I liked the analogy of thinking of the result of a test procedure to be either knowing to turn left or turn right. For nerds you can describe it as a binary search, the result is 0 or 1 which tells you which way to go. Another way to think of it is like the 'Guess the number game'. Your friend chooses a number between 0 and 100 and you try to guess it, they will tell you if you are hi, low or you got it. You learn pretty quickly to start with 50 and if your low then you guess 75, if that is high you guess 62. You always go up or down by half of the span.
Trying to explain how a partial short can load a circuit can be hard when someone does not have a good grasp of electronics yet. A good analogy is to think of the 'pull ups' and 'pull downs' as springs. If you have a spring that pulls up on a trunk release you have to pull down against that spring with sufficient force to move the mechanism far enough to release the latch. When you stop pulling the spring pulls the mechanism back to the latched position.
Our computer controlled output is the same, the output is pulled down to ground through a resistor (a spring), the driver transistor has to be able to pull up against it with sufficient force to overcome the pull down (the spring) and pull the output up to 12V. In this type of circuit, if you had a partial short to ground (extra pull down, or extra spring tension) the driver transistor might not have enough drive (strong enough pull) to pull the output up to 12V. Your test light would have a very strong pull in comparison.
Noah wrote: I like the spring analogy, that's a fairly apt comparison and it makes for a good visual concept.
Second this.
I REALLY liked this video, and the example that Paul used of the aftermarket anti-theft system. Resistive shorts can be one of the toughest things to diagnose, IMO, and have kicked my butt more than any other reason.
