# using hall proximity sensors with tachometer



## oldplanecollector (Feb 14, 2014)

After seeing the cheap tachometer mrpete222 demonstrated on his youtube channel I want to add it to my mill and lathe but I've got a question about the hall sensors.  The tachometer readout he used measures based on one pulse (one magnet) per revolution.  There is another readout available in the same price range that is adjustable/scalable so you could have say 4 magnets (4 pulses) and the readout would measure one rotation only after it received all 4 pulses.  
I'm mildly competent with electrical systems but no expert by any means so my question I hope someone can answer is; I thought that the more pulses you had for a single revolution, the more accurate a tachometer is at lower speeds.  Am I correct or am I mistaken?

BTW, I don't have a direct link to readouts on ebay, but all I did was paste the search parameters mrpete gave in his videso  "Hall Proximity Switch Sensor NPN+ 4 Digital Red LED Tachometer RPM Speed Meter"


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## Eremius (Feb 14, 2014)

oldplanecollector said:


> After seeing the cheap tachometer mrpete222 demonstrated on his youtube channel I want to add it to my mill and lathe but I've got a question about the hall sensors.  The tachometer readout he used measures based on one pulse (one magnet) per revolution.  There is another readout available in the same price range that is adjustable/scalable so you could have say 4 magnets (4 pulses) and the readout would measure one rotation only after it received all 4 pulses.
> I'm mildly competent with electrical systems but no expert by any means so my question I hope someone can answer is; I thought that the more pulses you had for a single revolution, the more accurate a tachometer is at lower speeds.  Am I correct or am I mistaken?
> 
> BTW, I don't have a direct link to readouts on ebay, but all I did was paste the search parameters mrpete gave in his videso  "Hall Proximity Switch Sensor NPN+ 4 Digital Red LED Tachometer RPM Speed Meter"



You are correct, the more pulses, the more accurate at slower speeds.  I would venture to guess that unless you reprogrammed the IC you won't be able to divide the readings by x.

As they can, relatively accurately, read down to ~120 RPM (+/-2 RPM), is there a great need?

I suppose if you were really in a pinch you could just add 2+ magnets and do the math in your head...


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## oldplanecollector (Feb 14, 2014)

Eremius said:


> You are correct, the more pulses, the more accurate at slower speeds.  I would venture to guess that unless you reprogrammed the IC you won't be able to divide the readings by x.
> 
> As they can, relatively accurately, read down to ~120 RPM (+/-2 RPM), is there a great need?
> 
> I suppose if you were really in a pinch you could just add 2+ magnets and do the math in your head...




From your responses "unless you reprogrammed the IC you won't be able to divide the readings by x" and "do the math in your head"  I think I may not have been clear.  
I was asking about the number of pulses vs accuracy to determine if I went with a tach model EXACTLY like mrpete used which only accepted 1 pulse per revolution.
OR choose another similar tach (it actually looks like the same circuit board but with adjustment pots added to the back of the board) that is scalable from 1-100.  So I could set a scale of 1-100 and it would measure that many pulses for each revolution.  I have an application that I would like to measure down to 30rpm up to 3000rpm.


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## John Hasler (Feb 14, 2014)

oldplanecollector said:


> From your responses "unless you reprogrammed the IC you won't be able to divide the readings by x" and "do the math in your head"  I think I may not have been clear.
> I was asking about the number of pulses vs accuracy to determine if I went with a tach model EXACTLY like mrpete used which only accepted 1 pulse per revolution.
> OR choose another similar tach (it actually looks like the same circuit board but with adjustment pots added to the back of the board) that is scalable from 1-100.  So I could set a scale of 1-100 and it would measure that many pulses for each revolution.  I have an application that I would like to measure down to 30rpm up to 3000rpm.



30rpm is one revolution every other second.  Thus with one pulse per revolution it will take you a minimum of two seconds to find out how fast the machine is going.  With more pulses you can either get the speed more quickly or average over several and get it more accurately.


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## GK1918 (Feb 14, 2014)

Yes I saw the vidio and after I watched it, I bought the same one as Lyle's..  I remember working on Detroit 6-71 marines, that had only
one magnet on the flywheel and the readout did the math.  We will see.  It must be programable, question is does circumferance matter?
My plan is the mill quill pulley from the top-;I think???  If not it will end up on my Model T-


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## Pmedic828 (Feb 14, 2014)

I have a similar tachometer except with a blue readout - I can measure down to 20 rpm and faster than 1000 rpm (i'm afraid that if i turn the spindle faster than 1K, then the magnet will fly off.  I have not epoxied the magnet as of this time.  Comparing other tachs, this readout is within 1-2 rpm at 700 and just about right on slower than 50 rpm.  A hall effect sensor uses a magnet to cause a pulse.  The magnet needs to be within 10mm of the pickup but works great.  I am ordering another pickup so I can switch from lathe to mill.


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## oldplanecollector (Feb 14, 2014)

GK1918 said:


> Yes I saw the vidio and after I watched it, I bought the same one as Lyle's..  I remember working on Detroit 6-71 marines, that had only
> one magnet on the flywheel and the readout did the math.  We will see.  It must be programable, question is does circumferance matter?
> My plan is the mill quill pulley from the top-;I think???  If not it will end up on my Model T-



No programming available on the basic tach.  The one mrpete used will only operate correctly with one magnet. If you add another magnet the tach will display twice the actual RPM (since for every revolution two magnets will cross the switch).  That's why I was confirming that more magnets will increase the slow speed accuracy because it will require buying a different tach module than the one he used. A tach with scaling allows you to program how many magnets you want to add, this tells the module how many magnets will cross the sensor every revolution so it won't measure wrong.

As far as circumference, it doesn't matter. Your not measuring surface speed, only RPM (or how many times the magnet crosses the hall sensor)


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## Ray C (Feb 14, 2014)

BTW:  I have one of those cheap hand held optical LED tachometers that works off of reflective tape.  The user guide suggest adding additional, evenly spaced pieces of tape and dividing the result accordingly, if you wish to have greater accuracy at low RPMs...

Ray


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## mhguy (Feb 14, 2014)

I just built a MachTach for my lathe.  I put 6 reflective strips on the tail end of my spindle and it will read down to about 5 rpm.  Yes I got it that slow to test it.  It is programmable from 1 to 60 strips (or magnets, or slots - depending on the sensor you use).  They point out that more pulses per revolution do not increase accuracy, but only lower the minimum speed it will read.  It seems to work great and reads exactly what a portable tach reads.


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## Hawkeye (Feb 14, 2014)

I have MachTachs on three of my machines, all using hall sensors. I use six magnets on each one and get good results. Really easy to set the number of pulses. Plus, you get the bragging rights because you put it together yourself.


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## oldplanecollector (Mar 14, 2014)

Update and request for help.  First I'll update with results of adding more magnets then I have another problem.  Keep in mind I purchased the display unit that allowed changing the scale factor to select the number of magnets located on the circumference. I opted to go with 12 magnets and it read good at slow speeds, but as the speed got up to 500+rpm the last couple digits bounced around a lot.  I changed out to only 4 magnets and I still read good at low speeds but had a much more stable readout at speeds 500+rpm without the last digit flickering.  So I'll probable stay with 4 magnets.  By the way, anyone who uses a unit like this with the hall proximity sensors needs to verify the polarity of each magnet before gluing them in place.  The sensor will only pick up on one side of the magnet, if you flip it over, the unit won't read it.

Now the problem, the unit reads great up to 950-1050RPM then in that range it stops displaying speed until speed is dropped back down to 950rpm or less.  It's not like you have a great deal of flexibility with these types of units so they should be fairly idiot proof, but I have now reached the end of my technical ability.  So far I've tried the following things with no change in results;
1. Switched from 12vdc wall wart to my bench power supply to see if maybe unit wasn't getting enough current. Still won't read beyond ~1000rpm.
2. Cut down number of magnets from 12, to 4, then to 2 then to 1. (and YES I changed the prescale factor accordingly as I reduced the number of magnets) Still won't read beyond ~1000rpm.
3. I increased the distance from the magnets to the sensor to the maximum gap that it would still pick up at. Still won't read beyond ~1000rpm.
4. I decreased the distance from the magnets to the sensor to about 1mm. Still won't read beyond ~1000rpm.
5. I have two display units so I swapped them out just in case one was faulty. Still won't read beyond ~1000rpm.
6. I started out using 8mm dia x 3mm thick magnets, but also tried using 25mm dia. Still won't read beyond ~1000rpm.
7. I have an email message in the supplier of the display to see if they have any ideas.  Not holding out much hope in this one since they're in China and more than likely just a reseller, plus a lot can get lost in translation.  I only gave it a shot because on their page they said 'tell us your requirement, we can custom design for you.'  So I was hoping they had someone with technical assistance.

Maybe someone with more technical expertise can think of something I'm missing.


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## John Hasler (Mar 14, 2014)

oldplanecollector said:


> Update and request for help.  First I'll update with results of adding more magnets then I have another problem.  Keep in mind I purchased the display unit that allowed changing the scale factor to select the number of magnets located on the circumference. I opted to go with 12 magnets and it read good at slow speeds, but as the speed got up to 500+rpm the last couple digits bounced around a lot.  I changed out to only 4 magnets and I still read good at low speeds but had a much more stable readout at speeds 500+rpm without the last digit flickering.  So I'll probable stay with 4 magnets.  By the way, anyone who uses a unit like this with the hall proximity sensors needs to verify the polarity of each magnet before gluing them in place.  The sensor will only pick up on one side of the magnet, if you flip it over, the unit won't read it.
> 
> Now the problem, the unit reads great up to 950-1050RPM then in that range it stops displaying speed until speed is dropped back down to 950rpm or less.  It's not like you have a great deal of flexibility with these types of units so they should be fairly idiot proof, but I have now reached the end of my technical ability.  So far I've tried the following things with no change in results;
> 1. Switched from 12vdc wall wart to my bench power supply to see if maybe unit wasn't getting enough current. Still won't read beyond ~1000rpm.
> ...



You've got a 16 bit counter in there.  There is nothing you can do about it.  That's the limit.


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## oldplanecollector (Mar 14, 2014)

John Hasler said:


> You've got a 16 bit counter in there.  There is nothing you can do about it.  That's the limit.



The display says it will display 5 to 9999 RPM and can prescale 1-9999.


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## oldplanecollector (Mar 16, 2014)

UPDATE: I got a response back from the ebay seller in China less than 6 hrs after sending it off.  They asked for pictures of the installation so they could have their engineer review.  I sent pictures Saturday evening at 7pm, thieir response came back at 10pm. The engineers recommendation was for them to send me different sensors. Rather than use the magnetic hall sensors, they recommended a proximity sensor that would pick up iron vs magnet.  So now I wait for replacements to see if they will work. 

I'm horribly impatient and hate having to wait weeks to get the replacements, but I will say that I was impressed I got an answer at all let alone the response time from the seller 'TMALL'.  



I don't know if the placement of the magnets is a problem or what.  I placed my magnets on the perimeter versus adhering on the face of the pulley like 'mrpete' did in his video.

I even tried using just one magnet but the out of balance vibration at 500+RPM was too bad.


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## GK1918 (Mar 16, 2014)

My additional 2 cents.  My first attempt was a 110v trans down to 12vt dc.  The skecthy instructions say 8-24 vts dc so I thought I was
safe, only lasted about 5 min and the display went brain dead.  So I got another but using a 9vt common battery. The new one no directions
at all.  The air gap I'm guessing now I find it to be around 10mm.  So I placed the magnet turned the mill on and hand held the sensor and
it read 179-180 about what the mill chart says.  Mr Pete says its a battery eater so I put a push button, no need for displaying all day.
Hate to drill a hole but I think one hole to hold the sensor and display box. Again I'll see it this one lasts.


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## twr (Mar 18, 2014)

- - - Updated - - -

Hi i bought 2 from the same seller as you for my lathe but mine are not adjustable and use one magnet. One works fine but the other one losses part of the last diget. Seller is sending me another display. I dont think i have tried it passed 800 rpm yet but i will now. I am using a 120v down to 12v adphter and no problems yet with lighting up.


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## oldplanecollector (Mar 27, 2014)

Another Update:
The new inductive proximity sensors arrived from the seller and I gave them a try.  They sense iron versus magnets. The results got worse. This time the display maxed out at around 350 rpm before it stopped picking up.  So the "engineer" the ebay seller has seems to know about as much as I do about this problem. 

As you could see from my previous picture with the original sensors I had been placing the magnets on the perimeter of the ring and only got to about 950-1000 RPM, I decided go back and try the original sensors and place the magnets on the top surface versus on the perimeter. With this change I actually hit about 1450 RPM now before it stops reading.  Much improved but something still not right.  I even tried increasing the magnet thickness to see if that would help pick up at higher speeds, no change.

Since the digital tach is really more of a novelty I've about reached my limit for dealing with this and scrap the whole idea.


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## hman (Mar 28, 2014)

What may well be happening is that, though the tach circuitry can handle inputs up to 5000 Hz, the sensors you are using may not be able to respond fast enough.

I've used quite a few Turck proximity sensors in previous years.  Looked them up on the web just now, and (at least the models I looked at) are limited to ~2000 hz switching frequency.  I'll offer a SWAG here and say that because proximity sensors need to sense a change in frequency response, they'll all have an inherent speed limitation.

I've not used Hall effect sensors.

My suggestion would be for you to consider using a photo sensor.  Photo sensors generally have the simplest circuitry (an LED emitter and a transistor receiver), so they should be pretty fast.

Just for fun, I went to the Mach Tach's documentation page ... http://machtach.com/Docs.html looking for speed specs on their sensors.  All I could find was the rise and fall times of their Hall sensor, which are 1.5 microseconds.  This would be plenty fast enough for tens (or hundreds?) of thousands of RPM.

Another possible problem (mentioned in MachTach's documentation), would be interference from VFD wiring.  I don't recall your mentioning if you have VFDs, but it seems you are able to easily vary the speed of your machines.

If nothing else, I'd suggest you go to MachTach's documentation page and download their User and Assembly manual.  It contains a wealth of info.

PS - Not associated with MachTach, but I'm on their Yahoo group and have bought a kit (not yet assembled).

Best wishes!


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## GK1918 (Mar 28, 2014)

oldplanecollector said:


> Another Update:
> The new inductive proximity sensors arrived from the seller and I gave them a try.  They sense iron versus magnets. The results got worse. This time the display maxed out at around 350 rpm before it stopped picking up.  So the "engineer" the ebay seller has seems to know about as much as I do about this problem.
> 
> As you could see from my previous picture with the original sensors I had been placing the magnets on the perimeter of the ring and only got to about 950-1000 RPM, I decided go back and try the original sensors and place the magnets on the top surface versus on the perimeter. With this change I actually hit about 1450 RPM now before it stops reading.  Much improved but something still not right.  I even tried increasing the magnet thickness to see if that would help pick up at higher speeds, no change.
> ...



Interesting, I almost think the sensor air gap could be critical. I set mine .032 with a brass gauge and works fine. sam


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## oldplanecollector (Mar 28, 2014)

GK1918 said:


> Interesting, I almost think the sensor air gap could be critical. I set mine .032 with a brass gauge and works fine. sam



So your able to measure faster speeds than 1400RPM? Are you just using the magnet they sent with the sensor? 
I'm mounted around 1mm or so but I didn't measure like you did. I will try setting with feelers like you did and see if it get any better. 

BTW; I had downloaded the Machtach manual (even though I didn't want to buy one of their tachs) to see if they had any info I could use about the sensors. I also did some searching for other info online about the switching frequency of sensors.  The original hall effect sensors have a frequency of 320kHz and the new one they sent, the inductive sensor is only 0.5kHz.  Somehow I did feel that the frequency was part of the problem.   By my thinking, 320kHz is 320,000 pulses per second so It should be more than fast enough, right?


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## GK1918 (Mar 28, 2014)

oldplanecollector said:


> So your able to measure faster speeds than 1400RPM? Are you just using the magnet they sent with the sensor?
> I'm mounted around 1mm or so but I didn't measure like you did. I will try setting with feelers like you did and see if it get any better.
> 
> BTW; I had downloaded the Machtach manual (even though I didn't want to buy one of their tachs) to see if they had any info I could use about the sensors. I also did some searching for other info online about the switching frequency of sensors.  The original hall effect sensors have a frequency of 320kHz and the new one they sent, the inductive sensor is only 0.5kHz.  Somehow I did feel that the frequency was part of the problem.   By my thinking, 320kHz is 320,000 pulses per second so It should be more than fast enough, right?



I guess all I can say is the price was right.  And as far as 1400 rpm that I dont know.  I usually run around 800 rpm and the magnet has not
flown off 'yet'.  The magnet is on the circumference of the spindle timimg belt pulley. The instructions are horrible it was all guess work
on my part.  My first one blew up using 12vt transformer I left a note with ebay seller and ordered another and then they sent me two
(one for free). This time i used a 9vt battery with a push button switch.  Its only needed when changing belts anyways.


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## iron man (Mar 28, 2014)

oldplanecollector said:


> Another Update:
> The new inductive proximity sensors arrived from the seller and I gave them a try.  They sense iron versus magnets. The results got worse. This time the display maxed out at around 350 rpm before it stopped picking up.  So the "engineer" the ebay seller has seems to know about as much as I do about this problem.
> 
> As you could see from my previous picture with the original sensors I had been placing the magnets on the perimeter of the ring and only got to about 950-1000 RPM, I decided go back and try the original sensors and place the magnets on the top surface versus on the perimeter. With this change I actually hit about 1450 RPM now before it stops reading.  Much improved but something still not right.  I even tried increasing the magnet thickness to see if that would help pick up at higher speeds, no change.
> ...



 I have used the sensors to pick up iron I used a steel gear and it worked fine what did you use?? The hall effects sensor has to be the right one for high speed I use to use for engine tach's.. Ray


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## oldplanecollector (Mar 28, 2014)

iron man said:


> I have used the sensors to pick up iron I used a steel gear and it worked fine what did you use?? The hall effects sensor has to be the right one for high speed I use to use for engine tach's.. Ray



With the inductive sensors I took the plastic ring I originally made with magnets on the perimeter and trimmed down to remove magnets and tapped in a couple #10 hex heads on perimeter.

After reading some online info about the magnetic sensors I question if it would need an even stronger magnet to read at higher RPM.  Either way I'd really have to have at least 2 opposed magnets to keep it balanced. My last run at 1400 RPM with one magnet on the top face of the drill press pulley caused a terrible oscillating vibration.

I couldn't see spending $60+ on the MachTach units because it really is only a gadget to me and I could thing of a lot of other useful tooling I could spend that money on. I only thought to try these stupid things because at $15 I could add one to both the mill and vise and drill press. It's the gadgets that seem to always get me in trouble.


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## GarageGuy (Mar 29, 2014)

Like Ray C, I just got a $30 HF optical tachometer with the reflector strips, and it works great.  With my belt drive mill, I only have a few RPM choices anyway.  It takes longer to change pulleys than it does to check the speed.  Best of luck,

GG


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## chip maker (Mar 29, 2014)

I have two of the MachTachs and they really work great to me. You kind of got what you paid for with the cheap tachs. After all the problems you have had with them you spent more time than they were worth to me. Just bite the bullet and give the MachTachs a try you won't be disapointed. I really think you will be very happy with the tachs plus you get to put them together yourself. I also beleave that if you contact Henry at MachTach he may be willing to build you one for an added cost if you don't want to build it yourself. Good luck. !!!


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