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?