Airbag requires about 6V only to set it off, so setting 5V as your ceiling is probably pushing it, I think I remember it needed less than 200mA, so a fuse is actually a waste of time, if not a bad placebo. I only know this because I once helped build an airbag tester to be used for Volkswagen and BMW on the Uitenhage plant in South Africa, AND had to wait patiently while everyone still messed about with antistatic gear (which I thought was overkill, but I may be wrong). The trouble with relays or switches, is that they need to have a good bit of juice running through the contacts, else they do not make good contact (they are designed to actually arc, and in low current applications, a small ceramic is used to sometimes make a tiny spark to weld the contacts together). DC is also not that great, since the arc carries metal with it. 

So a transistor switching (boosting) circuit (which you have designed) is also getting my vote, you can use the transistors to drive a 12V relay (small one), ro regulators needed, and since car battery is 13.5V and the darlington-style circuit (which you drew) often looses just over a volt anyway. The voltage (and needed current) for the BASE of the darlington is going to be low enough to be safe. Just remember to place a diode across the relay coil for back EMF taking your transistors out.

How can I build a switching circuit that scales several 12v signals down to <5v?
I'm trying to compare the spatial distribution between two groups of fishing locations (for each of several different years) in the Bering Sea (lat's ~53 to 65N, long's ~ -178 to -158W. My preliminary explorations have used kernel density estimation (with an unconstrained plug-in bandwidth matrix) and then examined different overlap indices of their home ranges (quantified via utilization distribution overlap index and Bhattacharyya's Affinity) to compare similarities/differences (all performed in R). 

However, I'm concerned about the interpretation of these data when the data may span more than one UTM zone. Most of my data are in UTM zone 3 with some data points seeping into zone 4. In more extreme cases, my data may span from zone 1 to 4. 

Am I being really ignorant in thinking that I can compare across zone boundaries? 
Since I am interested only in relative differences between calculations with the same distortion, will it not matter? 
Should I simply make sure that I have my UTM origin set at the southwesterly most point of the data and I'm fine from there?
I've been reading StackExchange and other sites all morning and have seen several discussions about measuring across UTM zones, but I haven't been able to logically extend such responses to a method such as this one. 

Thank you for your time and expertise!
