DescentBB

I don't know anything about resistors but I think this should work.
Pic I found online:
[url removed]
If I hook up an ir led to a battery with too much power I'm sure the battery would overheat and catch fire. And I think the same could be said if I draw too much power from the battery with a single ir. But I think I have some leeway with a AA battery. I guess I could keep it on a metal pan while I use it.

LED's have strict current limits. Go outside this range and *POOF*, but they are simple devices to understand.

Each LED (diode) conducts electricity in a single direction. It also will have a specific amount of voltage dropped in this direction. Unlike a device like a resistor, the voltage dropped across a diode is fairly uniform over a wide range of currents.

How do you use this information to drive an LED?

Let's use an example...

Forward Voltage of LED (Voltage Drop): 1.0V
Maximum Current of LED: 20ma

If this were the LED's specs, I would run the LED at 10ma, not the max, so we have...

1.0V @ 10ma

Now, how do we use this? Ohms Law.

V=I*R

Ohms law states that voltage is equal to current times resistance. This equation can be written to satisfy any of the 3 variables.

Also, you will need to know that in a series circuit, the total voltage across the circuit is equal to the sum of the voltages of each element.

The total current in a series circuit is the same in each element of.

So, armed with all this, we can figure things out now.

Voltage across battery: 1.5V
Voltage across LED: 1.0V (from example)

This leaves 0.5V. That would be the voltage across the resistor.

Total current: 10ma

Since current is the same in all elements in a series circuit, that means the LED, resistor and battery would all have 10ma flowing through them. We only care about the resistor.

Resistor Voltage drop: 0.5V
Resistor Current: 10ma

Now we need to know what value resistor to use. We use ohms law.

V=I*R
0.5V = 10ma * R

or...

0.5V/10ma = R

Do the math and we get 50 ohms. If you can not find a 50 ohm resistor, you can use the nearest value.

If you change the voltage of the battery or use a different LED, you will need to recalculate the value for the resistor.

Lastly, if the voltage drop of the LED is higher than your battery, you will need to place another battery in series.

Cool!
Based on my experience with holes in buckets I assume a higher voltage and resistor, will elongate the battery life.
(cheating with an ohm calculator) So if I had a 9v / 10ma = 900ohm I could use a nine volt battery on a tiny ir led as long as I had 900 ohm worth of resisters attached somewhere in between.
Thanks a ton for your post. I was going to spend big money on something and instead will buy a breadboard and a few parts.

Great! Just remember the voltage across the resistor equals the voltage across the battery minus the voltage across the LED.

Battery life is based on Amp Hours(AH). Typically, you will get more AH on a AA,AAA,C,D than a 9V, so you would rather use those.

Typically, a 9V battery only has about 0.3AH, where as a AA has anywhere from 1-2AH.

Since your LED is consuming a fixed amount of current, you can easily tell how long approximately a battery will last.

For 10ma draw...

9V (0.3AH) = 30 Hours
AA (2AH) = 200 Hours
AAA (1AH) = 100 Hours

However, when using the 1.5V batteries, you may need 2 or more in series to provide the amount of voltage required.

Also, get yourself some visible LED's so you can experiment before frying your IR LED's

Do not look directly into the newer high power LEDs!!!!

I know that sounds out of context, just don’t do it.

Spidey wrote:Do not look directly into the newer high power LEDs!!!!

I know that sounds out of context, just don’t do it.

lol. Listen to Spidey!!! High Power LED's will leave you crying for yo mamma!

Even the super bright ir leds?

Isaac wrote:Even the super bright ir leds?

No. Although you might get a warm and fuzzy feeling from it.

Just because you can't see it doesn't mean it can't still damage your eyes, actually the results can be even worse because the protective blink reflex is only triggered by visible light.

Yes, Krom is right if the LED is powerful enough. Basically, looking into an IR LED is equivalent to looking into a Red LED although without your ability to detect the brightness. Hence, experiment with your visible LED's first.

Thank you all very much for this excellent information. I'm now capable of building this simple yet important module. Especially you SuperSheep! Thank you!

Isaac wrote:Even the super bright ir leds?

These are even more dangerous since your eye doesn't register the wavelength and won't contract your iris (edit: or cause you blink) -- you'll get the full load, possibly damaging your retina and/or lens.

This might be the start of something new for me.
While spending a few hours in the circuit board section at frys electronics I came across so many different little parts including programmable chips and prebuilt modules.
In the resistor isle I found other things like 8bit lcd character displays. Blank pci boards and big 2'x1.5' circuit boards. PC power supply plugs. It made me wonder if someone could build their own video card...

I still don't fully grasp how something like my motherboard works but that could change if I started building clock radios and rc cars from scratch.

lol...you have been bitten by the electronics bug!

I would strongly suggest getting good at the basics first. You will make a lot of mistakes starting off and your most impressive creation for a while will be a smokestack generator.

Buy a breadboard, light gauge hookup wire, resistor assortment, a small selection of capacitors, some 9V batteries (or a power supply), and of course, some LED's.

Get yourself a good electronics project book (for beginners) and buy the rest of what you need from their parts lists.

Microprocessor based projects are wickedly cool, but having a firm grasp of the basics is essential. Start off by building your own multivibrators, timers, comparators, amplifiers, and integrators. Learn how to bias transistors, what logic level high and low are. Oscillators, feedback circuits, transmitters and receivers.

I started with a \"555 Project Book\" and dozens of 555 timers and built all sorts of things. If I had to go back and change anything, I would have learned transistors better and analog design.

I strongly agree. Especially with this:

SuperSheep wrote:...but having a firm grasp of the basics is essential.

Here's a LED tester that one of our engineers hacked out for testing polarity. Every so often it changes if we get a different supplier. Appearantly, there is no industry standard for negative and positive with LEDs.

Ha! Mine's better! :p
viewtopic.php?t=15876

LOL, all our leds low voltage and were burning out if I held the button for longer than a couple of seconds. The Pot was added to remedy that.

I could have used that little tester. I wanted LED's for my three garage door openers and their respective switch buttons so that you could see the buttons in the dark. Fortunately, the circuit that powers the activation button is very low voltage, so some 1.5V LED's from Rook Shaft worked just fine, plus I got three different colored LED's to denote which door the button opens. Since there's always a small current in the circuit, the LED's are lit even when the door isn't being activated.

Great job Isaac!