# How to calculate the resistor value necessary for the columns 8x8 led matrix?

I am building an 8x8 led matrix using 5mm LEDs (3.2V, 20mA). I connect it to the arduino UNO through the I2C using the GPIO expander MCP23017. The circuit is as follows:

A matrix column is made up of 8 LEDs connected in series (am I right? are they not in series?). The forward current of an Led is 20mA and its forward voltage is 3.2V. I connect the VDD pin of the MCP23017 to the 5V pin of the arduino uno.

So, the resistor value I need to connect to a column so that the LEDs don't die out is (5V-(3.2V*8))/0.02, which is a negative value! Shouldn't I multiply 3.2V by 8? if not, why?

• See if lednique.com/display-technology/multiplexed-display helps regarding whether they're in series or not. (They're not.) Commented Mar 13, 2021 at 22:00
• something is wrong ... the leds appear to have three terminals ... two of the terminals are shorted for some reason Commented Mar 13, 2021 at 22:13
• @jsotola if you're talking about the blue lines, I've just realized I didn't draw them correctly. The anodes of the leds of one column are wired in series (column wise), and the cathodes of the leds of one row are wired in series (row wise) Commented Mar 13, 2021 at 22:24
• that description is unclear ... google led matrix schematic Commented Mar 13, 2021 at 22:58
• You should draw a proper, correct schematic and delete the existing one. Commented Mar 13, 2021 at 23:36