Transistor controlled LED ring and help with resistor calculations

So a few days ago I posted this:,100214.0.html and after a quick trip to Fry’s and some experimentation I have some more questions.

First, the refined project description:
50 LEDs will be arranged in a circle that can be worn like a necklace (I plan on using some sort of clear tubing to house them) and driven by the arduino in groups of 5 to create a sort of light show. Digital output pins 1-10 will be connected via 10k ohm resistors to PN2222 transistor base pins, and 9V will be connected through a 180 ohm resistor (I’m not sure about this resistance, see questions below) to the collector pin. The emitter pin will be connected to 5 of these LEDs ( in parallel, which will then be connected to ground. The “5 LEDs” is an arbitrary number; 3 or 4 would probably work too but I figure the more the merrier and/or brighter. Now for the questions!


  1. The LED packaging says “1.85V @ 10mA”. Does this mean 10mA is the maximum current or the normal operational current?
  2. Assuming the LED voltage drop is 1.85V, the resistor should drop 7.15V, right? Originally to find the resistance I used R = (9v)/(.05A) = 180 ohms (.05A is from 10mA x 5 LEDs) but using 7.15V think I should use a 150 ohm resistor. I am leaning towards using the 180 ohm resistor, but is a 30 ohm difference in resistance even that significant?
  3. After building my circuit without the arduino or transistor, I learned that my little resistor (I was using a 330 ohm 1/8W or 1/4W one because it was the closest one above 180ohms I had) heated up quite fast. Am I correct in calculating that P = (9V)(.05A) = .45W (again I use 9V because I would rather guess on the high side to be safe)? In this case, I assume that I need a 1/2 watt resistor, or should I get a 1W resistor? And are 1W resistors even common at electronics stores?

4) Instead of using 10 transistors would it be easier to buy a shift register? Correct me if I’m misusing “shift register”, I have no idea how they work/what they do exactly.
5) If the wattage of the resistor is almost 1/2 watt, do I need to be concerned about the power/heat dissipation of the transistor too?
6) I have no idea what to buy here; “2N2222” or “PN2222” transistors seem to be common, and my project appears to be within the voltage and current limits for the transistor so I assume they are a safe buy? (Based on this datasheet:

  1. Finally, I am worried that connecting the LEDs in parallel may be a bad idea because if one were to break/get disconnected/whatever, then more current would be delivered to the other 4, potentially damaging them. Maybe I could connect them in a series somehow, or do I have an irrational fear?

I know have a ton of questions so I don’t expect any one person to answer all of them, but any help at all would be very much appreciated!

LED ring sketch.png

I would read this:

Also LED current calculator:

I believe connecting LEDs in parallel is a bad idea. I can’t help thinking that putting a few in series kills a few birds with one stone. For one thing, you don’t need to drop 7.15 V any more, thus saving on quite a lot of heat.

Please keep your threads together. Start a topic, and stick with it.

You want 10 groups of 5 LEDs each.
Use your transistor idea and arrange the LEDs like this:

I’ve included the voltages and suggested currents. Tweak as you like & solve for resistor values.

For 2 LEDs: (9-1.85-1.85)/.02 = 265 ohm. Use a 270.
Resistor power: P = IIR, so 20mA * 20mA * 270 as an example = 108mW.
So a 250mW resistor (1/4 W) would be just fine.

Sorry about my lack of forum etiquitte; I am bit of a newbie. Based on CrossRoads' design I think I will connect 2 groups of 3 LEDs to 1 transistor collector pin for a total of 6 LEDs controlled by 1 arduino pin. Each group of 3 LEDs will need a resistor, so R = (9V - 3*(1.85V)) / (.01A) = 345 ohms (or the next highest resistance available if 345 isnt common). Is this correct or is there a voltage drop accross the transistor that I need to account for as CrossRoads did for the resistor between the arduino and the transistor? I would post a circuit diagram but am currently using my phone for today as I do not have internet access on my laptop. The design I described is the same as CrossRoads' but with 6 LEDs instead of five for symmetry and so that the two resistors can be the same resistance. Any answers to my original questions would still be appreciated as well! Also, how long could a 9V power 50+ LEDs? Would I kill the battery within the hour (in which case I will look into buying some rechargeable batteries) or within a full day of lighting all of the LEDs or can the battery life even be guessed given the current draw of the LEDs?

Yes, voltage drop across the transistor as well. Typically 0.7V. (9 - 3*1.85 - 0.7)/xxmA = Resistance Think I got distracted & left it off. Vce @ saturation is typically around 0.7V.

9V battery are typically only good for a couple hundred mAH. So 18 strings of 3 LEDs, each string drawing 10mA, is 180mAH. So maybe you'd get 2 hours of life? With LEDs getting dimmer fairly quick I'd imagine.

Ok, so I think I could get by with a 9V since all LEDs won't be on at all times. Tomorrow I'll go buy a bunch of transistors and resistors and see how it goes. Thanks for all of your help, so far I am loving this forum!

One last thing: in the circuit diagram the emitter pin is connected to ground. Does that mean that the negative terminal of the 9V and an arduino ground pin should be connected together along with emitter pins?

Thanks again for helping a newbie get into electronics!

Grounds - yes, all connected.