7-segment display problem

May 11, 2008 3 Replies
C

Hi all. I'm just starting out trying to get my head round this electronics thing and am building a few simple circuits that I find on the Internet. I got a 7-segment display (common anode) and tried to build this -

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to repeatedly count from 0 to 9. I've tinkered with the resistors and have one as a 1meg pot so I can change the timing and it works, except it does it in reverse. I mean instead of lighting all the outer LEDs for a 0 it lights only the centre line, and for a 1 (when it should be lighting the two rightmost LEDs) it lights all the others. So, instead of each LED in the 7-segment display being on in each configuration, it's being off. I hope this makes sense, and I'm sure there's a very simple answer to why it's doing it, but I am very new to this stuff!



Any help would be greatly appreciated.


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J

--- Sounds like what you've done is connected the anode of your 7 segment display to +9V.

If that's the case you'll need to either change the display to common cathode and connect the cathode to ground, as shown on the schematic, or use the display you have and invert the outputs of the driver.

If you invert the outputs of the driver you'll probably need current limiting resistors between the inverted outputs and the segments in order to keep from damaging the display.

You can make the inverters out of transistors, one for each segment, like this:

(View in Courier)

+9V | | +---[390R]---[---[1kR]---B 2N3904 | E | | | GND | . . . . . . | | | +---[390R]---[---[1kR]---B 2N3904 E | GND

or you could make them out of gates.

JF

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R

The display in the schematic is common-cathode, while you said you have a common-anode display. I'm kind of surprised that you're seeing anything at all. You might need either a different display or something like a ULN2003 to invert the signals.

Good Luck! Rich

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J

I guess appreciation doesn\'t go as far as acknowledgement of help received... JF
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