7 segment display with common anode

Question

I am brand new to electronics. I am trying to use a seven segment display breakout module with my arduino uno. My problem is that the display has a common positive instead of negative common. How do I make this work? As far as I am aware, all the arduino pins output positive with the exception of the ground, which is the negative common.

• PhillyNJ: You might want to add what driver you are using...Max7219?

Answer 1

This answer assumes Arduino Uno. For other types, refer to the documentation.

Ardino pins are not positive per se: you can change the voltage with digitalWrite() to either 0V (LOW, equal to the GND pin) or 5V (HIGH, equal to VCC).

The difference between a common cathode or common anode will be the flow of the current. With a common cathode (negative), the pins will need to provide current ("source" current), with a common anode, the pins will need to receive current ("sink" current).

There is a description for the limits of sourcing and sinking:

Pin SOURCE Current Limitations:

20 mA at VCC = 5 V

Pin SINK Current Limitations:

20 mA at VCC = 5 V

Please also read about the maximum total of 150 mA sourcing and 100 mA sinking.

You'll need to compare those current limits with the current requirements of your 7 segment display. My guess is that your a 7 segment display has 1 VCC pin and 7 output pins that you connect to the Arduino. The limiting factor is the total maximum here. The Arduino can handle 100 mA/7 = 14mA in the worst case. If that's enough for your 7 segment, it should work.

For example, this one works with 10 mA, so it would be compatible with Arduino. Make sure you have resistors in place.

• pvGuy: Thank you very much for your sagely wisdom! I was so wound up over trying to get an answer to my problem, that I neglected to mention that I am using an Arduino Uno! Again, thank you so much for your time and effort.