I am working toward some electronics for model rockets and I've come up against a problem that I'm not sure how to solve. As part of the error checking in the ignitor circuit I want to be able to check the continuity state. To me that means three possibilities: valid, open-circuit and shorted. The main problem is that a valid resistance will be in the range of .5 ohms to 10 ohms, somewhere out at the end of a 2' wire. I think I can handle the open-circuit to valid transition, but I just can't figure out how to deal with the low end. A compounding problem is that since this is an ignitor, I want to use the absolute minimum current through it possible to make the reading. I can take it in short bursts of course and minimize the heating, but to be on the ultra-safe side, I would like to keep the reading current down below 5mA.
So, are there any ideas on how to determine the difference between .5 ohms and 0 ohms at the end of what might as well be two foot antenna?
Martin Jay McKee
P.S. Lest anybody think that I'm leaving all the hard work to everyone else, in future I'll need to figure out the protection from the ignition voltage and an analog method ( as opposed to the three-state ) of reading the resistance proper at a later date.