Buttons, LOW vs no connection

Question

I'm working with a very simple circuit to understand push buttons. The code in the tutorial uses the INPUT_PULLUP pinmode. I understand the idea of the pinmode, but I have two questions:

There must be a difference between a LOW reading and no connection, correct? How does the arduino differentiate between the two?

I tried to experiment with this idea. So my thought was that you could create a circuit where the LED only stays lit when you hold the button down. In this setup, I used just normal INPUT for pinmode. So the logic was: if LOW then turn light on, else light off. The turnout was the light was almost always on and just really finicky when i moved the board. Why?

Thanks!

• Nick Gammon: To us to fully answer this question you need to post your schematic and your code.
• Michel Keijzers: Upvoted for the good question which I learnt from by the comments and answer of Nick.

Answer 1

There must be a difference between a LOW reading and no connection, correct?

No, the input circuitry measures a voltage. The exact details are in the datasheet, but how could it know there is nothing connected? There is not some sort of sensor that detects the insertion of a wire into the input. Therefore it is trying to read a voltage all the time.

With nothing connected then stray voltages from nearby circuitry will influence the reading.

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... and just really finicky when i moved the board ...

Yep, you are influencing the voltage by capacitance from your hand.

You need to have a definite voltage for "no switch pressed" which can be a resistor to ground (eg. 10k) or a resistor to +5V.

I have a tutorial on this.

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It sounds like you have wired your switch like this:

Floating input

Because you don't have a definite voltage there, the input could "randomly" switch between LOW and HIGH.

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You could add a pull-down resistor:

Pull-down resistor

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Or a pull-up resistor:

Pull-up resistor

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Or use the INPUT_PULLUP mode to create a pull-up resistor in the processor hardware. (There is no INPUT_PULLDOWN mode).

Then you will get consistent results.

• Michel Keijzers: Upvoted ... thanks Nick for this great answer... I learnt a lot from it (and thought the Arduino also had an INPUT_PULLDOWN mode.