using a digitalpin to check for 5v on a wire

Other Hardware General Electronics
1

I'm looking to check for a 5v "high" on a wire. The wire is attached to a seperate circuit operating at 5v.

I was wondering how to wire it up to my proto board? The wire is actually a pair of wire, vdd and gnd.

I think I am correct in saying I need to tie the gnd to my arduino's ground in order to be able to measure this 5v high.

But how should the thing be wired in? Do I need to pull the input down? What needs connecting to what?

With buttons I know how to do the wiring and how to use a pull down resistor but for some reason I can't get my head around this, and even managed to short circuit my arduino last night fiddling, causing it to momentarily reset! I did this by connecting the vdd (at 0v) to both ground and a digital I/O pin.

2

Simply connect the wire that you want to measure to a digital pin, and the grounds together. The you can do a digital.read. You may need to pull it low when the line is low but that depends on the source of the signal you are trying to measure?

3

Hmm. I tried to do that, but my LED (using it as a visual indicator) was always on!

With nothing connected to a digital pin, and it is used as the basis for a digital read, will it show as HIGH?

That's what was happening with mine when I did a bit of debugging.

4

If a pin is set as input, it will be high impedance (ie, high resistance, neither high or low).
How was your LED wired? Was the pin sinking or sourcing current? Was the "other circuit" powering the LED?

5

Directly give it a connection via some small 150-200ohm resistance and use digital.read() in the code offcourse after setting pin as input.
please make sure the input is max 5v DC.