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Topic: pull up resistor question (Read 1 time) previous topic - next topic

RedSquirrel

I understand that for digital inputs I should use a pull up resistor going from the pin to 5v, in order to drain any noise power that may find itself on it, that way if I don't apply voltage it's 0 and when I do apply some voltage it will be 1, otherwise noise voltage may cause it to go to 1 when I don't want to.  I'm just wondering, should that resistor be close to the board, or can it be remote?  Ex: say I have 100 feet of cable going to a remote sensor of some sort somewhere, can I put the resistor at that sensor?

majenko

You can put it at the remote end, yes, but it is simpler to just turn on the internal pull-up resistor inside the chip.
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RedSquirrel

Oh right, may as well do that. Saves me from trying to find 10k resistors on random old electronics.  :smiley-mr-green:

Grumpy_Mike

Note that 100ft of cable is going to be a problem.
What sort of proble, depends on what sort of sensor.
The internal pull up is only 35K or so, it might need to be bigger if it is a long way off like you have.

RedSquirrel

I have not actually measured so 100ft may be generous, but basically my goal is to rig up the smoke detectors and other devices around the house to send a +5v to the arduino if they are set off (either by completing a circuit, or sending it's own power) to activate the digital circuit. 

So if I do end up adding a bigger resistor could it still be at the far end or does it have to be at the board?   And would I want to go higher ohm or lower?  My gut feeling tells me I want to go lower so it draws more current, thus draining any power that comes from noise, as a longer cable means more noise.  I would be using twisted pair cat6.  Power and ground on one pair and data on another wire.

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