I'm using a CMOS chip (CD4098B) for a small circuit I'm making which will have quite long wires (2m or so) going of from the inputs through a switch and back to Vcc. The pins are pulled down with a 100K resistor.
Now, when I just touch an input pin with a wire (or my finger) it goes high despite the pull-down resistor. I really want to avoid this to make the circuit more reliable and I would prefer not to use shielded wires.

I tried to put a 0.1µF cap in parallel with the pull-down resistor and it seems to work but I don't know why so I thought I'd ask anyway.

My guess is that it has something to do with the capacitance in long wires?

 

That or long wires make decent antenna's. There is a lot of electromagnetic radiation floating around out there.

 

Yeah, it might be that as well...
But how do I prevent it from happening?

 

Sounds like you have already done so. However, you can try a twisted pair of wires. Cat5 and other cables use this technique.

 

As Bill mentioned, twisting the wires together will help a great deal to reduce noise.

Wires that long will also have a good bit of inductance. You may find it necessary to add a resistor (perhaps 100 Ohms) in series to "snub" any tendency for the circuit to ring.

100k is a bit large of a pull-up resistor, particularly for such a long run. Consider decreasing the pull-up resistor to the range of 10k to 50k. 10k is pretty much a standard value for 4000-series CMOS. If Vdd=5v, that results in a 500uA current.

 

OK, so twist wires, change pull-down resistor, add series resistor.
I'll keep my caps though since they really works like a charm!
Thanks!