light with a very bright, relatively narrow beam. The second design
approach uses multiple LED lights than can be equipped with wide or narrow angle
lenses.
Now you might immediately say: “Why bother? There
are plenty of excellent LED tail-lights available at quite low cost.”
And that’s true.
Except these lights makes all those others look
like just mere toys...
1. High Intensity Narrow Beam
The fundamentals of both last week’s handheld
spotlight / bike headlight and this article’s High Intensity Narrow Beam
tail-light are the same. That is, both use:
Salvage Parts
To reduce the cost of the light, we’ll use some
recycled and salvaged parts. So what can be scrounged at low cost? Three things:
the heatsink, the glass focussing lens and the housing.
Any discarded PC has heatsinks that are suitable
for Luxeon LEDs. In fact, if you’re happy to chop up a heatsink, you’ll find
something to suit in nearly any old hi-fi amplifier or receiver. As with all the
scrounged components mentioned in this story, it makes sense to collect
heatsinks over a period – they take up little space, and having a variety makes
it more likely that you will be able to select just the right heatsink for the
application – and, if you decide to get rid of them all, any scrap metal dealer
will give you cash for the aluminium!
While the 3-inch magnifying glass used in last
week’s project works extremely well, if you want a more compact light, or one
with a wider beam, you can do even better. How? Well, whenever you see quality
glass lenses available at low or zero cost, grab them.
Old slide projectors have heavily curved glass
lenses in their condenser assemblies, ‘projector’ beam car headlights have even
better quality curved glass lenses (these are often unbroken even when the front
glass lens is damaged, so can be salvaged from discarded headlights at zero/low
cost
[but beware the sharp glass edges when doing this!]
).
But perhaps the best compact and effective glass
lenses are the front elements from 35mm SLR camera lenses. These are optically
brilliant, have very high light transmissibility and are multi-coated. At garage
sales and the like, the lenses can be picked up for a few dollars – then it’s
just a case of pulling them apart.
Last week we used a stainless steel cup as the
housing for the light, but lots of other objects can also be used. Small
stainless steel containers (eg for food), small and large aluminium drinking
cups (where the cup can also become the heatsink!) and many other kitchen and
household items are suitable. I find the best collection place for these items
are the kitchen bric a brac sections of charity shops.
Building It
As it was built, the design used:
-
3 watt red Luxeon high power LED (Jaycar
Electronics ZD-0432)
-
Luxeon 15 degree collimator (Jaycar Electronics
HP-1292)
-
Salvaged heatsink
-
Shortened stainless steel drinking cup
-
Stainless steel screws
-
Ex-Cannon zoom lens front element
A stainless steel drinking cup was
shortened and a large diameter hole placed in its bottom. A further hole was
drilled in one wall for the mounting bolt. ‘Working’ stainless is difficult so
you’ll need to be patient and careful. The heatsink was drilled and trial-bolted
to the cup.
The 3W LED was glued to the heatsink with
thermally conductive heatsink glue (Jaycar Electronics NM2014 - don’t use
ordinary glue!). using heatsink glue saves having to bolt the LED in place. A
large amount of silicone was then used to glue the collimator and its housing to
both the LED and the heatsink. A vibration-resistant structure was required and
this achieved that.
The heatsink/LED/collimator assembly was bolted to
the cup with an O-ring placed in between. A ‘dressing’ comprising a cylinder of
foam rubber was slipped over the LED to hide the silicone.
The Canon front lens element (complete in its own
housing) was then siliconed into the mouth of the cup. The lens housing was
tapered, making for a good match with the cup.
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At
least an hour of experimentation was undertaken before the above design was
settled on. With these focussed lights, the brightness and width of the beam
depends on the optical relationship between the LED output, the collimating
lens, the glass lens and the distance between the lenses.
I
find the easiest way of devising a good design is to have the LED mounted on a
heatsink and powered-up appropriately. I then place the assembly on the bench,
aimed at the ceiling above. In a semi-darkened room, different combinations of
collimators and lenses result in clear variations in beam width and brightness.
It’s
important that this part of the development process is stressed: it’s quite easy
to select what looks to be a good glass lens only to find that the lens absorbs
a lot of light (ie its transmissibility is low) or the resulting beam pattern is
nothing like you’d guessed it would be!
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2. Multiple Wide-Angle Lights
The tail-light (or roadside emergency beacon, or...)
described above has an extremely intense beam. However, it is most visible when
viewed on-axis – that is, by someone directly in line with the light. This is a
quite common trait of high intensity LED lights, and in many situations is no
disadvantage. However, there are situations where you want the light to
be visible from wide angles.
The best way to achieve that is to build multiple,
small LED lights and position these to cover the required viewing angles. By
using Luxeon LEDs and off-the-shelf collimators (internally reflecting lenses),
it’s quick and simple to build very effective lights. They’re also extremely
light in weight – much lighter than the designs covered so far in this series,
that have both used glass lenses. And don’t worry about the simple aproach –
these are still extremely bright lights.
To keep total power consumption down, it is best
to use multiple 1-watt LEDs. Two different sized collimators are available –
20mm and 30mm. The smaller size is also available in a special wide-angle design
that gives an oval-shaped beam, while both sizes are available with various
round beam angles.
You can buy collimators (and the LEDs themselves)
from Jaycar Electronics
www.jaycar.com.au or directly from Luxeon –
see
www.luxeonstar.com. Buying direct from Luxeon works very well –
the prices are cheap, the international service good and the variety of special
parts (like oval beam collimators) is better than you’ll achieve elsewhere.
Building Them
The first step is to solder the wiring to the
LED.
Select a plastic chair leg tip to suit the size of
the collimator.
The collimator housing may need to be modified
with a file to clear the LED wiring. (Pop the acrylic lens out before filing the
housing!) Drill holes in the cup for a mounting bolt and to allow the wiring to
exit and then using clear silicone, glue the collimator in place – it should
firmly hold the LED in place.
Here is a narrow beam unit made with a 30mm
collimator...
...and here is an oval-shaped light made with a 20mm
collimator.
Driving the Lights
Note: you cannot power a Luxeon LED by
connecting it straight to a current source eg a 12V battery. If you do so, you
will immediately kill the LED!
The 3W Luxeon LED used in the High Intensity
Narrow Beam design requires a drive current of 1 amp at about 3V. The voltage
may vary a little from batch to batch and with LED temp, but the 1A current must
be maintained. It is this requirement that the LED be driven from a constant
current source that means the LED simply cannot be connected straight across a
battery.
As with any LEDs, a resistor can be used in series
with the LED to limit the current flow. This calculator
led.linear1.org
allows you to easily work out the required
resistor. For example, to drive the 3W LED from 12V you need to firstly look at
the specs – a forward voltage drop of 3V and a current of 1A. Plug these figures
in and the calculator suggests a 10 ohm, 10 watt resistor (or a 10 ohm, 20W
resistor if you want the resistor to stay cooler). Note that the resistor is
dissipating (wasting) about three times as much power as is being used to light
the LED!
For much higher efficiency, we’d again recommend
the Luxeon Star Driver kit (Jaycar Electronics KC-5389), which can be configured
to suit a 3W LED. (See Building a High Performance LED Lighting
System, Part 1.)
Don’t
Buy This One
It
is much more efficient to use a dedicated LED driver module rather than a
resistor – well, that’s the theory, anyway. Last week we used a Jaycar kit to
power the LED but this time we thought we’d take the easier path of buying a
pre-built module. We ordered a driver module from the LEDsales – see
www.ledsales.com.au.
It
promptly arrived but bench testing showed that it had two major problems.
Firstly,
the fixed output of 300mA is too low a drive current for a 3W LED. The module
outputted this current at a measured voltage of 2.5V, resulting in a LED drive
power of (0.3 x 2.5=) 1.7W. However, the input power was indeed 3 watts –
eg, 0.4 amps at 7.5V (0.4 x 7.5 = 3W). Now if you have an input power of 3W and
an output power of 1.7W, 1.3W (or 43 per cent) is being wasted inside the
module! That makes the module only 57 per cent efficient...
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One of the beauties of using multiple 1W LEDs is
that they can be wired to make the best match with the battery voltage, allowing
the use in most applications of just cheap resistors without then wasting a lot
of power - or those resistors getting hot.
Using the series/parallel calculator at
led.linear1.org is the easiest way of seeing the options.
Punch in the specs for a 1W LED of forward voltage of 3V and current of 350mA,
list a supply voltage of 12V and use four LEDs, and you’ll see that a series
array pops up that requires only a single 1 ohm resistor and has a calculated
efficiency of about 97 per cent!
Change the supply voltage to (say) 7.2V and you’ll
see that a different wiring diagram appears. And it’s still 81 per cent
efficient – far better than the LED driver module described in the box above!
(But LED driver modules will keep the LED current constant, even as battery
voltage falls. A simple resistor won’t do such a good job.)
Whatever the approach to driving the LED(s),
remember that a LED is a polarity conscious device – it needs to be connected
the right way around.
Conclusion
So how bright are these lights? The answer is:
very!
The High Intensity Narrow Beamlight is
visible from hundreds of metres – and that’s in broad daylight... At a night
viewing distance of 400 metres or more, it is super bright – and you can expect
to be able to see it from something like a kilometre. When fitted to the back of
a pedal machine, it will light up reflective signs and roadside reflectors for
hundreds of metres behind the cycle – massively improving a cyclist’s visibility
to oncoming traffic.
The range of the Multiple Wide-Angle lights depend
on the collimators that are used – a narrow beam collimator will give extended
range but at a tighter viewing angle. A good approach is to use a variety of
collimators – for example, in the four-LED system covered in detail next week, I
used two large diameter, narrow beam collimators and two smaller oval beam
collimators. (Note: the ‘narrow beam’ collimator is still much wider than the
High Intensity Narrow Beam light described earlier in this story.) Arranged
suitably on a pedal machine, the relatively narrow beam rear lights are visible
for many hundreds of metres while the wide-angle side lights can be seen from
anywhere within a near 180-degree field.
Next week: flashing high power LEDs, and a
complete lighting system installed on a Human Powered Vehicle.
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