I'm playing with a 7 segment display.

The following works on my breadboard:

5V -> 330 Ohm Resistor (same row, row 9)
330 Ohm Resistor -> Com Port (row 9 to row 15)
h led (the dot/row 17) -> ground rail

I get the dot to light up.

However, the following does not light up:

5V -> h led (the dot/row 17)
Com Port -> 330 Ohm Resistor (Row 15 -> row 38). 
330 Ohm Resistor to negative rail (row 38 -> negative rail)

I realize that the row numbers are meaningless -- things just need to be on the same row. I've included them to make it easier to visualize.

So basically, if the circuit goes:

5V -> Resistor -> Com, then I can use the ground wire to light up any part I want.

If the circuit goes Com -> Resistor to Ground, I can't use the 5V to light up any part.

I'm not sure why given that all the stuff is in serial?

1 Answer 1


Because you have a common-anode display and you're trying to use it as a common-cathode display. LEDs, like all diodes, only conduct freely in one direction.

  • I was just stating to suspect that -- as it wasn't labeled. Thanks! At least now I won't forget :-). Sep 25, 2015 at 7:16

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