I'm doing something in where I need to multiplex 6 7-segment displays. About doing it, I found things like this:

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And things in where the same thing is done without the transistors, just by connecting the cathodes directly to pins of the microcontroller. What's the difference? When should I need the transistors? Is it because the microcontroller couldn't handle current for all the LEDs?

I'm using an Arduino with an ATMEGA328, if that's needed for something.

  • \$\begingroup\$ This doesn't answer your question but you might want to look at the MAX7219 chip to drive the LEDs. Check this tutorial for using it with the arduino. \$\endgroup\$
    – Craig
    Commented May 16, 2013 at 16:27

1 Answer 1


Many MCUs won't handle the current for seven segments unless a transistor is used. You might be OK with the AVR (40 mA maximum per output, IIRC), but you need to check it, and the total current.

  • \$\begingroup\$ While 40mA should be enough for a 7-segment display (maximum of 5mA per LED, set by the series resistors) only one digit will be on at a time with this multiplexed control. That means that the total 200mA chip max isn't a factor, but because each digit is only on 1/6 of the time (because you are multiplexing 6 digits) the effective brightness is reduced to 1/6 of what it would normally be at 5mA. \$\endgroup\$ Commented May 16, 2013 at 21:05

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