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Designing a 4-bit non-sequential synchronous counter

  • #26
JJBladester
Gold Member
286
2
Is there any way to ensure a constant voltage/current into the seven-segment display pins? I looked at various decoder/driver combos and the "driver" part seems to ensure a constant supply current. Since I built my decoder from scratch out of logic gates, it obviously doesn't have any bells and whistles.

The displays *are* common-cathode. The appropriate display is enabled when the 74139 decoder outputs a 0 on the CK pin of the correct seven-segment display. One of the example circuits in my book actually uses the same technique for enabling the displays one-at-a-time. The example does note quite clearly that "additional buffer drive circuitry may be required". This is what my circuit seems to be missing and I don't know how to rectify it.
 

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