homemade LED flashlight/torch

have, being a non-scientist.

The key is to understand that you must control the current flow through the led to keep it from burning out. The resistor acts as a crude current regulator.

Here are some good sources:

USING LED's

Current limiting resistor calculator

LED Apps

LEDs 101

I need to get approx. light of a 2xAA pocket incandescent flashlight?

I'd get a catalog from a place like

and pick some led's with the high MCD ratings. Stay away from super duper LEDs ("Luxons") for the time being.

As a practical matter, for these simple circuits you will need at least

3 volts to run red or yellow leds. At least 6 volts to run white ones.

Stephen Powell

Electronic Hobby Information Stuff that should be easy to find, but isn't

Lots of alternatives to consider. If you want to keep this dead-simple, you'll need to provide a voltage source that exceeds the required voltage needed for the LED and use a dropping resistor to help establish a predictable operating point.

LEDs require different voltages depending on their color, so reds will need about 2V while blues will be more in the 3.5V to 4V area. You need to find a specification sheet for the LED you want to consider and if you are going to consider several, you'll need several such sheets. Yellow and orange are probably in the roughly 2.5V to 3V (just guessing, as I haven't seen a sheet on these.) You will need at least this voltage for those to operate. Whether or not you'll be satisfied with the results is entirely a different question. But incandescent bulbs provide blackbody distributions in their light, which is quite different from what you will get from a yellow LED, so you may not like the LED as a replacement. That's why the white LEDs are often worth the bother. On the other hand given your 'red night vision' point, you may like it better.

The actual resistor you'd use can vary. The calculation is something like this, though. Assume you read from the data sheet that a certain LED has 2.15V when

40mA is running through it. Also, assume that you are using 2 AA batteries for your supply and that you want to guess that the combined voltage will average about 2.8V during most of its normal operation. Then:

R = (2.8V - 2.15V) / 40mA = 16.25 Ohms

This is an estimate, based on assumptions and you don't need to have an absolutely exact value like that. There are two nearby standard values of resistors, one at 15 Ohms and one at 18 Ohms. Either of these may be just fine to try. If your estimate of 40mA for running the LED was a bit on the high side, then try the 18 Ohms. If you really wanted a little more current, then try the 15 Ohms. Also, keep in mind that the specification sheet tells you typical values, usually. So the 2.15V mentioned (hypothetically) would be a typical number. The actual voltage for some LED you actually try may be a little different, as well. The point here is that these calculations are approximate. They get you into the ballpark, but you may want to have several nearby values to try out and see what you get in practice.

You can, however, operate an LED entirely well from a single alkaline AA. But you will need three components in addition to the LED. The circuit can operate any of the LED types, including the higher voltage ones. It does so through the use of a small transformer (coupled inductors) to get the voltage up. The nice thing about it is that it will drain the battery to near-dead state, so your lifetime of operation is pretty good. It doesn't regulate well, though, so as the battery gets lower your light output will similarly decline. The problem here is that to understand it you need to know more about ideas in electronics.

Jon

Thanks for your help people. FYI I'm in Taiwan. I used to think this stuff was cheaper here than the US but it looks like you got good deals if you look at the right places. I am aware that yellow or orange LEDs aren't great matches for incandescent color but will be adequate for finding my way in a blackout. Now if it's not a dumb question: what are the volt/current/other specs for an LED (let's say orange) that should have roughly the same light output as an 2xAA incandescent flashlight? Unless I missed it, don't think there was this info in the FAQs. Otherwise now I do know how to calculate what resistor I need for a given setup. Since higher volatge tends to be safer, should I use a 9v batt instead? No thanks, don't feel like playing with transformers yet. :)

Seb

Have a look around:

and

For red, amber, yellow green 2 AA will be fine, blue, white, true green need a bit more voltage.

You can just stick a red Led in a 2AA maglite and it will work, internal resitancew of batteries stops LED getting toasted immediately, Adam

I don't know exactly. I have the CIE curves describing human responses to colors and brightness and I could probably calculate this from the spectral emissions of some LED. But I believe if that the usual measure in candelas already incorporates this detail of human perception -- so, as a layperson on this subject, I'd suggest that you simply compare the candelas and also at the same time look at the dispersion angle of the LED, too. You can get specs from various incandescent lamps in candelas, I think, for comparison purposes, but you will also have to realize that most flashlights use a parabolic reflector so that you will get an apparent concentration. I'm guessing here, but I believe you need to get an idea of the effective dispersion beam of a typical flashlight, get the candelas for the lamp itself from a spec sheet, and then somehow increase the effective candelas on the spec sheet to take into account of the fact that the reflector puts a spherical radiation pattern into a tight, forward beam (which should increase the effective spec.) It's that figure you can compare with the LED values for a similar dispersion pattern, I think.

Jon