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I'm designing a circuit which has to display 8 numbers. I pretend to do it with 7-segments displays. I dont have enough pins in my MCU to control every single display independently, thus, I will use a demultiplexer.

The thing is, using a demux, only 1 display can be "on" per instant of time (i.e. if I want to show numbers from 1 to 8. In one instant of time I will send the data to show the "1" to the first display, and in the next instant of time I will send the data to the show the "2" in the second display, and so on). Obviously this change is going to be so fast that It will seems that every display is on all the time, but my question is, the luminosity will be 1/8 of the total? (As each display is "on" only 1 instant of time and there are 8). I think about it as a PWM with a duty cycle of 1/8 (12.5%). I would like to have all the displays with the max luminosity.

Any help is appreciated.

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  • \$\begingroup\$ what is your question? \$\endgroup\$
    – jsotola
    Commented Apr 24, 2022 at 6:06
  • \$\begingroup\$ Is the luminosity will be 1/8? The quesiton is written in the second paragraph \$\endgroup\$ Commented Apr 24, 2022 at 6:09
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    \$\begingroup\$ sorry, from what you wrote, it appears that you already know that \$\endgroup\$
    – jsotola
    Commented Apr 24, 2022 at 6:20
  • \$\begingroup\$ aren't there demultiplexers with latching features? I managed to find one here electronics.stackexchange.com/questions/550756/… \$\endgroup\$ Commented Apr 24, 2022 at 6:37

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As you have surmised, the multiplexing affects the display brightness. The common solution is to use a higher peak current, but there is a limit to how much you can do this - refer to the datasheet of your led displays to determine the peak current the display will tolerate.

if you decide to use a higher peak current, then ensure your drive circuitry can provide this. If using the GPIO of a microcontroller to drive the displays directly, then it is possible you can exceed the available current. In which case you need extra driver circuitry.

Note that there are chips that do all the work for you like the ICM/MAX7219 and others.

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  • \$\begingroup\$ Thanks, I just wanted to be sure how it works before starting the desing. The recommendation about MAX7219 helped a lot to reduce MCU pins used! \$\endgroup\$ Commented Apr 24, 2022 at 7:05

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