Input pin

Hi
When i set some pin as digital input and not connect any external singl to it
The pin keep change his signal from low to high and Vice Versa
I need to know why?
And what is the normaly set of the pin?

An input pin floats. You can use a pull up or pull down resistor if you want to stabilize it.
e.g. pinMode( myInputPin, INPUT_PULLUP ) ;

What is the normal set of the pin?

If you mean is it normally a 0 or normally a 1, the answer is neither, it does not have a 'normal' state when not connected. It can read either 0 or 1, or change between them on each read.

This is a common misunderstanding with people new to Arduino or electronics generally.

The following diagram may help to get a little bit idea on the meaning of a 'floating input pin'. In this example, PIND2 is the floating pin.
ddq.png

An input pin should have a known logic level which can be achieved in one of the following three ways:
1. Connect internal pull-up.
2. Connect external pull-up.
3. Connect external pull-down.

BTW: For K1 to be active, the DPin-2 should be connected with which resistor -- internal pull-up or external pull-up or external pull-down?

ddq.png

Watch this: