Sending PWM signals down cat5 cable

Question

I working on an LED lighting project where I will be installing 20 boxes in a space, each containing 4 LEDs (RGB and IR.) I want to connect all of these boxes to central board which will drive all of the LED's using PWM signals.

----- Generic Questions:

Can I utilize all of the wires in the twisted pairs to send PWM signals, or should I only send one signal per twisted pair?

How badly will the PWM signal degrade over distance if 4 or 8 different signals are sent?

Will increasing the current (to 200ma or 400ma) hurt the signal (I am considering driving the LEDs with the signal, directly out of the cable)?

Should I send the same signal down both wires in a twisted pair, or should I pull one of the wires high (5v) or low (0v)?

----- Bonus Question and project specifics:

I will not be filtering the signal. Regarding precision, cost is a major concern for this project (we have a shoe string budget, so complexity will limit the number of boxes we can install.) 10% variance in LED output between the boxes is ideal (that will allow us to almost display #fff hex style colors.)

I will be using transistor arrays receive the PWM signal and supply power to the LEDs. Can I avoid he need for a circuit within the individual boxes by placing my transistor arrays on the central board, 20 feet away?

Answer 1

If I were to try and send PWM over twisted pair, I would take each PWM signal, feed it to a differential driver, and then connect that to a twisted-pair. So, you could get four at most in a single Cat5 cable if you do it this way. This is assuming a PWM frequency of <100kHz or so.

What I get from your question is that you have a bunch of remote boxes that contain LEDs that are driven by some type of transistor array that respond to the input PWM signal. My initial thought is to "up-layer" your problem / design such that instead of sending PWMs over a long distance, you send a command via some bus (RS-422/485, CAN, etc.) to each box saying "go to PWM x", and then inside the box, the path for the PWM to its driver is very short. This way, you can send your "real" information (desired PWM duty cycle) in a potentially error-correctable, resilient form over long distances. A local MCU or processor than takes it, and generates the requisite PWM signal with a short trip to the LED drivers. I realize this increases complexity, cost and requires more hardware.

As for distance limits, if you do what I suggested and utilize a RS-422/485 based system, you can reference application notes on that subject that can describe the network in terms of termination, maximum unit-loads, and distances -- TI has a RS-422 design guide that is useful. If you have many boxes that do the same thing, RS-422/485 will let you do "multi-drop" as well, and you can have many nodes tap-off the signal as needed.

EDIT: I see you updated your question:

1. I would not send two separate single-ended signals down a twisted-pair. So in a conventional Cat5 cable, I would say you're limited to 4 unique signals. Use the shield as your ground return, or sacrifice a pair for ground.

2. The number of signals shouldn't matter as much if you follow what I said in #1.

3. Yes, there is a practical distance limit. It will depend on how you choose to send the signals (i.e. if you use RS-422/485, reference that specification / app notes)

4. You should use each twisted pair in a differential fashion.

5. I wouldn't separate the transistor arrays from the LEDs by a large distance, because then you're going to have high currents switching over a long distance which is not a good idea.

Answer 2

If you use very inexpensive CMOS LVDS drivers and receivers, with proper termination, you can easily drive 1 signal per pair, and perhaps reserve one pair for ground reference. A few tens of meters should not be a problem, but the LVDS may not be as resistant to damage from ESD as bipolar RS485.