using hall proximity sensors with tachometer

[oldplanecollector]

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"

 

[Eremius]

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...

 

[oldplanecollector]

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.

 

[John Hasler]

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.

 

[GK1918]

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-

 

[Pmedic828]

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.

 

[oldplanecollector]

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)

 

[Ray C]

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

 

[mhguy]

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.

 

[Hawkeye]

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.