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Rev counter (Tachometer)

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John MC24/02/2019 17:38:56
142 forum posts
21 photos

I have recently been given a couple of electronic tacho's, cheap things that you will see for sale on Ebay. I want to use them on my milling machines.

The question I have concerns the mounting of the magnet. Initially I wanted to mount it on the side of an aluminium disc, this being fixed to the driven pulley. Problem is that it would put the (Hall) sensor in a position that could vunerable to damage.

I now want to mount the magnet on the periphery of the aluminium disc, this will move the sensor to a better position.

My question is does it matte how the magnet is presented to the sensor?



JasonB24/02/2019 17:48:14
15171 forum posts
1548 photos

Face to sensor and one face usually works better than the other

Edited By JasonB on 24/02/2019 17:48:59

The Novice Engineer24/02/2019 22:21:28
48 forum posts
20 photos

I have fitted one of the eB*y Tacho's to my Emco, although I used a smaller sensor than the one supplied [originally it was fitted to the emco Steppers]. The sensor is fitted into a housing that is stuck around the spindle with industrial double sided foam tape.

I have placed the magnet side on to the sensor, its position is not ideal but it works.



Plasma25/02/2019 07:35:51
192 forum posts
21 photos

How do you wire these things up? I bought one and got a display and sensor in a box with no clue how to power it etc. I'm a complete electronics dunce I'm afraid.

JasonB25/02/2019 07:38:32
15171 forum posts
1548 photos

Magnifying glass and you will see the colours of the sensor wires against the connector. Also google has images.

+ve supply also goes to 1 and -ve to 2

Edited By JasonB on 25/02/2019 07:39:50

Philip Rowe25/02/2019 11:23:10
167 forum posts
14 photos

I recently fitted one to my lathe and although it is recommended that the magnet face the sensor on its flat face I have found that it does work with the magnet on edge. It just means it needs to pass closer to the sensor, in my case around 4mm to guarantee reliable operation.


Plasma25/02/2019 18:38:16
192 forum posts
21 photos

That's fantastic. Mine is for a wind speed station I built for my garden, rather than fitting to a machine.

I will get it wired up now.


John MC27/02/2019 09:08:57
142 forum posts
21 photos

Many thanks for the replies. I have tried the magnet in both orientations, both work but positioning is far more critical with the magnet on the periphery of a disc rather than the side.

The device is quite slow to respond to speed changes, I guess thats reflected in the price! Also seems accurate compared with a hand held "Smiths" type tacho.

One final question, does the "strength" of the magnet effect the performance of the device?


John Haine27/02/2019 09:34:04
2455 forum posts
132 photos

These sensors use a Hall effect device to sense the magnetic field. The ones that give a "digital" on/off output, which the tachos generally use, are sensitive to the direction and strength of the field. The chips inside generally want a "north" pole (IIRC) to be presented to their top face and won't respond to a south pole. It's the direction of the field that's important, and they may respond if the magnet is presented in various orientations if there's a field component in the right direction.

Ideally if using one of the common rare earth disc magnets try to mount the magnet and the sensor so they are "face to face" as the magnets are magnetised through their thickness. You will need to experiment to see which face is the right one to work your sensor.

They will be slow to respond to speed changes because probably it counts pulses in a fixed time. So at 1 pulse per rev and 60 rpm, if you wanted to get a 1 rpm resolution you'd need to wait a minute to get 60 pulses. Alternatively they may measure the time between pulses and take the reciprocal, but even then they will average over a number of calculations which takes time, and of course the calculation takes some time too.

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