aster94:
someway yes,
"Normal npn transistors can't really handle that much current"
it looks like that the current of the lcd is provided by the arduino pin!
No that does not say the LCD's current is provided by the Arduino pin. It is a comment on if the transistor you attach to the Arduino can handle the current. As you have not got a clue about how much current the LCD takes the point is moot.
Note if turning off the LCD in this way also make sure that any outputs connected to the LCD are changed to inputs before you turn it off. This will prevent latch up and parasitic powering through the LCD's input pins.
thank to all of you, i can't measure the consuption of the lcd since i have a very bad multimeter (5€ in chinese shops) and i am waiting for a good one (still chinese but this time is better) to arrive
or will the entire display be OFF and turns ON only when the alarm rings or when you press a button?
this is my idea
BUT check first if disconnecting power from the LCD then reconnecting will make the LCD initialise and communicate with your Arduino.
i need to check it
as agreed by all i will put all the pin connected to the lcd as input before powering it down
yes i have the irlz44n but not the 2N2222A but it is not a problem to buy a few in the future they could be useful, i will use the 2k/150 resistor
anyway i will try to use the irlz44n since as noobian said i already have it. I understand that the "circuit" in the previous post isn't ok. It is better to put the transistor between Vcc and the LCD, isn't it?
I will explain my idea of allarm clock:
i don't need to wake up at a stated moment of the day (example 7a.m.) i just need to do 6 hours of sleeping, so there is only a small error since the starting moment is when i go sleeping