16 LEDs On makes the LCD display weak?

Just checking a silly thing. I'm using the USB power, so I wonder if when I use an external power supply with more mA it will work better?

What happens is that when I light all my 16 blue LEDs the LCD display gets a bit dim. But not the LCD backlight. Its like the contrast gets weird somehow. Its a blue LCD with white digits. But the "white" gets a bit gray.

Is that the mA power output of the USB that is weak?

Just checking, not a really big deal for now...

Wk

How much current is each LED drawing?

The USB can only give you MAX 500mA, if the FTDI and the Atmega chip draw a total of 150mA(and this is being conservative) and the lcd another 100mA, 16leds drawing 20mA each, will give a total of 150+100+(16*20)=570mA, more than the maximum allowed by the USB specs, they should shutdown when there is overcurrent but they may not do that and just burn you one port or your motherboard usb hub.

Thanks guys.

I don't know yet the current for each led. I'm using 220ohm resistors and a shift-parallel out chip.

Can I use an external power and still have the usb plug, or I have to keep the usb unpluged while not using it, unplug the power, plug the usb, upload, unplug usb, plug power, ... ??

Wk

I don't know yet the current for each led. I'm using 220ohm resistors and a shift-parallel out chip.

But you should. Assuming your LED drops 1.8V (for blue it's more, measure it if you're unsure) you get: (5V - 1.8V) / 220Ohm = 15mA

Korman

Can I use an external power and still have the usb plug,

Yes modern versions of the arduino switch the power source automatically.