question about 7-segement led display on a homebrew computer

liquidpaper · Joined: Nov 23, 2007 Posts: 93 Location: venus

Greetings!

This question is about homebrew computers rather than homebrew synths, but hopefully someone will know the answer. If anyone could suggest another forum on the net that I could ask, feel free to do so. I'm building an ultra-simple homebrew computer. It's from the book "How Things Work" by Neil Ardley. You can enter two 4-bit binary numbers, and the result is displayed on a 7-segement led display. If the result exceeds 9, the display goes blank (see what I mean when I say simple?). I have it all breadboarded up, but I am having a problem with the seven-segment L. E. D. display. The computer seems to add numbers correctly, but the L. E. D.s in the display light up opposite to the way they should. So for example, when the result should say 0, only the horizontal L. E. D. in the middle lights up (that's the only one that is supposed to be dark). When the display is supposed to be blank, all the segements light up (so it looks like it says "8"). It works this way for all the numbers. Do you know what might be causing this problem and how I can fix it? Thanks so much for any help!

Tim Servo

Hi Liquid,

It sounds to me like you might have the power hooked up backwards on the display (+ and GND) or more likely, the schematic was written for a "common cathode" display, but the part you're using is a "common anode" (or vice-versa). You might want to double check the display, and maybe rig up a simple test for one segment to see if it lights up in the way you expect.

Tim (lit up) Servo

liquidpaper · Joined: Nov 23, 2007 Posts: 93 Location: venus

Yeah, I had a common anode, when it requires a common cathode. Can you fix it with NOT gates?

liquidpaper · Joined: Nov 23, 2007 Posts: 93 Location: venus

Oh, and thanks for the reply!

marinedalek · Joined: Sep 30, 2008 Posts: 6 Location: Stoke-on-Trent

There should be a common power/ground pin on the 7-segment display.
Just swap that to the other rail and it should work OK.

EDIT: Actually, I don't think it will - shouldn't post after an 8-hour shift! Not gates would work, but it'd be far easier just to get the right type of display.