Trigger Lead Lengths

[CoffinBound]

When creating our own trigger cables, are there any guidelines we should keep in mind? Especially regarding cable lengths?

For example, if I use 22AWG wire, how long of a run can I make that will function properly?

TIA.
CoffinBound

[JonnyMac]

Here's how I would do it.  Granted, I'm a very conservative designer, but then, the things I build tend to work. 

A. The resistance of #22 stranded (7/30) is 0.015 Ohms per foot
-- reference: http://www.ece.seas.gwu.edu/~ecelabs/appnotes/PDF/techdat/swc.pdf

B. The input threshold for the Prop-1 interpreter is 1.5 volts.

C. The pull-down (SETUP = DN) resistance is 4.7K

Knowing these things and taking the conservative approach, I would ensure that the length of wire plus switch resistance plus connections allowed for 4v at the input pin when the button is pressed.  Keep in mind that the longer your wire, the more likely you'll pick up electrical interference (you're creating a big antenna); you can mitigate this by using a twisted pair and software debouncing of the input.

Okay, how do we solve for the wire resistance?  Technically, we can breakdown the circuit like this:



(The +5v in this circuit comes from P6.R)

Again, being very conservative (always best with these things), we will allow for no more than a one volt drop through the wire, button, and connections -- this means that we want 4 volts at P6.W when the button is pressed.  We can find the total (R_T) resistance of the circuit plus the pull-down with a bit of algebra:

R_T / 5 = 4700 / 4 ... R_T = 4700 / 4 x 5 = 5875

So, if we subtract out the pull-down (4700) from the total we get 1175 ohms.  And here's where the wheels fall off for most people.  That's the TOTAL resistance for wire, connections, the button, etc. -- not just for the wire.  Let's continue to be conservative and cut it in half for the wire, 588 ohms.

588 / 0.015 = 39,200 feet

And now the joke is on me... because even though we have to cut that value in half (for the trip out and back), we still have a conservative theoretical distance of 19,600 feet.  That doesn't mean, though, that we should go crazy.  Use twisted cable; be very careful with all your connections; don't run the wire near AC wires or near solenoids and other inductive devices (that can create a magnetic field that can induce a current in your trigger wire). 

All right, JonnyMac the Science Wack is off to Hollywood to hang with friends!

PS: Just because things worked out so rosy with #22 doesn't mean you should drop to a smaller gauge wire (unless it's part of a cable assembly); #22 is tough to break and this is important when you're dealing with long runs; nothing is worse than trying to troubleshoot a wire with an internal break.