# How many LEDS?

I need to build a circuit that will light 36 LEDS at the same time. I only need one flip switch to turn the thing on/off. I'm not sure if I even need an Arduino, but I don't what circuit I would need. If someone could give an example circuit with an explanation, I would greatly appreciate it. I know this is probably uber-simple, but I'm new to this and really need this to work

You need to figure a few things out first:

a) What's the specs on the LEDs? Voltage? Amps? b) How do you plan on wiring them, serial or parallel?

At the very least, you need a power supply to drive them, its output will be determined by the above two questions.

If you have a high-enough voltage you can string them all together in series and use one current-limiting resistor.

If your voltage is lower you can make strings of several LED's, each string with a current limiting resistor, and put the strings in parallel.

What is your supply voltage? What is the forward voltage of each LED? What is the current requirement of each LED?

http://led.linear1.org/led.wiz

Look at the datasheet for your LED and find the forward voltage. It should be something like 1.9 ~ 2.5. If these are regular leds, you can just enter 2.5 to be safe.

Source voltage is your battery. It needs to be greater than the forward voltage of the LEDs, so a single AA battery won’t work. Two will give you 3v, so that would be fine. 9v would be okay as well, but the higher the voltage above what the LEDs need the higher the wattage rating you may need for your resistors because they will generate more heat.

Forward current is how much current you want to put through the LEDs. This defines how bright they are. You can safely enter a value up to 20mA here.

And for number of LEDs in the array just enter 36.

Now click wiring diagram, and click “design my array” and the wizard will spit out a drawing showing you how to wire the LEDs to your switch and battery.

Note that if you use 3V to drive the leds then you’ll see there’s one resistor for every two in series. I oversimplified when I said earlier when I said a higher voltage would dissipate more heat. What I really meant to say was the more voltage you have left after adding all the forward voltages of he leds in series and subtracting them from the source voltage, the larger a resistor you would need. In other words, if you use a 3v battery with one 1.2fv led, you’ll have 1.8v remaining that the resistor has to turn into heat, unless you stick a second led on there, in which case there would be 0.6v remaining to dissipate. But 2 leds would be the limit since you’d have run out of voltage. If you have 9v though, 9v / 1.2v = 7.5, so you can run 7 leds off a 9v and 9v - 7*1.2 = 0.6v, so you would need the same resistor for a string of 7 leds on a 9v as you would for 2 on a 3v.

I screwed up my example though cause 1.2v is way too low for a single led. 2.1 is what I meant to use. So 3v would light one led in series, and 9v around four.

There’s one last thing to know about using 9v though and that is that let’s say you want to light 5 leds. Well you can only put 4 in series, so the 5th one has to be off by itself. Which means that one will be getting the full 9v, and it will need a much larger resistor in series with it to dissipate the heat. The wizard will take care of this for you though. It will show you a bunch of possible circuit designs, and one will be with the number of leds on each string balanced as well as possible. But that does mean you might still end up with 3 leds on oen string and 2 on another, so you will still need two different resistors if you want the leds to have the same brightness.

I think that about covers it. Just try that led wizard, it will make things clear.

You probably definitely don't need an arduino.

Use the link by the poster above me, it's really good. Most LED's are about 1.5V-3.0V forward voltage with about 15-25mA forward current

Thanks a heap. That calculator is just what I needed.