By dave | May 4, 2015

If you've got a 7 segment display but are not sure if it is common anode or common cathode, then you need to check the polarity of the display. This is easily done with a low voltage supply (9V or less) from a battery, bench PSU or Arduino board.

 

7 segment polarity cicuitFirstly and before anything else, connect a 1K resistor between the common pin and the battery. This will avoid potential damage to the LED's in the display. Next, connect the resistor to the positive terminal (either battery or supply) and then connect any of the A-G segments to 0V or GND. If the LED lights up, it is common ANODE.

If no segment lights up then you need to reverse the wiring. So swap the two wires over on the battery or supply, if the LED lights up now it is common CATHODE.

If the LED has not lit up in either example then it is potentially broken. You can check all segments are working using the same technique, just connect to each of the A-G pins in turn (using the correct polarity of course).

If your display is common ANODE, then you'll have to reverse the logic when you write to it, as a HIGH value will be OFF, and a LOW value will be ON.

Pinouts used for most 7segment displays

For most displays, the wiring will look somewhat like the example. Below is a graphical diagram that shows the standard letter assigned to each of the segments and a very common wiring arrangement.

graphical diagram of a 7 segment device wiring

Hopefully, this has helped you work out what type of display you've got, and now you can get back to business.

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