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Measure spindle speed with a Multimeter???

Les Fackrell

Nov 26, 2016
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Nov 26, 2016
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I am in the market for a replacement multimeter and I have noticed that some have a frequency counter. This made me wonder if a simple circuit using a light dependent resistor, some shiny sticky tape and a battery would make a signal that the meter could Count? If the reflective tape were stuck on the spindle and a light shone on it, the LDR would change its resistance depending on the reflected light. Would this make a signal that the meter would pick up?
You will probably guess that I am a complete novice in electronics. (I am aware that you can buy a tachometer cheap as chips on Ebay but I just wondered)
Les
 

Bluejets

Oct 5, 2014
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Think you will find it requires a changing voltage.
A hall effect (A3144), magnet, collector resistor and a battery should work.
Take the signal from collector to ground.
 

Alec_t

Jul 7, 2015
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LDRs are rather sluggish, so you would get unreliable results at high spindle speeds.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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If you wanted to go with an optical solution, an opto-transistor would be a better bet.

Some problems you might have to deal with are ambient light levels, 2 x mains frequency flickering of lights, and enduring you get enough light to the sensor.

Some form of chopper that interrupts a light source pointing directly at the sensor may be better if you can achieve it.
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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Does your spindle have any ferrous gears or other ferrous protrusions?

Chris
 

Nico Orru

Jun 7, 2017
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Jun 7, 2017
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I have obtained reasonably accurate results:
- gluing a tiny neodymium magnet to a broken bit
- wiring up a U18 hall sensor (latching, but should not be necessary)
- using an oscilloscope with frequency meter to read the result. You can probably achieve the same by using a multimeter (with freq.meter) instead.

20170607_213312.jpg 20170607_213329.jpg
 

FuZZ1L0G1C

Mar 25, 2014
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Mar 25, 2014
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Many DMM's these days have built-in frequency measurement.
Found (on Major-Tech and Top-Tronic DMM's) that they read sinewaves more reliably than square (maybe as square contains harmonics*).
Also, according to their own manuals, voltage level should be at least 10V RMS AC.
* Reads the fundamental of square-wave, but 'hunts' up & down as well.
 
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