Hi,
i want to connect a large number of LEDs to an Arduino Mega (45 pcs.). From what i could find out the board itself would not be able to power them all at once, so i plan on using a sensor shield with a seperate VCC to power them. Would having the negative ends of the LEDs connected to the digial pins work to control them, or will something like transitors be neccessary?
Any input would be appreciated.
Post a diagram of what you are proposing. How much power do the LEDs require? How many LEDs would be on at a time. Are you opposed to LED drivers? What else have you not told us?
No controller what so ever, is a power supply. External power must be used and the controller controls the load via drivers.
20mA per LED. I need all of them to be turned on at the same time. I havent made a diagram since it would just be the LEDs connected to GND and the Digital Pins x45. I would like to avoid LED drivers, yes
ok let me rephrase, the chip would not be able to pass on that much power without blowing up. The limit i believe is 200mA
Read the specification for the controller! Yes, digital outputs are designed for maximum 20 mA, but there is a limit for the maximum I/O current. 45 x 20 mA == 900 mA is way over that limit as far as I can judge, being an UNO guy.
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So no matter if theyre connected to the positive or negative of the LEDs, the pins cant handle all the of them at once?
The term you're looking for is "sinking". That's vs sourcing if providing the + terminal. 
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right, thanks.
so can it sink that much power?
Correct. You'll need additional hardware. There could be ways to increase the voltage and reduce the amount of additional hardware, but we'd need to know more about your project.
Nope.
Be warned that you can't connect leds just to power and GND, you definitely need a separate resistor for EACH led.
Consider the possibility to decrease a current for each led. The led brightness is not linear function of current, perhaps the leds on the 5 or 10 mA will bright enough
The conducting copper strips on the board have a limited capacity. 900 mA is way too much in my opinion. The heat from too many transistors/circuit parts is another factor.
Again, read the technical spec! Don't rely on one "happy helper" saying "Yes"!
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