At 04:31 AM 5/26/2006 -0400, you wrote:

Hi, Josh:-

>Hi all. I have a bit of a design challenge I'm grappling with. Here's
>the deal. I have a friend who wants to make some remote indicator
>lights. In the beginning they will be controlled by switches, but I
>will be building a PIC based system in the future.
>
>We've talked about how to deal with long cable runs in the past, but
>this application is slightly unique in that the indicators are
>bi-colour LEDs. These are the ones where they are two diodes wired
>back to back. At the moment I'd like to avoid having to put in a
>remote PIC mainly due to the tight delivery schedule he needs. So how
>best to provide protection when the polarity of the line will be
>changing? Let's guess that the line length will be between 50 and 400
>feet of cat5 cable.

A 1000' pair of AWG24 wires has a loop resistance of about 51 ohms at 20=B0=
C.
If you're going to be using, say, 470R series resistance, the voltage
drop for 50-400' is hardly significant as far as visual brightness goes.

>Also, I haven't done the calculations yet for voltage drop. If anyone
>has done this in the past, tips on good voltage to use (to deal with
>line drop) would be appreciated. In the future I will be moving to a
>more solid state design and I'm thinking about using an open collector
>or similar driver with a slightly higher voltage to drive the long
>line.

Maybe you could use an RS485 driver such as the SN65HVD20. Or if you feel
lucky, just use a couple of PIC pins and split the total resistance into
four, with a couple of unipolar TVS's to ground in a T. Or just a
single resistor if switches are doing the driving.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff



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