Circuit Test
Imagine that you have been hired by Acme Engineering Co., and your job is to analyze an unknown electrical device. “But… but I have no knowledge of electronics!” you tell them. Well, no big deal… Acme thinks you’re qualified. Plus, the salary is immense! So why not give it a try?!?
The electrical device is housed in a purple plastic box with a set of wires sticking out each end. One set is labeled “Input.” Presumably, the other set is the output, so a voltmeter has been attached as shown in the diagram.
Your goal is to characterize the device (and possibly determine what’s inside the box) by applying different signals to the “Input” terminal, and seeing what happens at the output (by reading the value on the voltmeter).
Since you’ve only been given rudimentary lab equipment, you see that you have three ways to apply inputs to the device:
- Short the “+” and “-” terminals of the Input together using a metal wire (essentially applying a zero-volt input).
- Connect a 9-volt battery between the terminals of the Input.
- Connect a 12-volt car battery between the terminals of the Input.
Which of these will you try first?
Posted: February 19th, 2008 under Engineering.
Comments
Comment from Eloprah
Time 2008/02/19 at 9:35 pm
Car battery? Why stop with a car battery? I was thinking jet engine, at least.
The bigger, the better is my motto when dealing with electricity. Of course, if by “short” you meant that I got to use an oxyacetylene torch to weld those two bad boys together – plus I got to wear the ultra-cool helmet – well, that would be another story all together!
Comment from Jed
Time 2008/02/19 at 10:57 pm
Josiah, alas, it does not have LEDs on it. It’s just a plain purple box.
Haha, Eloprah! Nice. Yes, that is an acceptable way of shorting the two terminals together! 
Comment from Bethany Wheeler
Time 2008/02/19 at 11:05 pm
I’d put it in the microwave to see if it blew up.
Comment from Jed
Time 2008/02/19 at 11:11 pm
Hahaha! Well, you can try that, Bethany… but I don’t think Acme will be amused.
Comment from MattTheMusician
Time 2008/02/20 at 7:07 am
I would try to plug it into the wall…
Comment from jeff
Time 2008/02/20 at 8:17 am
Since we don’t know what’s in the box, we don’t know whether any of the proposed inputs would damage it. I would, however, be suspicious of the jet engine, microwave, and wall outlet.
Knowing as little was we do, I’d probably start by shorting the two inputs together. From this, I would hope to learn if the input is expected to be a switch and whether there is any internal power source within the purple box. If the output changes upon shorting the inputs, then I’d have learned that there is a power source within.
The 9V battery would be my next choice, because it cannot supply much current. It would be unlikely to damage any but the most sensitive components.
A 12V car battery, on the other hand, can supply lots of current. While the purple box might eventually be found to require this, it isn’t a good starting place.
Comment from Jed
Time 2008/02/20 at 9:09 am
Sounds reasonable, Mr. Wheeler. It looks like Acme has hired at least one trained engineer.
Comment from Josiah
Time 2008/02/19 at 8:07 pm
Does it have any shiny LEDs on it? I will short the inputs together, as I have experienced bad results from car batteries.