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Long ground wire

Let's say you have a circuit that uses an opto-isolator to trigger a microcontroller with simple logic high/low. (npn output)

The opto will have a steady DC voltage (through a resistor of course) on it's led but will use a switched ground to trigger the led.

Now, let's say the pair of wires going to the switch is a couple hundred feet or even more.

Everything works fine -- but of course, the led isn't going to be falsely triggered by noise, etc -- it needs it's 5 ma or so to "kick on".

Wouldn't it still be good practice to do something to eliminate or reduce the effects of the long ground wire (acting as an antenna?) which would connect directly to the ground plane of the PCB?

Certainly a well gorunded shield covering the pair would (probably?) suffice but would you also want possibly an inductor of some sort between the PCB and the ground (common) wire?

To keep noise out of the PCB ground circuit?

Actually what I have breadboarded works perectly, no issues, but I don't have a very noisy environment either.

Thanks for any help.

Reply to
mkrnews
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Use a twisted pair. This could be shielded, but it's probably not necessary.

To protect the LED from glitches, just put a .01 ceramic cap in parallel with the LED.

Good Luck! Rich

Reply to
Rich Grise

Everything works good until a lightning storm. Ken

Reply to
Ken S. Tucker

Using a twisted pair.....and like I say, all is fine.

Of course, the opto is the way to go with something like this....I'm just sketchy on what happens when the entire ground plane of the PCB has a few hundred foot long "leg" on it.

I'm talking about the IC pin grounds and everything else....I didn't know whether a choke in series would be a good idea?

Of course, plenty of .1 caps on the supply buss.

thanks.

Reply to
mkr5000

Well, one possibility is to split the dropping resistor into four (two in each lead) and use a low-side switch. That way nothing needs to connect directly to your power or ground points, and you could have two places to bypass the daylights out of, if you really needed to.

How worried you should get about this depends on where the wires are going to go. You can get some really ugly spikes due to inductive coupling from nearby lightning strikes, for instance.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Phil -- That's brilliant.

You're saying just supply direct DC to the LED and then parallel 2 resistors to the ground connection on the board (or even one would be an advantage).

I'm just being a perfectionist and although this circuit has worked fine for a year or so, I was always concerned about this....at least doing it that way would do no harm and you'd have the peace of mind of a "buffer" from your ground to the outside world.

You're a smart guy.

Have you seen this done in any equipment?

Reply to
mkr5000

Haven't looked.

I was actually suggesting this:

----------RRRRRR----*------RRRRRR------0 Vcc | | L ===, transient suppressing zener, E | etc, to taste D GND | /

----------RRRRRR----*------RRRRRR--------0 0-----GND | ===, zener, etc, to taste | GND

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

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