quick, messy, & cheap spindle RPM meter / tachometer

Rick, I see you eyeball-to-eyeball on the magnet issue. With all that swarf and what-not, I'm a bit concerned. I may look into a different type of sensor system.

There's four types of magnetic sensors. One uses a coil that produces a tiny current when the magnet swings by. The second operates a 'reed' switch. The third uses a 'reluctor wheel', which may be how your millhead was setup, The other is this 'hall' device. It's simply a different type of transistor that gates 'on' when in the presence of a strong magnetic field. It essentially acts like a grounding/shorting switch, like the 'reed'. The 'hall' devices used in these sensor/tachometer applications are actually tiny IC's that have supporting circuitry to make this thing work cleanly. If you like tech data, here ya go:

http://www.alldatasheet.com/datasheet-pdf/pdf/55092/ALLEGRO/A3144.html
 
Thanks for the explanation. Looks like most use magnets in some form or other. I haven't looked closely at the mill, but if it used a reluctor set-up, seems like there should be some tell tale signs on the pulley.

Thanks again.
 
I put the same LED tach on my wood lathe with a transformer that gets powered up any time the speed controller is turned on. Works great, or at least it did for a few months. Now one of the LED segments has burned out. 5 and 6 look alike. Also 8 and 9 look alike. It is a bit annoying, but not so bad that I want to replace it yet.

My only advice is to put in a stronger magnet than the tiny one that came with it. This allows it to work with a gap of about 1/8". Also, the magnet works better on one side than the other. Maybe the hall effect sensor is polarity sensitive. I made sure to glue it in with the proper orientation.

Steve
 
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