Electronic – arduino – Long three cables connection: can I use an audio stereo cable

The solution was to physically separate the power connections for the audio components. While @Dave mentioned in the comments something about a star ground topology, the problem was that while the ground connections were all centralized, the Vcc connections were also, and shouldn't have been.

In the original design, power was supplied to the entire assembly via a small PCB with header pins for each separate component. Two rows of male header pins, one for 12V DC and one for ground. In this arrangement, all of the noise was being dumped onto the audio components each time the servo motor was actuated. Adding capacitors helped slightly, but certainly not enough.

The main two wires supplying the small power distribution PCB are about 10 cm long. I added a second pair, and created a second power distribution PCB. The original one was used for the servos and receiver, while the new one was used for audio and amplifier. The "Y" split in the power cable ultimately had the effect of keeping the power clean on the audio side.

I also removed the ground connection between the RC receiver and Arduino, though this was unnecessary. Its presence did not add noise, nor did its absence cause triggering problems.

An electrical engineer coworker advised me that it was important to have separate power supplies for motors versus digital components. While this project still only uses one power supply, the solution was to split the power as early as practical, and dedicate each to its respective purpose.

How practical this is depends on how far you want to run the signals, their frequency, the environment, the voltage and current, and a host of other factors. But in general, it should work quite well. For only 30m, I doubt you'll need to do anything more complex than what you've already done.

Some things to consider to improve the performance (if you need to):

1. Ethernet cables are made with twisted pairs. For best results, each signal should use one pair. Often, with signalling between ICs, each signal uses one trace/wire and they share a ground trace/pour/whatever. If you give each signal it's own ground wire, the current flowing in the two strands is the same and that reduces crosstalk. It also reduces noise due to currents flowing in the ground wire.
2. Going one step further, you get the best from twisted pairs with balanced signals. An unbalanced signal is two wires with one at ground and the other at a varying potential. A balanced signal is two wires at a potentials symmetric about ground. You can convert signals from balanced to unbalanced and vice versa with a balancing transformer, also known as a balun. This is particularly valuable with small AC analog signals. Note that baluns only work with AC signals within their rated band - balancing DC is more awkward and rarely worth it.
3. If you are using high frequencies, and need to worry about EMC, then using balanced signals will help, as will using shielded twisted pair (stp) cable.

If you do both 1 and 2 above, and use cat6 cable, there is no reason you can't push >100MHz digital signals down 100m of the stuff. That is after all exactly what it's designed for. And for lower frequencies, the distance would be essentially unlimited. For analog, it really depends on the characteristics of the signal.

Also, some non signal-related suggestions:

1. If you're sending power along the same cable, make sure you have decoupling capacitors at the sensor end.
2. Your tape and solder (presumably?) system is functional, but a bit messy and I doubt it'd be reliable long term. Terminating the ethernet cable with a connector with strain relief is probably a good idea.
3. If you use voltages that would damage standard ethernet hardware, use a different connector so people can't plug it into a NIC by accident.

Edited in at Andrew Morton's suggestion. For the question asked, you are unlikely to need STP, but in the interests of completeness:

If you are using sheilded twisted pair (STP) cable, then the shields give several advantages: reduced crosstalk; less noise from the environment; controlled capacitances; less radiation and EMC concerns. To work properly, the shields need to be connected to something, and that something is almost always ground. Any ground gives big improvements over UTP, but for best results:

1. Connect only one end of the shield. Heatshrink or tape the other end so it doesn't touch anything. Connecting both ends is will cause ground loops.
2. Don't use the shield to carry return currents, even if it has a drain wire.
3. If you're using baluns with a centre tap (such as the cheap and versatile autotransformer balun, but also 1:1+1 isolating baluns) then connect the shield to the centre tap, and arrange things so that is at ground.
4. wherever you attach the shield, the usual concerns about PCB layout and noisy grounds apply. Don't have the path from the shield to ground pass close to e.g. a SMPS, or it won't stay at true ground.

Remember that there will be a relatively large capacitance between the wires and the shield. This replaces the uncertain capacitance between pairs you have in UTP. For particularly sensitive or high frequency circuits, you'll need to take this into account. On rare occasions, such as when transmitting a differential signal with a large common mode, you may want to connect the shield to something other than ground. The above points, especially 3. still apply.