max effective resistance for pull down resistor?

I am monitoring several mouse traps using arduino, and I find at times the contacts in the trap itself may cause enough resistance that my 10k pull down resistor is the path of least resistance. I usually have to go up and reset the trap them I'm good for a bit.

Could I get away with using say, 100k resistors? Or would that be too high and no longer be effective?

Wire the switch between input and ground and use a pull up resistor like the rest of the world would. There is a myth that electricity flows down the path of least resistance it flows down all paths. It puts current down those paths according to the resistance values of the paths. The internal pull up rsistors are 20 to 30K. It is likely that the switch contacts are becoming high resistance. You might have to use a transistor or maybe even use the analogue inputs to detect changes.

I want to stick with NC loop, as there's always the possibility mice chew on the wires, so that would trigger an alarm too. (sends emails). I decided to go ahead and try the 100k to see what happens. Two of them are now in alarm but I'm thinking they tripped when I put them up so I'll have to check later. Pain in the butt since it's blown insulation up there and makes a huge mess. :grin:

If you move to high-resistances you will need to protect against electro-magnetic pick-up in the cabling, which in practice just means adding a few nF of capacitance between the input pin and ground at the Arduino end.

Hmm so just throw in a small capacitor between input and ground? What side does the + go on? I guess I'll leave it as is for now to see what happens and if I start getting false alarms.

I want to stick with NC loop,

What has that got to do with anything? You simply change the code you run.