Wednesday, June 01, 2011

LED Torch with a difference

This LED Torch has a difference, when most people convert torches to use LED lights they are usually interested in super bright LED's and illuminating entire football pitches with a brilliant white light using something that can be stored in their pockets.

This torch I actually need to be intentionally dim. I also don't want white light, I want red light.

This torch is a star chart reading torch to be used when outside and looking at maps.

it could also be used whilst in a car (assuming that you're not driving) to read maps whilst you're going along the road.

The theory is actually quite simple.
When you switch off the lights at night, you can't see anything, then after a while, you find that you're able to see quite a lot. after ten minutes you're able to see a reasonable amount of things, after half an hour you can see so much more.

How it Works
This is why, the retina of your eye is made up of photo receptors (things that sense light), these are broadly divided into two groups called rods and cones, seemly like an odd name for bits of your retina, but under a microscope the rods are rod shapes, and the cones are, well cone shaped.
Your rod cells contain a chemical called Rhodopsin, it's this chemical that is senses the photos in light and lets you see. though strangely this chemicals actually breaks down into two different molecules (retinal and opsin), these two molecules, do re-combine, but at a very slow rate (hence why it takes time for your eyes to adjust to the dark!)

Why it's Important
If you're looking up at stars this is especially important, some stars are extraordinarily dim this is why light pollution is a problem for anyone looking up at the stars, the night sky is effectively partly illuminated, for the brighter stars that's not a problem they cut through the light shroud that covers us, dimmer stars don't have enough light coming our way to do this, so we don't see them.

A Simple (no tools or skill required) Experiment
If you are interested in stars, and even if you're not I highly recommend that you drive out to a field somewhere (actually the middle of nowhere is best!), then leave your car and walk into the field, you might need a torch to light your way to get to the middle of the field. When you are in the middle of the field turn off the torch and look up-you'll see loads of stars (more than you'd normally see when looking up when in a town or city. stay there for a while, after ten minutes, look up again, there are seemingly more stars.
Wait half an hour and look up again, try not to fall over backwards as now there will be so many stars in the sky that you'll be amazed at how they all got there!

-you can go back to your car now. and by this point your eyes should be so well adjusted to the dark that you don't even need a torch.

Red Light
The rod cells in your retinas do not perceive red light, if when you were out in the middle of a field you switch on a normal white or yellow torch the Rhodopsin will break down into its component molecules, but since the rod cells don't perceive red light, looking at something with a red light will leave the Rhodopsin (and your night vision) perfectly intact.

Making a Red Light Torch
Even though red light does not make you loose your night vision, you ideally still want to keep the light output down to the pretty much absolute minimum that you can.

Parts required
You will need:
>a torch body (if the bulb is blown then don't worry about replacing this) the torch body must be intact, and the glass front (for keeping out water) and the switch to turn it on or off
>Batteries for the torch. (I have a torch that requires 2 AA batteries, this give a total voltage of 3Volts when using non-rechargable batteries, or 2.4Volts when using rechargeable batteries.
>Red LEDs I used 4 normal 5mm (5v) LEDs, you may choose to use just a single one. bear in mind the low voltages listed above, you may want to use some low voltage LEDs
>A "midrange" value resistor. think above 1k below 10k you can figure out the values for optimum use using the data sheet for your LED.

Ohms Law and Resistor Values
Figuring out the resistor values is easy. you use Ohms law (or at least a part of it).
Ohms law states V = IR (or V/R=I or V/I = R depending on how you write it)
V is voltage
R is resistance
I is current

lets say that LEDs have a maximum current carrying capacity of 3milliamps.

basically you just put those numbers into the equation.
V = 3v
I = 3mA (0.003 amps)
V/I = R

3/0.003 = 3000 Ohms (3Kohm).

I places 3 LEDs in parallel (therefore splitting the current load between 3 devices). so my total maximum current draw is now 9mA and my resistor needs to be at least 1k.

That would be the resistor value needed to illuminate the LEDs to their fullest brightness, which I didn't want to do anyway, so I chose a trusty 4k7 resistor.

Beam Angle
Now you may be wondering why I say don't illuminate your LEDs to their fullest brightness, but use multiple LEDs which will increase the light output. that's to do with the beam angle.

A single LED has a relatively narrow beam angle, try illuminating an LED and looking at it from the top compared to looking at it from the side. you'll see that the beam is focused into a reasonably narrow channel, this is good for some applications, but for throwing some light onto a large sized star chart, not so good, so I used 3 LEDs arranged in a flared out triangle to increase the beam width pattern.

Unless you're building this into the body of a mini MAG light using miniature LEDs I would recommend this, if you are using a mini MAG light then, it's not the torch that I'd have picked (and I did have a broken one of these I could have picked), but I understand it's going to look a bit nicer than my slightly more bulky plastic wolleys torch.

Putting it all Together
There is no real skill involved in putting it together, -you don't need a doctorate in electronic engineering, but it can be a bit fiddly, so have some patience!

Start with the LEDs take the short lead (or flat side leg) of each LED and put these together being careful not to pull the legs off any of the LEDs. now solder them together.
take the positive side and group then solder these together also. to one side of your LED bulb attach the resistor that you're chosen.

Take the front part of the torch containing the bulb and reflector off of the torch.
remove the bulb.
Carefully push the LED assembly into the bulb housing, (you may need to pull at the assembly and poke in one LED at a time if the hole that the bulb went through isn't big enough).

Inside the reflector you should now have all your LEDs, poking then through one by one means that they've likely splayed out by themsleves anyway, but if not try to make the tops of the LED point towards the reflector in the torch.

Now you can either choose to try to solder wires to the bulb and connect these straight to the switch, or you can position the legs of the LED and resistor into the existing bulb holder in place of the torch -i.e in the future if you need a torch you can just take the LED red bulb out and use a normal bulb again. I went for positioning the wires so that the resistor was in the middle and would make contact with the centre spot (+ve) connection and the short legs of the LED touched the outside of the bulb holder (-ve).

Try turning it on. you should get a light that looks pretty useless indoors, and doesn't really light up a lot, because it's so dim.
But outside, in the dark, it should provide just enough light for you to be able to see to set-up your telescope, or see star maps -without ruining your acquired night vision!

This cost me nothing but my time, because I already had batteries, a torch LEDs and resistors
however, a rough costed parts list is here.
torch £5 -Wolleys is closed now, but your cheapest supermarket/hardware store/pound land will probably have a torch. don't go overboard and you should get a reasonable torch for not a lot of money.
LEDs -these are in the order of pence when buying in bulk however if you go to Maplin they will be £0.60 each. you'll want three (£1.80)
Resistor -again pence, really look up Rapid Electronics or CPC, don't be afraid to create an account and get parts delivered to your door, from Maplin £0.24 for a single resistor. (twice the price for a single resistor at Rapid, or 3 times the price of bulk buying at Rapid!

Total cost (maplin price) = £7.04. (includes torch)

Which is not a lot to pay when you consider it'll save you half an hour of standing waiting for your eyes to adjust!

Alternatives -and why I didn't use them

Red filter - I tried a red acetate filter, too much white light was escaping around the sides and would have lessened the acquired night vision.

Red filter with torch body covered in Gaffa tape -would have worked but just looked messy.

Red Nail Varnish -I didn't have any, and buying some would have meant spending as more on a bottle of nail varnish as I would have on buying 3 LEDs and 1 resistor anyway! for a substandard result and permenant destruction of a torch -I can take my LED bulb out and have the original torch back. -but if your don't care about the torch and you have access to red nail varnish then you may as well try it that way.