QUOTE(jim_hoyland @ Nov 18 2012, 10:21 AM)
...Given the resistance/rpm chart above, can an rpm activated switch be made from Radio-Shack parts ? How might that be done ?
Those two thoughts aren't related...
The resistance is what the MSD-6AL rev-limit circuit uses in its comparator circuit. Each comparator circuit design will be different.
For instance, I believe the Crane HyFire is actually digital.
I'd guess the MSD circuit is probably charging a capacitor of a known value through the resistance. Or possibly discharging at a known rate through the resistance, since no pill == no limit...
But either way, you have something that's reset by each trigger, and a resistance-proportional charge/discharge rate, and an analog comparator (just a transistor?) that turns on the rev-limit circuit when the charge level is at the threshold.
I bet a real EE could sketch up such a circuit on a napkin in 10 minutes. I'd have to re-learn a bunch of stuff to figure out appropriate values for the resistors and capacitors, as well as the relay drive part to do whatever it is you want done when the rev limit is achieved.
You could also complexify it with microprocesor control and digital counters and such, but it's really not that complicated and the application tolerances don't require it. (E.g. - if you had a couple of rpm drift from cold to hot, it'd be fine...)
I gave up on finding any useful parts at Radio Shack a long time ago. Now they just sell cell phones and such...
Mouser and DigiKey get my business now...