PIR on a Long Cable, Part 2

[MMayberry]

Hi all,

I saw an old thread regarding this topic, but couldn't find the final solution.  I'm looking to put my PIR out about 20' from my MIB prop.  What was the acceptable wire size?  Was a standard cable cut and spliced onto the ends of the new wire?

First ever EFX-TEK post but have been reading (and learning from) the forums for awhile.  You are a great group of folks!

[BigRez]

I'm sure Jon and others can cite the actual specs, but from experience I've connected one of my PIRs on a long cable of about that length and had no problems with it.  I made the cable from cat-5 cable and connectors purchased from Jameco, but yes, you could splice into the standard cable for both ends.

Welcome to the boards!

[JonnyMac]

Welcome to the forums.  Calculating cable is pretty simple if you understand Ohm's Law.  If you don't, get a copy of "Getting Started in Electronics" by Forrest M. Mims -- it's a great resource.

There is no definitive spec on the current required by the Parallax PIR so I'm going to use 100mA.  This is probably overkill, but going into war I'd rather have a bazooka than a BB gun.  Overkill tends to mean reliability in harsh conditions.  When calculating wire size you should not allow more than a 10% drop from one end to the other.  Allowing for this you'd have about 4.5v at the sensor which is within its working voltage.  Coming back you'd have about 4v which is above the "1" threshold on an input pin.

We need to find a wire size that will drop no more than 0.5v in 40' when 100mA is running through it.  Using Ohm's Law (E = IR) we can find that R is 5 Ohms (0.5/0.1), or 0.125 Ohms per foot (or 125 Ohms per 1000').  Using this chart:

http://en.wikipedia.org/wiki/American_wire_gauge

...we *should* be able to use #30 or bigger.  That said, #30 -- unless it's in a cable -- is very fragile for these kinds of applications.  Go with something beefier (e.g., #22); it won't cost much more and will better withstand the abuse of Halloween prop building.

This information is provided as-is, with no warranty, expressed or implied.  ;D

One final note: Using a long wire subjects the controller to more EMI.  Make sure that your controller is doing some sort of debouncing on the PIR input otherwise you may get a lot of false starts.

[JackMan]

 

I'm looking to put my PIR out about 20' from my MIB prop.

If you're using a Parallax PIR they're pretty sensitive and trigger from about 20' out unless it's field of view is choked down. Try it out, you may not need to place it 20' away from your prop.

[MMayberry]

Hmmm - Hadn't thought of that.  I'm going to try using the house 68" cable and Scary Terry's PIR choke mod and see what happens.  I'm hooking up to the EZ-8 board that has - according to Jon in another post - some built-in debouncing protection.  We'll see what happens!  The wire info was still good to know; thanks everyone!

[JackMan]

A good cheap method that has worked for me is to use a plastic cap from a can of spray paint. Punch a 1/2" to 3/4"  hole in the center of the cap (depends on how much you need to choke it down). The PIR fits nicely into the inner portion of the cap, a little hot glue secures it. Drill a matching 1/2" to 3/4" hole in whatever is convenient for your prop and hot glue the cap to the backside. This will pinpoint your "victim" trigger zone.