Designing a 7-segment hex decoder


Building a combinational logic circuit to decode 8 bits and display a 3-digit decimal number on 7-segment displays is complex. In this video, I build something much simpler—a circuit that decodes 4 bits to a one-digit hexadecimal display—and even that’s pretty complicated. In future videos, we’ll look at much easier ways of doing this, which we’ll use for our computer’s output register and then as a key element of the control logic.

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You don’t really want to build this do you? If you do, you’ll need some breadboards, 5x 74LS32s, 9x 74LS08s, 1x 74LS04, 1x common anode 7-segment display, 1x 100Ω resistor for the display, some DIP switches, LEDs, and 1k pullup resistors for the input, and hours of podcasts or something to keep you from losing your mind. 🙂

If you do build this, you’ll _really_ appreciate the elegance of the next few videos.



  1. can this circuit be programmed in assembly code?
    with my project, i am not allowed to use any IC's (including logic gates). is it then possible to write a code in assembly that gives specific patterns to the I/O pins of an arduino to light up the 7 seg without any circuitry?

  2. Как получить любой файл
    как получить всю информациюГенератор всех файлов,генератор всей информации

  3. BTW Each of the cathodes should have a resistor. Using 1 resistor in the common Anode causes the display's brightness to change depending on how many LEDs are lit. Common Anode should be connected to VCC.

  4. BCD to 7-segment decoder driver. 74HCT47 does the same thing, all in one IC package. I began using the 74LS47 in the late 70's as I recall. They are dirt cheap. 🙂

  5. The way I typically drive 7 seg displays is by wiring it to an octal D-latch like the 74HCT373 to sink current rather than source. Sink is harmless abuse of the D-latch. I can then use a software lookup table to define the binary representation for each character and set the data value to the display the same as you would a store to ram, as the latch is wired up as addressable memory on the data bus with the chip select. If I have say two displays, I consume two D-latches which is cheaper than using an eeprom. So from a software standpoint, the display is updated by storing a value from a lookup table into what amounts to a ram location — effectively. Simple, cheap and easy. 🙂 Plus you can redefine the lookup table in software, if you want to change things without needing to reprogram a hardware device.

  6. 3:00 hmmm AVR and some code to control more than one digit display, but yes, I'm curious so I'll watch your way. Result 12:25 @%@#$% you got some serious bad sector in your head (positive manner) 😀 Damn cool to watch anyway! Thumbs up from me…

  7. Your breadboard circuits are pieces of art, Says a guy who has done unthinkable messes on breadboard. I studied all Digital circuits 10 years backs in Engineering and yet your explanation teaches something new! Good job, Ben!

  8. * he literally only uses blue connectors.. *
    my computer logic professor would have a fit if we turned in a project with only blue wires.

  9. when I was young I did design that circuit but with diodes and a rotary switch. so using diodes and maybe two 74138 should to the trick

  10. I build this a few years ago in wire mod for gmod, just by wits, and it was very big and very messy. I didn't bother sharing gates, I just made a "complex" for each segment. But, it worked 🙂

  11. I am grateful for this resource, as I have now made a 7-seg out of LEDs because I'm impatient… though I don't want to do that again lol

  12. You are really passionate about Electronic Engineering! Nice video.
    I used to find it to be very complicated, but you simplify and explain stuff! Keep up the good work.

  13. Very nice video. I'm loving your explenations for logic. But.. there's some IC's that are 7 segment display decoders…

  14. Ben's "messy" diagrams look like "final copy" works of art compared to my notebook sketches. Guess I should use a ruler. LOL

  15. i was looking 28c16 eeprom chips online. shop i last time bought 6502 stuf dont sell them so i look 28c256 that last time cost like 7euro.

    NOW 1 pcs cost 149€
    25 pcs cost 143€
    i dont want spend 143euro buying 25pcs even it is 5.75€ piece LOOOOOOOOOOOOOL
    when only would need 1 or maybe 2,3 to play with.allready have one.

    so someone want buy 1 28c256 from me i give you discount 50%. i sell it 70€ LOOOOL

  16. Hey I just learned about this in school! I’m not awesome at it, but I can make this other table (forgot the name) that simplifies this. I understood these videos before, but now I have an even greater understanding!!

  17. Ive been thinking if a circuit that takes 7 inputs and converts it into 3 bits binary. I'm stuck on it. Im just learning circuitry so bear with me.

  18. OH good old days.
    40 years ago my project to my college where I used to study was a digital clock using 7 segment LEDs.

  19. Dude you need a new mic, I can hear every time you swallow, great video, but that audio is very off putting. Not looking for asmr.

  20. I made the full Kmap solution for this 7 segment display problem, check it here:

  21. I'm done with this kind of circuit for our lesson on logic gates devices and switching theory, by only using IC' s 74ls04, 08, and 32, and a cathode 7 segment. From zero to letter F it takes 3 breadboards

  22. Sir, thank you very much, i have always wondered how computers work at the root level and your videos have made me understand the beauty within them. Once again thank you very very much.


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