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Author Topic: Struggling to understand the basics of a 7 segment display common anode  (Read 1019 times)
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I have built something similar to this.



My LED works fine. The problem is I do not understand how the circuit works. The anode side, display pins 3 and 8, receive power. Lets say for now I have pin 2 of the display going to pin 2 on the arduino board.

Now in my mind the pin 2 on the arduino board should complete/ground the circuit. However the program sketch uses pinmode(2, OUTPUT) and I use digitalWrite(2, HIGH) to turn on that specific led.

How does the cathode side of this CA 7-seg display work?
« Last Edit: February 09, 2013, 11:02:12 pm by crosson » Logged

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Cathode needs to be LOW for a segment to turn on.

Perhaps it common cathode & not common anode?
Since you show what you think is the common anode connected to 3.3V, and the LEDs turn with 5V from an output pin, that would suggest a common cathode part.
Try moving the 3.3V connection to 5V - if the LEDs no longer turn on, that would prove my conjecture out.
Move the connetion to Gnd instead, and you should see working LEDs with improved brightness.
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You were right. I was blinking the leds confusing myself. To confirm I turned all leds on by letting it remain on digitalWrite(2, LOW). Also tried your experiment and you were double right. So when I set the Write to LOW does that send power in that direction?
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The last thing you did is where you should start looking.
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See attached images


* 7seg.jpg (141.1 KB, 561x469 - viewed 24 times.)

* CCCA.jpg (24.4 KB, 455x267 - viewed 21 times.)
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The way you have it in your schematic isn't the same as how you have it wired up! That goes for me too.

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See the common cathode diagram above - writing your outputs  high source current into the anodes to turn the segments on.
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Incidentally the 3V3 rail isn't a good choice for LEDs since

a) Low current only, might not be able to drive many segments at once
b) A bit too low a voltage for blue and white LEDs - may only be able
   to light up dimly...
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