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Author Topic: Using Reed Switch for building a rpm counter  (Read 2400 times)
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Chennai, India
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How good is the idea to use a reed switch to make an rpm counter. Or is it better to use a hall-sensor? what are the differences? I've built one using both and under normal conditions they both work alike only the hall sensor costs 5 times the reed switch.
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I belive that the hall sensor will be a better choise at high RPM's.

But if the reed switch is Ok for your need i can't see why you should not go for it.

I think i read somewhere (in here??) that some reed switches have serious bounce problems.
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Chennai, India
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Quote
i can't see why you should not go for it.

i was thinking perhaps the mechanical element might wear out over a period of time... just thinking aloud not sure.
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CT, USA
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Since coding wise, there's not a whole not of difference between using a hall switch or reed switch, I'd suggest going with a hall switch since it is much more reliable.
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As a serious skinflint, I'd consider a reed switch on a slow rotating object.  If its doing say 50 RPM, I think it would last a while and it wouldn't be too grevious to hardware debounce it.   If its zipping round at 6000 RPM its a whole different ballgame........
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Reed switch can take several milliseconds to pull in and the same again to release.  If each is 5mS then a complete cycle will take 10mS.  This gives a maximum repetition frequency of 100Hz. ie 6000RPM

Switch life is typically 100million operations so you'll get around 1,000,000 seconds of life at 100Hz.  1 million seconds equates to around 11.5 days.

Around 45 years ago I made a rev counter for my dad's van using a reed switch wired to the distributor.  At 4000 RPM engine speed (say 60MPH) this rotated at 2000 RPM.  2000 RPM is 33 revs/second so typical life calculates to 34 days at 60MPH, which is around 50,000 miles.  I cannot remeber how many miles it had done when we got rid of it, but the counter was still going strong

So, if your cycle rate is relatively low, say less that 50Hz, you should get a reasonable short term life.

jack
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Holland
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A reed switch can fail because the contact may stick (ie: won't release).
Haven't heard of a reed switch breaking mechanically under normal use.
A hell sensor has no mechanical part, thus isn't susceptible to this type of breakdown.

That said, and as others have pointed out, I wouldn't care too much if its used at low RPM's (and is used sporadically... if it were to be 24-7, I'd use a hall sensor or hall switch instead).
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pracas,

I have had good success using a magnet and a reed switch at speeds up to 320 rpm for periods of several hours.

Here is a temporary setup that I used to count the turns on a sensor coil that I made for my Aurora Borealis Monitor:

http://www.flickr.com/photos/54986139@N04/5098434130/

However, despite good success with reed switches for these kinds of temporary setups . . . I would be inclined to go with a Hall Effect Sensor for higher speed or longer term installations.

My local electronics supplier sells both reed switches and Hall Effect Transistors in the $1-3 range so there really isn't a difference in price.
« Last Edit: October 19, 2010, 08:53:07 pm by CSingleton » Logged

Chennai, India
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i'm planning to use it for teaching to build rpm counters... i'm looking at 60 to 300 rpm motors... perhaps a robotics application of making 2 wheels move at the same speed...
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Then it's reed switches you need.  Teaching any form of physics should start with basic building blocks (reed switches) and then advancing to higher tech devices (hall effect sensors).  
If you teach high tech from day one then your pupils will have little comprehension of how to "make do" and develop little in the way of initiative.

jack
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