How do I find out what kinda LED resistor I need?

How do I find out what kinda LED resistor I need? Or even if I need one?

I created another thread about this already. I'm making a arc reactor (From Iron Man) for halloween, and 10 LEDs will go in a circle around the reactor molding. I thought itd be neat to control it using my arduino for the lighting (power on/off fade, press a button to trigger the center LED's UV light to glow on and off), etc. Or just have the entire thing just pulse.

Anywho, I read that I can power a LED with just a 3V Watch battery. But if I'm hooking 10 LEDs to my board (and maybe 3 center UV LED's), how do I find out what kind of resistors I should use? Or if I even need one? I'm scared to hook a LED up to the board while its connected via usb to my computer. They're all 5mm Single Color LEDs.

I dont know how Im planning on powering the board and LEDs also. I have about 25 3v watch batteries I ordered, and I'm also thinking about just buying a 4/8 AA holder from Radio Shack, and hooking that up to the arduino. Whatever one works for a longer time I'll do. Unless I can electrical tape a bunch of 3v watch batteries to the back of the arduino (that way nothing run in my pocket)

Anywho, thanks! Looking forward to the tips so I can start. Ill post pics too! Excited :D

LEDs Specs (what I can read on packaging): 20x 5mm Blue LED : 3.0-3.4V / 24mA max / 8000 mcd 20x 5mm Orange LED: 1.8-2.2v / 24 mA max / 5000 mcd 20x 5mm White LED : 3.0-3.4V / 24mA max / 20000 mcd

10x 5mm Cool White LED: Forward Voltage:3.2V; Forward Current:25mA; 4000mcd 10x 5mm UV LED: 200mcd; Forward Current:30mA; Power Rating:120mW; Wavelength:405nm; Output Power, Pout:120mW; Supply Current:20mA 10x 5mm White LED: Forward Voltage:3.2V; Forward Current:25mA; 23500mcd Intensity

And I was hoping I could use this to control the modes and power it on?: Tactile Switch 50mA, SPST-NO, DC Nom24V Contact Volt, Contact Max Current: 50mA

I already bought all of this

thanks! That helps me decide the resistor I need. All I know of my UV LED is listed above. Is there anyway to tell the Voltage of my UV LEDs? And for the other LEDs, im a little curious as to why it gives a range. (3-3.4V...) Idk which to choose.

And does anyone have any suggestions for me? (On how to hookup my LEDs and what kind of power to use? Aka: AA's or 3V's)

Honestly I wouldn't spend too much time thinking about it - LED's are reasonably tolerant. Close enough is almost always going to be good enough.

Whether you use AA's, 3V watch batteries, or something else comes down to how long you want them to run for and your portability concerns. Obviously watch batteries are smaller, but you won't run it for very long. Why not just go for a 9V battery, and carry a couple spare with you? Portable and simple.


can a 9v battery power 10 or 13 LEDs?

Is my way of thinking wrong?: Each LED needs 3V of power to run (since it says rated 3.0-3.4v). So by that thinking, a single 9V Battery can power 3 LEDs?

oh yeah, and its cool to laugh at how i thought it works :)

can a 9v battery power 10 or 13 LEDs?

Maybe, maybe not. Depends how they are wired.

It's not a matter of voltage, it's a matter of current. If you put the LEDs with resistors in parallel, each LED gets 9V. However, each LED consumes some amount of current. How much is a matter of applying Ohms law using the voltage and resistor values. If the battery is capable of supplying the total current, the battery can power 10 or 13 LEDs. If not, it can't.

Safe option that almost always works for any LED on any voltage up to 12V - 1k ;)

That should get the purists going....

Safe option that almost always works for any LED on any voltage up to 12V - 1k

Not safe, that's the gospel version. I heard from my cousin's girlfriend, who once dated a guy who cleaned the pool of someone who designed electric stuff, that for LEDs you need to use 1kOhm below 12V. Or was it that the resistor needed to have at least one orange or two red rings, the darkest ring facing towards the LED.

In contrast, the safe option would be to learn and understand what you do.


I always put the darkest ring away from the LED :o

Have a think about your home appliances. They all need 230v/110v and your supply is the 230v/110v. Can you run more than one appliance at once?

Can you run more than one appliance at once?

Well, I've never managed to run the radial arm saw and the table saw at the same time. Too far apart.

Shock, horror: A "Sensible" contribution to this delightful thread....

For more on LED resistors, though I've failed to say anything about which way the dark band goes...

(the band thing is, by the way, newbies, a joke. Resistors can be put in either way 'round)

... see...