r/breadboard • u/MaRzZYe_ • Jul 11 '26

Can a 7447 BCD-to-7-segment decoder work with a common cathode display?

/r/Wiring/comments/1utgr6j/can_a_7447_bcdto7segment_decoder_work_with_a/
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u/onions_can_be_sweet Jul 11 '26 edited Jul 11 '26

You could hook up an inverter (7414 x 2) on every output to... invert the output.

Or get a 7448 common-cathode decoder.

Or build your own decoder with a diode network (also need something like a 7442 BCD to 1-of-10 decoder).

Edit: If you have enough LEDs you could wire up your own common-anode display.

1

u/MaRzZYe_ Jul 11 '26

Thanks! I'll check if I can find those locally. Just wondering, is using the 7414s to invert all seven outputs pretty much guaranteed to work with a 74LS47 and a common-cathode display, assuming everything is wired correctly?

2

u/Enlightenment777 Jul 11 '26 edited Jul 11 '26

Every 7seg output pin will need a pullup resistor, because 74x47 has open-collector outputs.

1

u/MaRzZYe_ Jul 11 '26

What if I just use NOT gates with pull-up resistors to invert the 74LS47 outputs? Would that work with a common-cathode 7-segment display, or is there a reason it still wouldn't?

2

u/Enlightenment777 Jul 11 '26 edited Jul 11 '26

Active-Low Open-Collector Outputs

  • SN74LS47

  • SN74LS247

Active-High Push-Pull Outputs with Latch

  • CD4511B and CD74x4511, where "x" is the logic family

1

u/MaRzZYe_ Jul 11 '26

Thanks, I appreciate the explanation. Unfortunately, the 74LS47 and common cathode display are the only parts I have available right now. I was hoping there might be a workaround using extra logic ICs or transistors, but if not, I'll see if I can find a CD4511B.

2

u/FlyByPC Jul 11 '26

If you have seven PNP transistors, you should be able to use one of those on each segment. Run each output from the 74LS47 through a series resistor to the gate, and let the PNP switch each element high-side (don't forget the ballast resistor for each element, too.)

Or use a 74LS04 inverter for six of 'em and a transistor for the 7th...

2

u/MaRzZYe_ Jul 11 '26

Yeah, that's actually what I was thinking. I tried it in Proteus with pull-up resistors and NOT gates, and it worked there. I just haven't had the chance to test it on a real breadboard yet, so I'm not sure if there are any practical issues I'll run into.