# Multiplexing LED's resistors

Hey Guys,

I am totally new to electronics and I am trying to make my own LED speedometer with as guidance this DOT matrix LED playground example. However instead by 8x8 I will use 8x10.

I am kinda lost in the overload of new information and I want to do this project as good as possible. So what formula do I need to calculate the resistors? I have the following information about my dot matrix:

• DC: 20mA
• Voltage: 1,8
• Illumination: 25mcd
• Wavelength: 660nm

(Vsource - Vled)/20mA = Resistance
Ex, (5v - 1.8v)/0.02 = 160 ohm, use standard size 180 ohm

25mcd is not very bright. You can get 5000mcd for the same 20mA and ~2.5V.

Thank you CrossRoads, That is actual quiet easy!

I will look into a brighter LED screen, this is indeed not very bright...
So lets say with a new matrix: (5v - 2v) / 0.02 = 150 Ohm, better of using this standard now?

That is the formula, Voltage/current = resistance. Voltage is the voltage across the resistor, and not the LED.

LEDs usually have range of "forward voltage drop". Use the lower value for calculating the resistor needed.

I don't think you will find an assembled 8x10 matrix. Are you planning to make your own?
http://www.dipmicro.com/store/index.php?act=viewCat&catId=511
really bright LEDs for pennies each here.

You could use several (4) of these and a MAX6953 to make a larger display,10 x 14
http://www.mpja.com/prodinfo.asp?number=18460+OP
http://www.maxim-ic.com/datasheet/index.mvp/id/3291

These guys have a bunch too

for good prices.

Thanks alot for clearing that up! I was also trying to find some high brightness yesterday but eventual I found out that most 1,2" dot matrix are about +/-20000?cd which is 20mcd, right? I assume if I want higher brightness I need to make my own LED matrix.

I also want to hook up two 8x5 matrix, I already have ordered 2 dot matrix's in this dutch webshop.

Yes, 20,000ucd = 20mcd. For brighter, then I agree you need to make your own.
For testing/debugging while all that soldering is going, the preassembled ones are nice.

Thanks again Cross!
If it's also not to bright I might consider making my own.
Also I forgot to mention that I want to have a few millimeter of distance between the 2 matrix so that you can read the numbers instead of seeing a block of LED's

The resistors & jumpers are on the way with the post so the fun is about to start soon