arduinoledmultiplexer
I'm doing something in where I need to multiplex 6 7-segment displays. About doing it, I found things like this:
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.
The problem is you have on and off mixed up.
The pnp transistor will be at cutoff when the output is high, not low.
So you are turning both of your displays on during the 1000us wait.
The transistors are needed if the current required to drive the LEDs is higher than the microcontroller can handle. This means either a single pin current rating, or the maximum rating for all the pins together (will be given in the datasheet)
Most typical 5mm and 3mm LEDS have a maximum current rating of around 20mA and can be driven directly from a microcontroller pin (10mA is fine for most applications)
However, if your microcontrollers maximum rated current is 200mA and you want to drive 20 LEDs at 15mA, then even though the individual pins can supply the current you are out of spec for the maximum current rating.
The LEDs should always have a current limiting component (e.g. resistor) in place when driven from a voltage source, so you are right to suspect the first link. It appears that it is simply relying on the pins drive not being high enough to cause damage to the micro or the LED, which is definitely not a good idea.
Here are the the Absolute Maximum Ratings for the ATmega328. Notice there is a per pin current rating and also a maximum total Vcc current rating:
Also see notes 3 and 4 on pg.314:
Also, you can get high power LEDs with current ratings of >500mA, so a transistor is obviously the only option in this case.
Best Answer
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.