Help me understand basic electronics, please!

Hello there,

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 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.

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)?

that works but it will cause dimming like you describe, you have one resistor limiting current for the LED display circuit, which is really 8 LED's packaged up in a box.

if one segment is lit up, that one led is allowed to consume 20ma give or take based on tolerances, but when two segments are lit up you are still only allowing 20ma to pass though the circuit, in a perfect world each LED would be consuming 10ma (which they wont)

so to get a uniform brightness out of each segment you would need to connect a resistor to each segment (or use a driver), and even then it might take a little tweaking to get them all even depending on the tolerance of the resistors and the quality of the display.

and yes your math is correct

Thank you Osgeld, that makes sense!

But one thing I don't think you mentioned is whether or not I have to account for each display's voltage drop, or since they are the same they only count as one?

Yes ground the cathode on each display. Use 1 resistor per segment on all the displays.

each display is in parallel to each other so the voltage drop doesn't add up, that happens when they are in series