It’s easy to make light – lots of light.
Car headlights are particularly good at it – they typically have a power of 50 watts each headlight and produce a broad, even beam. But 50 watts is a helluva power draw if you have to carry the battery or generate power from human effort.
Some night racing mountain bikers use miniature 12V halogens, typically with a power of about 20 watts. Again, they make lots of light – but 20 watts from 12V is a continuous power consumption of 1.7 amps. To get more than a few hours of light, you’ll need a bloody big battery – even if it’s a state of the art lithium ion.
So how’d you like a decent light that draws that has a power of only 5 watts – one-tenth of a car headlight and one-quarter a 20W halogen? It will be bright enough that if you mount it on a bike, cars coming the other way on a dark country road will often flash their headlights, thinking you’re on high beam. I can vouch for the ability to ride downhill at over 60 km/h on a moonless and starless night, road illuminated solely by the 5W headlight. Used as a handheld spotlight, it will match a typical 50W sealed beam for penetration (although it will have a narrower spread).
And you want this amazing light? Here’s how to make it.
The headlight – and in fact all the lights covered in this series - use Luxeon LEDs. If you’re used to only normal garden variety 5mm diameter white LEDs, you might wonder what all the fuss is about. The excitement is because Luxeon LEDs are nothing like those littlies of yesterday. You can’t look directly at a Luxeon LED – they’re simply too bright. And when you add a collimator (a special internally reflecting lens) and an external 75mm glass focusing lens, well, the beam is simply fantastic. And yet the light weighs little and (it’s worth saying it again!) draws only about 5 watts.
To make the headlight you’ll need:
To power the headlight you’ll need:
Note that in most cases, the mouth of the stainless steel cup will have a diameter of 75mm (3 inches), making it easy to source a suitable plastic cap and magnifying glass. Single wall stainless steel drinking cups can now be very hard to find, but Coastal Kitchen and Cutlery on the Gold Coast, Australia, (phone: 07 5526 9399) has them in stock at AUD$5.50 each. A double wall (ie insulated) cup can also be used but it is heavier and a more difficult to drill.
So how does it come together? In short, the plastic plumbing cap has a large diameter hole cut into it and the magnifying glass is mounted within the resulting flange. The flange/lens combination then fits over the open end of the stainless steel cup. The 5W LED is fitted with its internal collimating lens and bolted to a heatsink. A hole is drilled in the base of the cup for the LED/collimator assembly to project through and the heatsink is then bolted to the base of the cup. This pic shows the completed light, although in this case instead of using a plastic PVC cap, I machined off the bottom of an old aluminium fire extinguisher and used that to form the rim.
The LED driver module efficiently supplies the right current to the LED. The battery – well, you know what the battery is for. Both the battery and the driver circuit board should be mounted in a weatherproof box.
So the project is not for the fainthearted – there’s fabrication and electronic kit building and soldering and boxes and brackets. Total cost also adds up – say near AUD$150. (Of course, this will be decreased if you have a battery charger available, or the battery, or a suitable box, etc.)
Cut a hole about 65mm in diameter in the centre of the plastic plumbing cap. Sand the edges of the cut-out smooth and then use silicone to glue the lens within the cap. This assembly forms the focussing lens.
Drill holes in the heatsink to allow small nuts and screws to be used to attach the LED to the heatsink. Drill an additional pair of holes in the heatsink to allow the power supply wiring to the LED to pass through the heatsink (or alternatively, these wires can pass through a hole drilled in the stainless steel drinking cup). Use a file to shorten the plastic legs of the collimating lens so that it sits squarely over the LED, legs resting against the heatsink and centre of the collimator in firm contact with the LED.
Place some heatsink compound under the LED and then attach the LED to the heatsink with the small screws and nuts. Ensure the heads of the screws do not short-circuit the power supply connections to the LED (you may want to use nylon nuts and bolts). Glue the collimating lens securely to the LED and heatsink. Pass the wiring through the heatsink and solder it to the LED. Seal the holes through the heatsink with silicone.
Cut a 35mm hole in the centre bottom of the stainless steel cup. At this point also drill the cup for any mounting brackets that will be needed. Position the heatsink on the bottom of the cup so that the LED and collimator project through the 35mm hole. Mark and then drill appropriate holes to bolt the heatsink to the cup, sealing this join with silicone.
Powering the LED
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!
As with any LEDs, a resistor can be used in series with the LED to limit the current flow. This calculator linear1.org allows you to easily work out the required resistor.
For example, to drive the 5W LED from 12V you need to firstly look at the specs – a forward voltage drop of 6.84V and a current of 700mA. Plug these figures in and the calculator suggests an 8.2 ohm, 4W resistor. Note that this means that the resistor is dissipating (wasting) about the same amount of power as is being used to light the LED! To avoid the resistor getting really hot, it’s wise to double the wattage over the suggested value – eg use a 10W resistor. Powering the LED via a resistor is the cheapest and easiest approach to running the LED.
A resistor will work but it is much more efficient to use a dedicated LED driver module. The Luxeon Star Driver electronic kit available from Jaycar Electronics is a dedicated DC/DC high power LED driver. It will waste far less power and so will not get as warm as a resistor; furthermore, the drain on the battery is lower. If using the Luxeon Star Driver kit, now is the time to build it – it comes with instructions and needs to be configured to suit the LED being used (in this case, 5W).
Connect the LED to the power supply (making sure that the polarity is correct), and the power supply to the battery. Alternatively, if using a resistor, place the 10W, 8.2 ohm resistor in series with the LED.
Test the operation of the LED light with the focussing lens in place. The assembly should throw a very bright spot of light about 60cm in diameter on a wall 3 metres away. This beam angle is ideal for a long-range bike headlight or a handheld spotlight.
If all is working satisfactorily, use silicone to glue the lens assembly in place.
The performance of the prototype was – and remains - outstanding. On a country road lacking any street lights, and tested on a very dark night with no moonlight or starlight, sufficient illumination was provided by the headlight to allow the safe pedalling downhill at over 60 km/h. Yep – sixty kilometres an hour! That’s one bright bike headlight...
Used as a handheld spotlight, you can illuminate objects an easy 50 metres away – and pick up reflectors at more like 200 metres. Because the spotlight weighs so little (in this type of application you’d wear the battery on a belt), it’s also quite wieldy to use.
With the quoted battery and using the Luxeon driver kit, the light should stay at full brightness for about 7 or so hours.
Next week – building two different high intensity red LED lights.