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[on:]

Situation... If I build a small array of LEDs via the circuit below and mount that at the far corner of my back yard, then run the wires across the yard what kind of noise/interference would be produced if I wanted to use a varying PWM signal to fade in/out the array? Right now there would be 3 wires, power, ground and signal.

I was considering using CAT5 for this since it's easy to use, ready to go and easily available.

So 3 questions:
Would using a twisted pair eliminate most/all of the noise?
What would be twisted? Ground and signal?
Could I smooth out the signal with a cap and eliminate the square waves and thus the noise?

Thanks in advance.

(P.S. I know the values and probably the schematic are off a bit. This is just a concept so far and am researching. I'll crank more numbers before I test this out. I also think I'll change back to 1 resister per 3 LEDs like I had it before.)

[Reply #1 on:]

I think the arduino output pin might struggle driving that long of a wire.
Use a line driver to the base signal out there.  Heck, use another 2N2222 as the driver.
As for noise, I doubt you'd even see any LED flickering over the distance IF the transistors happened to oh so briefly turn on/off when not intended to.

Isn't 2N2222 an NPN? I would go NPN and pull the cathodes low to turn on the LEDs, less voltage drop across the transistors that way.
Since they are all getting  the same signal, one transistor woud suffice, only have maybe 180mA thru all 9 diodes?
With 6V you can probably 2 in series also,  so 4 pairs +1 for 100mA current flow when on.  Drive all 9 with just 1 transistor.

You are burying the wire? Exposed wire is at risk of animal chewage.

[Reply #2 on:]

[Reply #3 on:]

Yes, buried wire.

Good advice on taking it low to power on, thanks. And ya, as I was typing that up I realized that one transistor could probably handle the current draw on it's own. Guess I was being overly cautious using so many. I expected that correction.

I guess I was more worried about noise to other devices. Cell phones, radios, etc... If it's running under a patio and you set a radio over the wire or even run a wire out temporally for listening to music outside is it going to give out an audible interference?

I'm thinking back to my car stereo days and you had to watch where you ran speaker wire or you'd get some nasty buzzing in the speakers.

[Reply #4 on:]

Since they are all getting  the same signal, one transistor woud suffice

Likewise one current-limiting resistor, albeit a big one.

What about using RS-485 to drive the signal, less EMI and ground shift issues.

______
Rob

[Reply #5 on:]

I don't think you can go to just 1 resistor.  Variations in the LEDs will cause some to take more current than others.  I do agree with rs485 being a potential solution if UTP alone does not work.

[Reply #6 on:]

PWM is only 500 Hz tho, pretty sure most stuff will filter that out, especially if the signal is buried.

[Reply #7 on:]

I have picked up 400hz from airplane electrical systems.  However I would not worry about interference unless it became a problem.  Especially since the wire will not have to change, just the driver/receiver.

[Reply #8 on:]

400 Hz is the frequency of the AC power lines in airplanes, not the tens of milliamps that will be switching in this application.

[Reply #9 on:]

PWM is only 500 Hz tho, pretty sure most stuff will filter that out, especially if the signal is buried.

Thanks. I'll have to try it out and see how it works.

[Reply #10 on:]

Hey @crossroads, how'd you do that neat quick schematic????

[Reply #11 on:]

Using the software from expresspcb.com.
Being an electrical engineer helps