Ok, so I've started reading and trying to understand basic electronics. I purchased an Arduino UNO to play around with and try and figure some stuff out, but it seems I am getting stuck on the basics.
I am currently working on a project that has two 7-segment (red) LED displays (datasheet http://www.rct.ru/pdf/indicator/fys-5611ax_bx.pdf) and four buttons, one button to increase the displayed number by one, one to increase the number by 5, one to decrease the number by one and one to decrease the number by 5. I have wired everything up to the best of my knowledge using resources online and written a sketch and uploaded it to the board, and everything seems to work. However, the displays have problems with dimming slightly and variably.
Now, my question is am I in the correct thinking that (based on the datasheet) each display has a forward voltage of 2.20v, a maximum Forward current of 30mA (20mA on average I have found from various sources) and with the source voltage at 5v, the correct resistors to use would be 140ohms (I only have 220ohms, which I am using)? I got this based off the formula V=IR, V = 5-2.2 = 2.8, I = 0.02. Is this correct? Or do I need to account for BOTH displays on the initial voltage, ie V = 5-4.4 = 0.6? I think I am just confused about the whole voltage drop/forward voltage concept.
And the resistors are placed on the cathode, correct? Or do I need a resistor for each anode pin on each display (16 resistors total)?
Thank you for any help you can offer, I greatly appreciate it.