I'm working on a train layout and have built 2 automatic switches for the switch track. Each switch track is powered by M-Motor. I'd like to connect the 2 motors to a single IR receiver and battery pack with 6 AAAs. Unfortunately the longest extension cable I can find is 20 inches. I need a much longer cable than that; more like 8 feet. Is there a limitation to the length? Is it possible to make or buy such a cable? I'd like to avoid adding more Power Functions equipment if possible.
As the PF plugs are specific, the simplest thing to do is to buy a short extension cable, cut it in half and solder another cable in the middle. Current in PF cable can be pretty high, so to avoid too much drop it's best to avoid thin cable, I recommend 0.5mm² of copper section.
I found two independent tutorials how to create electronic contacts compatible with the original Lego contacts, which are located at two knobs of a 2x2 brick:
Two not-so-good (but where no LEGO parts are harmed) solutions for the sake of completeness:
You could daisy-chain a few extension cables; if you just need the one long cable in one instance it'll do the trick.
You could use older 9V cables (the longest being nearly 10 feet long) and converter cables. A bit awkward for a PF-based system, though.
I've been using both solutions when displaying GBC modules, as the placement of modules is always different and the length of the cables isn't fixed; but since the power source comes from a 9V transformer, it makes more sense to use also 9V extensions before converting to PF.
I'm not sure this would be the best solution for you, but it all depends on circumstances.