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I've hooked up my arduino to a 7 segment display with a common anode..

Awesome schematic

At first I thought it wouldn't work at all because I was setting the pins on the cathode side of the LEDs to high.. then I discovered when I set the pins to the cathodes high the LEDs turn off..

Which is good, because I just needed to do a binary NOT (pretty much) on the output.

However, I don't really understand what's going on.. when the cathodes aren't set high, how is this the same as them being ground? Can someone please tell me what's going on here? I don't quite see how the circuit is complete..

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    \$\begingroup\$ Please share a schematic, it's easier to understand at a glance than a verbal description. \$\endgroup\$ – Anindo Ghosh Mar 20 '13 at 14:28
  • \$\begingroup\$ dropbox.com/s/jj9jkgl71htv5sc/2013-03-20%2014.41.35.jpg - Please note there are resistors between the 595 and the display too for each pin out of the 595. \$\endgroup\$ – John Hunt Mar 20 '13 at 14:44
  • \$\begingroup\$ Also apologies for how bad that 'schematic' is :p I'm meant to be working.. \$\endgroup\$ – John Hunt Mar 20 '13 at 14:44
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    \$\begingroup\$ Congratulations on the most awesome schematic I've ever seen on this site :-) ... Also, we don't do the "Thanks, me" stuff on posts around here. To thank someone, upvote their answers. \$\endgroup\$ – Anindo Ghosh Mar 20 '13 at 14:55
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    \$\begingroup\$ dropbox.com/s/724yvvbfrk1b29g/2013-03-20%2015.10.11.jpg < Updated diagram with pin numbers, more care (but not loads).. \$\endgroup\$ – John Hunt Mar 20 '13 at 15:11
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This is how the output of most logic chips works. When they are high, they are connected to the 5 volt supply, and when they are low, they are connected to ground. You saw this yourself; you just didn't believe it.

Equally important is why the LED is off when the output is high. When the cathode and anode are both at 5 volts, there is no voltage difference between them, and the LED does not light. Many beginners have trouble understanding the nature of voltage.

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  • \$\begingroup\$ Excellent, that makes sense. I suspected this was what was happening, but I am just starting out so it helps to have someone confirm my assumptions. \$\endgroup\$ – John Hunt Mar 20 '13 at 15:14
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Not setting the cathodes high doesn't mean they are grounded. It depends on the circuit connected to the cathodes.

If the circuit is open-collector transistors, when active they will connect the cathodes to ground but when they are inactive they will go open circuit (like a relay contact) and the cathodes will float at a voltage near the anode and the segments of the display will not be "on".

Maybe you are using a different drive circuit?

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