How to build LED lighting

[elShoggotho]

Now that I built two of them, I can finally give some standard advice regarding LED circuits suitable to put light to goggles.

First off, you need some basic materials:
Suitable LEDs. If you get them in the 3.6 to 4 volts range, you need two of them.
Resistors. For the aforementioned LEDs, the value should be 170 ohms. DO NOT RUN THE LEDS WITHOUT RESISTORS!
A battery. I used an E block.
A battery socket. Duh.
Some wires, black and red.
Some solder or clamps.
A switch.
If you want to keep the battery pack somewhere else, a plug and socket combination would be of advantage.

Mount the resistors to the LEDs. Put them to the short wire.
Mount the wires to the combination. Black wire goes to the resistor, red wire goes to the long wire of the LED.
Put the black wires together, same goes for the red wires.
The switch goes to the black wires, the other end goes to the black wire of the battery socket. If the switch has three connections, one of the wires goes to the middle connection.
The red wires go to the red wire of the battery socket.

Now switch it off, connect the battery and switch it back on. If you did it right, it looks like that:



...well, without a switch. It's too small to be mounted with clamps. I'll solder it later.

If it doesn't work, disconnect the battery at once. Wrong polarity LEDs are destroyed pretty soon.

[jringling]

One word on the need for a resistor. I have single LEDs in both my goggles and my wife's. I use 3v button batteries without resistors. I am not sure if this is going to limit the life of the LED, but it isn't on very much anyway. If anybody doubts that this will work, pull apart any cheap LED flashlight and you won't find a resistor.

[elShoggotho]

You don't need a resistor if the LEDs you're going to run are in the voltage range for the battery you're going to use. In my case, it's an E block running two LEDs which are individually suited for up to 4 volts, so I needed resistors. LEDs are quite sensitive regarding voltage, so I decided to play safe.

[Gaspard de Coligny]

You don't need a resistor if the LEDs you're going to run are in the voltage range for the battery you're going to use. In my case, it's an E block running two LEDs which are individually suited for up to 4 volts, so I needed resistors. LEDs are quite sensitive regarding voltage, so I decided to play safe.

Hemmm... Led are killed by current more often than voltage...
That's why leds lit by button cell works well directly but set my bicycle on fire when they were powered by high power AAs.
Voltage rating was the same... available current... not so much...

Usual leds need between 3 and 5 V. But no more than 20mA.

[von Corax]

LEDs (and, to be honest, most other diodes) exhibit a peculiarity called "negative temperature coefficient of resistance." In English? An ordinary conductor (like a piece of wire) shows an increasing resistance to current as its temperature increases. Pass a current through it, and the temperature increases, the resistance increases, the current decreases, and the conductor stops getting hotter.

Pass a current through a diode junction, and the junction heats up, the resistance decreases, the current increases, the temperature increases, the resistance decreases, the current increases, the temperature increases, and then the diode releases its magic smoke and the current drops to zero, never to rise again.

Put a resistor in series with the diode, and the current through the diode will be exactly equal to the current through the resistor, and that current will never be greater than

I = V / R

where V is the voltage across the diode-resistor pair. That's why that resistor is sometimes referred to as a "current-limiting" resistor.

The data sheet for a LED will have two numbers of particular interest to us: the forward voltage V_F, and the maximum averge forward current I_Fmax. I_Fmax is the maximum continuous current the diode can carry without risk of damage, and is typically around 20mA. V_F is the maximum voltage that will appear across the leads of the LED (until you roach it), and is typically in the range of 1 to 3 volts. Even if the diode is connected by itself, directly to the voltage supply, the voltage across the leads (and the voltage across the supply's terminals) will not exceed V_F. Connect a diode with V_F = 2V to a 12V battery, and the voltage across the battery's terminals will drop to 2 volts. (Just for a moment; then you get smoke, and then the voltage jumps back to 12 volts.)

To choose a resistor, you need to know V_in (the voltage that will be powering the circuit,) V_F, and I_Fmax (in Amperes; if it's given in milliAmperes, divide by 1000.) Then plug everything into the formula

R = (Vin – VF) / IFmax

to get the required resistance in Ohms. Add 5% or so, just to be safe, then buy the smallest resistor larger than your calculated value.

If V_in happens to be a large value, you'll also need to watch the power rating on the resistor. Make sure that the power (Watts) rating on the resistor is larger than V_in*I_Fmax; otherwise you risk having the resistor release its magic smoke and stop working. Most of the time, though, a typical quarter-Watt resistor should suffice.

I do hope that was at least moderately understandable, and that someone finds it to be of use.

Regards,

Prof. Darwin Prætorius von Corax, Executive Director & Lead Researcher
The Leverkusen Institute of Paleocybernetics

EDIT 4 Feb 2009: reformatted formulæ for readability

[Morningstar1981]

Just a correction to the previous post - if it's in milliamps, DIVIDE by 1000, not multiply, else your calculations will be way off.

[von Corax]

Just a correction to the previous post - if it's in milliamps, DIVIDE by 1000, not multiply, else your calculations will be way off.

Duly noted and corrected. 

(I'm a Computer Science major. We don't usually do simple arithmetic...)

[The Reverend Catmandoo]

Here is a link to a page that I find quite useful when working with LED's.
http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng
CMD

[JingleJoe]

Aparently that site has a new place from which to find your LED's resistor
http://ledcalc.com/

[evand]

LEDs?  You're all crazy. 

Neon

, the new gas just recently discovered in 1898, is where it's at!

Personally, I like the warm glow.  LEDs seem harsh in comparison.  They're a bit harder to power (they need ~ 90V, and they need a ballast resistor just like an LED).  They're cheap -- I got those NE2 bulbs for $0.22 each (in bulk, $0.35 in qty 1).

If other people are interested in using neon, I could document the power supply I used (90V from a pair of CR2032 coin cells), or possibly sell a few to anyone with a soldering iron phobia

[Der Tinkermann]

Yes,I'd be interested in the power supply to these neon lights(not sure how to get 90V from a pair of coin cells?)

[evand]

In general terms, one uses a boost converter.  I built that one from a 555 timer, a couple transistors, a diode, a few resistors, and a few capacitors.  I never bothered with a schematic; I'll draw one up and post details later tonight.

Edit: new thread started about neon.  Feel free to ask questions over there.