Connecting LED with Long Wires

[Brumby]

To answer the first question, for an application this simple, it shouldn't matter at all.

This is what I was thinking.  20cm is not that long at all - and, although the OP hasn't come back with a frequency, I wouldn't be at all concerned running it up to 10kHz or even more.  If you were really worried, twist the wires together.

I wouldn't be getting into some of the 'advanced' considerations mentioned here unless we were rocking along at RF frequencies or had some particularly sensitive circuitry that needed an electrically quiet environment.

The only thing you may want to take into account is that if the wires are REALLY long, they will have their own, no-longer negligible resistance which you should subtract from the value of the resistor.

If the wires were 20m long, then that would be worth checking - but even 2m isn't much of an ordeal for the LED itself ... unless there are strong EM fields from sources in the area that could induce problematic currents.

Yes, there are considerations for looong wires, but I wouldn't be too worried about 20cm unless you were getting up in the UHF band.

[Kjelt]

Long time ago for me but 20cm is enough for a perfect couple of hundreds MHz antenna.
Just make sure those frequencies are not in the signal, and if I remember correctly from college, the square wave with Fourier harmonics, you must take care that the flanks of your switching signal are not steep, the steeper the higher frequencies will be present, so use a cheap RC filter, ferrite bead or shield it.

[kmossman]

What is gauge of the wire?   That is the most important thing.  There are resistance per metre/yard charts available on the Internet. Just look up and determine the total resistance of the wire. Subtract that from the calculated resistor value. If it is negative then use a heavier gauge.  Simple.