Contributed by: James Alpigini, 2/28/01
I have recently made a one-ounce head lamp that I would like to share with you. The head lamp uses a 5600 mcd white LED and is along the lines of the Photon mini-light. I prefer the white LED - just my own preference. These are available at a very good price from http://whiteleds.net/ while the remaining components are available from Radio Shack, any sewing supply house, and a "dollar" store.
- 2 three volt lithium watch batteries
- 2 lithium battery holders (surface mount)
- 1 5600mcd white LED (20 ma, 3.6v)
- 100 ohm resistor
- smallest slide switch you can buy
- 1 inch elastic band
- smallest, cheapest plastic flashlight you can buy
- seam sealer
Here is the circuit. Essentially, the circuit is soldered in series using the wire as needed.
Now, prepare the flashlight. Hacksaw the flashlight at an angle like so:
Now, discard the back of the flashlight and drill small holes around the cut edge of the front of the flashlight. The holes should be about 1/8 to 1/4 inches from the edge.
Next, remove the flashlight bulb. Carefully break the glass bulb from the metal part and very carefully drill out the metal base of the bulb.
Insert the LED into the metal bulb base. You may have to strip some insulation from some wire to insulate the LED leads. Simply slide the insulation over the LED leads. You can use toothpicks to pack the LED leads tightly into the base, then give the assembly a shot of seam sealer. Now you have an LED light bulb.
Now, solder together your circuit. The lithium battery holders allow you to solder to the holder, not the battery, and also allow you to easily change batteries as needed.
Once you have soldered and tested your circuit, assemble the LED "bulb" into the flashlight piece. Next, carefully sew the circuit onto the elastic band. The flashlight was cut at an angle so that its lens faces downward when worn. Using a curved upholstery needle works well for sewing the flashlight to the elastic.
Now wrap the circuit, on its elastic, around your head, cut the elastic to size and sew to make a loop, and you are finished.
Total weight, 1 oz.
With the resistor, the bulb should last 10,000 to 100,000 hours.
You should get from 40 to 100 hours of use from the batteries.
You can reduce the resistance or eliminate it altogether to have a VERY bright lamp but I do not recommend this. It would exceed the rated current of the LED, resulting in a diminished lamp life. As we normally do not bring a soldering iron on the trail we want the LED to last.
Now for you fellow nerds out there. If you want to wire this for a different LED, the crucial component is to work out the resistor size. The calculations are simple:
VB = battery voltage
VLED = voltage rating of LED
VResistor = voltage of resistor
First find the voltage drop across the resistor: VResistor = VB - VLED
Next, use the current rating of the LED to calculate the size resistor to use. Remember that 1 ma (milli-amp) is equal to 0.001 amps.
Resistor size = VResistor / LED current
So, for the circuit above:
VB = 6 v
VLED = 3.6 v
VResistor = 6 v - 3.6 v = 2.4 v
Resistor size = 2.4 v / 20 ma
= 2.4 v / 0.02 a
= 120 ohms
Since resistors are typically accurate to only 10 or 20%, we select a 100 ohm resistor, which is close enough.
I am working on saran-wrap technology for waterproofing. :-)
James Alpigini, 2/28/01
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