Limit Switches for Dummies

A limit switch on an assembly line. The
switch stops the line when it is tripped.

It's eHow week, so we're devoting all seven days to the freelancers who've sold their mortal souls to Demand Media Studios, also known as DMS (ever notice that you can't spell "dumbass" without "DMS"?). The only problem in zeroing in on eHow for a week is picking which of the tens of thousands of available candidates to use...

Let's start out easy this morning with eHow.com's Mark Vallet, one of the site's myriad former English and journalism majors who choose to masquerade (too often doing so poorly) as someone who knows something about technical stuff. Mark's topic today is "What are the Functions of a Limit Switch?" (now hiding at HomeSteady.com¹).

It's pretty obvious that the gentleman had no earthly idea of the function of a limit switch. Hell, if he'd just understood the wikipedia entry (Wikipedia was verboten to eHow writers, though it's hard to understand why), he would have known that the "limit" qualifier means that the switch's main function is to open or close an electrical circuit when a device exceeds a set point, usually physical. One of the most recognizable examples is on a garage door opener: a limit switch turns off the motor when the door is fully closed. If the switch weren't there, the motor would continue to try to push the door into the ground until it burned out.

There are other kinds of limit switches, too: a gas furnace has a limit switch that turns the burner on and off depending on the temperature of the combustion chamber. A similar switch controls when the compressor in your refrigerator fires up depending on the temperature in the freezer. Mark's "wisdom": "The most common switches, mechanical limit switches keep track of items. When a device, or part of a device, reaches a certain location, its limit switch opens or closes. The switch depends on physical contact, or the lack of it, to open or close. An example is refrigerator light that switches off when the door closes and turns on when it opens."

The definition is OK, though obviously just reworded from something he didn't understand. But Vallet's example? The switch that controls the light in your fridge isn't a limit switch: it's a simple on-off switch. In order for it to be a limit switch, the switch would have to stop the motion of the refrigerator door, not just turn the light on. For managing to turn ignorance into misinformation (and pretty substantial cash), Mark Vallet is our first Dumbass of the Day during eHow week!

¹ The original has been deleted by Leaf Group, but can still be accessed using the Wayback machine at archive.org. Its URL was   ehow.com/facts_7501098_functions-limit-switch.html

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