935 Hall effect speed sensor for Tachulator

[kb58]

The Tachulator is a tachometer which can be added to any machine, and can figure Surface Feet per Minute (SFM) of cutters. It uses an IR optical emitter and sensor to detect rotation, so you're on your own to figure out where to place it. At first, I put it up top, near the spindle hex shaft. Not long after though, I saw that forum member mksj integrated a Hall-type (magnetic) sensor (NJK-5001C) into a spindle light of his own design, which was too sweet to ignore. The catch was that he used that sensor with a different tachometer, so it was a big unknown whether the Tachulator could be adapted to use it as well. Short story is that it can, but expect zero support from the manufacturer, who ignored multiple emails. What sucks for him is that I'm an EE, and love an electrical challenge.

Okay, the NJK-5001C Hall sensor has three wires, labeled on the unit in Chinese, of course, but which translate to:
Brown: +12V
Black: Output
Blue: Ground

Measuring how the existing optical sensor was connected and looking at the signals with a scope, it looked promising, and in fact worked, as follows:

Brown -> +12V on Tachulator
Black -> "White" input terminal on Tachulator
Blue -> "Blue" input terminal on Tachulator

The nice thing is that adapting the sensor requires no modifications whatsoever. Make sure to test it first before the magnet is permanently fastened in place, because it only senses one pole and ignores the other. Lastly, make sure you're okay with the size of the Tachulator; it's not exactly small, or cheap, for what it is, but there's little else out there. Sure, there are counters, but they'll read frequency and not RPM.

 

[mksj]

I did get a response on the Tachulator after about a week, but did not provide and details:

It could be used with a magnetic pickup. A slight modification might be required if the board is to supply the pickup as there is a 270 ohm resistor in series with the internal 5V supply for the IR LED. If that's shorted out it can supply 20mA or so with no problem 0/5V input is fine, but it should swing close to 0V. An NPN type should work.

Of course the pickup has to respond to whatever maximum frequency you require- the RPM divided by 60 multiplied by the number of pulses per rotation in Hz.

The unit can accommodate up to 16 pulses per revolution division.

We only have one size of PCB at present.

I inquired as to the location of the 270 ohm resistor but did not get a response, they are all surface mount. Most of the magnetic hall sensors are rated for 6-36VDC, but I have used these with 5VDC w/o problems. Another small (8mm body) NPN NO hall sensor model is the PR08-2DN. The other issue as you mentioned is the size of the Tachulator board/enclosure which takes up a lot more area. See attached dimensions.

 

[kb58]

Without a schematic, it's a guess how the thing is wired. I measured about a 3VDC float on the input, so figured that the optical sensor is probably a transistor (just like the Hall sensor is), and that the existing pull-up resistor should work okay. The Hall sensor output (black wire) was wired straight to the input ("White" terminal), and it worked without messing with any resistors.