0
\$\begingroup\$

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?

\$\endgroup\$
  • \$\begingroup\$ You have not mentioned current. Since you are planning on using transistor arrays, then it could be low current like 20mA. There is an old style intermediate distance interface call current loop, usually 20mA or 60mA. If you drive the LEDs directly, it would be just like current loop interface. At no more than 10 or 20KHz (comparable to 9600 or 19200 baud communications), it should not be any issue. I am not suggesting to use a current loop interface, just use that as a guide to what you can do. \$\endgroup\$ – rioraxe Apr 16 '16 at 7:41
2
\$\begingroup\$

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.

\$\endgroup\$
  • \$\begingroup\$ This sounds great, I will need to research RS-422/485. For this project, price is a major concern. A variance of 10% in LED RGB color is fine. \$\endgroup\$ – Hoytman Apr 15 '16 at 21:19
  • \$\begingroup\$ RS-422/485 drivers/receivers can be pretty cheap (<$1) in quantity, especially if you're willing to use some older, less fancy parts. TI has a few in that price range. Also -- I assume these are all running off the same power supply / ground reference? \$\endgroup\$ – Krunal Desai Apr 15 '16 at 21:24
  • \$\begingroup\$ Yes, they are. I would like to avoid designing a board for the box if possible. \$\endgroup\$ – Hoytman Apr 15 '16 at 21:26
  • \$\begingroup\$ The parts are available as 8-DIPs, so with a tiny piece of perfboard (just the transceiver + decoupling capacitor, 0.1uF or so + connector), you can probably get away with building a few of those by hand and putting in the box. Personally, I don't think it is a good idea to send single-ended CMOS signals for PWM over a long distance, especially since these are (essentially) clocks, almost. \$\endgroup\$ – Krunal Desai Apr 15 '16 at 21:28
0
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.