Power and data thru 2 wires?

[ColinB]

how about usb cable? 4 core and... shielded novel though, so you will not find it in any thesis or using local power supply at the motor/load location and only remotely send data signal? YMMV. i'm only brainstorming

What benefit would USB cable have over CAT5e UTP?  For starters, you can be CAT5e in bulk very inexpensively, while USB cable is very rarely sold or used in bulk form, only as finished cables.  Perhaps the USB cable would have heavier gauge power conductors, but that's the only advantage I can imagine, and you'd need to determine this for certain ahead of time.  CAT5e is 24-26 AWG (I usually see 24 AWG) but CAT6 is supposed to be 22-24 AWG according to Wikipedia.

Actually I think you could easily get away with using CAT5, especially if you bond a couple of pairs together for power/ground to decrease resistance.  I would also dedicate one pair for MCU power+ground, and other pair(s) for motor power+ground, to prevent the digital supply from getting noise and voltage drop due to the motor.

CAT5 conductor resistance is specified as less than or equal to 0.188 ohms/m.  Therefore if you have a 30 meter run, and your motor takes 0.2 A, that is about a 1.13 V drop.  At 5 V, this is not great, but if you bond a couple of pairs together for motor power and return, it will decrease significantly.  Also, you could use a 12 V motor and use higher voltages with lower currents to decrease power loss, if you aren't stuck using a 5 V motor.

[Erwin Ried]

30m ... even AWG22 will give loads of resistance, Cat5e is AWG 22

I tested the cable I got and:

cable length: 29,9 mt
red, black=14.91 ohm <- only 1 wire inside
green, white= 17.3 ohm <- multiple thin wires inside

It is bad? or good? I am very weak in any electronic concept

[SeanB]

Normal range for a random data cable. To drive a curtain you need a motor and gearbox that will run off a minimum of 12V at 1A, 2A or more at switch on. Either you use a thicker cable, or you send 48V down the cable and use a DC-Dc converter at the other end to make a 12V supply, and use enough input capacitor at the remote end to handle the switching surge.

[Erwin Ried]

OK Thanks!, the motors are very small, typical toy motor but with the worm gearbox they are very strong, silent and self locking, I have tested them in my supply bench:
no load = 0.03A 7V
full load = 0.22A 7V

So as I had 4 cables, one carrying 5V to the uC + one carrying 7-9V for the motors seems good solution even for 30 meters, right?? (it is like 2 meters from 1 courtain and 14 meters from the other one + 14 meters for a first floor sensor)

[David_AVD]

You really need to use a higher voltage down the cable and a DC-DC converter to get your low voltage (and impedance) for motor and logic.  The voltage drop (in the cable) is going to be cause havoc with the system otherwise.

Even if you accept that you will lose a couple of volts in the motor V+ wire, the common (to logic and motor) V- wire will also have a similar drop on it.  This will affect the 5V supply and shift the logic reference to boot!

[Uncle Vernon]

30m ... even AWG22 will give loads of resistance, Cat5e is AWG 22

I tested the cable I got and:

cable length: 29,9 mt
red, black=14.91 ohm <- only 1 wire inside
green, white= 17.3 ohm <- multiple thin wires inside

It is bad? or good? I am very weak in any electronic concept

220mA across 17.3? = 3.8V so your 7V motor will be getting 3.2V under load less at start-up. You could jack the supply voltage up but that will create other issues as your current requirement varies with load.

Your options are either to sulk like that tram guy or look at alternative arrangements. With a 24V supply you will still have 20.2V across the cable at the same current. See why they go to all that trouble of using regulators or DC/DC converters?

[Uncle Vernon]

I am impressed

Of course you are! I'm Way impressive!

you must have taken one of those online humor writing course, haven't you ? 

Hell no! Just natural skill, talent, charm and ability.