So a few days ago I posted this: http://arduino.cc/forum/index.php/topic,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 (http://www.weisd.com/test/WEISD_TBL_view.php?editid1=LINB4303F1) 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!
- The LED packaging says “1.85V @ 10mA”. Does this mean 10mA is the maximum current or the normal operational current?
- 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?
- 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: http://www.datasheetcatalog.org/datasheet2/8/0uhjxp5gawo2x3s668ahs717f3py.pdf)
- 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!