Can't run LCD off a digital pin?'

I have a 16x2 lcd panel, and I thought it would be a good idea to run it off a digital pin, that way when the arduino goes to sleep, the LCD automagically turns off. However even though the pin goes to +4.98V, the LCD will not work. The LCD works fine if I wire it straight to the +5v rail, though. I thought digital pins could source 40ma? Does an LCD panel really use that much?

Well I think that the LCD should take more then 40mA with the back light on. You should test how much current the LCD takes with a multimeter.

I don’t have a multimeter whose current meter works. The backlight of the display was not on; I don’t think it has a backlight; there are no backlight wires anyway.

The data sheet for your LCD will tell you whether it has a backlight, and if it does, how it is wired, as well as how much current the LCD draws. So, which LCD do you have?

A full datasheet would be great! I never thought a LCD would draw much more than 10ma. It’s actually a surplus LCD of uncertain origins. It says PMC1602D-SGR on the back. Googling doesn’t bring up much, but it appears to be similar to the “KFC model” display on this site, which is where I got the pinout:

There is a note on that page:

Actually I used the STK500-kit and mounted the LCD module on a bread-board, connected to the PORTD-connector of the STK500. I used a separate 5V power-supply for the LCD, since the backlight can be a little heavy on the power-consumption, and I do not know how much power the STK500 can supply.

That model does indeed have a backlight, and apparently draw a bunch of current. More than a digital pin can provide, anyway.

Maybe you could power it using a transistor activated by a digital pin, to turn it off before the Arduino goes to sleep.

If it has a backlight, it's not working, not off 3 AAAs anyway.

3 AAA batteries only put out 4.5 V. Maybe it needs more than that.

The schematic shows +BG and -BG on pins 15 & 16. Are they backlight pins? Do you have them connected to anything?

On my display, there is no pin 15 or 16; there is no spare hole next to the header where would might be either. It stops at 14. I suppose it could be the voltage.

I suppose it could be the voltage.

More likely the current draw. You really need to measure the current with a DMM. If you are going to do hobby stuff with an Arduino then buying a DMM is a root requirement, they are not that expensive.