Hall sensor vs. reed switch

Application here is a tachometer, I'm thinking I'll have 8 magnets on the perimeter of a drum, likely spinning no faster than 120 RPM, so signal will be no greater than 16hz. Just off the top of my head, given that some reed switches I've looked have open/close times in the millisecond range, that should be fast enough, even with debouncing taken into account. But I think that with, e.g., a lifetime of 20,000,000 switch cycles, that's 347 hours of operation (if I figured correctly) -- well, probably plenty for me, but if my idea for solid rocket fuel takes off, maybe not. So, I'm thinking Hall sensor. But other than the very basics, I don't know quite how to select one.

For example: Optek OHN3120U looks fine to me, but looking at the "425G Trip, 50G Release", I assume those are gauss figures. If I use the Neodymium Diametric Disc N35 6x2.5mm down at the bottom right of that page, it seems they're plenty strong to activate the sensor. But with 8 of them around the perimeter of a 5" diameter drum, will the flux density at the midpoint between 2 of them be low enough for it to turn off? The drum will be PVC. At the other end of the magnet spectrum, cutting bars or dots from magnetic vinyl stick-ons comes to mind, but there are no gauss ratings for those.

Any advice?

If the field strength is not low enough between the magnets all you need to do is alternate the poles of the magnets (N / S / N / S....).

Note that the 3120 only responds to positive gauss (whether that's N or S I dunno). I'd suggest picking one of the devices (3x30 or 3x75) that latches between N and S instead so you're certain you can get eight evenly spaced events when you're using magnets with their poles alternating.

cutting bars or dots from magnetic vinyl stick-ons comes to mind, but there are no gauss ratings for those.

You can't detect such low fields as that from a hall effect switch. You will have no trouble at all getting the hall effect switch to turn off.

120 RPM = 2 revs per second. With only 1 magnet that gives an update frequency of twice per second. Why do you seek an update of 16 times per second, or is 120 the maximum and you want to measure very much lower. 1 magnet at 120 RPM will give 2000+ hours of reed switch life so why not go for the simple solution. If balance is a problem you can always fit a "dummy" magnet opposite the real one.

I once used a reed switch on my father's car as a tacho pick-up, operated off the ignition coil so doing up to 2000RPM and it lasted the life of the car

Thanks for the info, guys. The reason for using 8 is ... it's more than 4? :) No, seriously, I don't know how slowly I might want to run this thing. My 1st ball mill, ya know. From what I've seen on the web, running faster is not very likely, and it varies depending on the grinding media and what you're grinding. So I'm just planning for high versatility. I do like the idea of alternating poles, simply because it seems a bit more robust. If I use a fairly weak glue, shouldn't be any trouble to flip the poles on 1/2 of them, but based on what Grumpy_Mike said, I'll try running them the same way first, though I guess that with a unipolar hall switch, I could run them alternating pole, and it wouldn't matter, then I could switch to a bi-polar if needed.

@jackrae: The 2 reasons I can think of for not using a reed switch are avoiding debouncing it, and just that I'm a bit leary of mechanical actuation in this case, even though, as you mention, they can be highly reliable.

ETA: The advantage of the reed switch is I can drive to Radio Shack, or maybe even home depot and buy one. Might still try that, since I really don't have any big reason to make another electronics order right now, but I also have this irrational :~ objection to software debounce, and I don't think I have any Schmitt triggers laying about.

Ah, ball mills; so 120 RPM might well be far too high. I once made a lapidary mill, around 100mm diameter, and the rotational speed was quite low, probably about 50RPM. If you go for a larger drum then the speed gets progressively less since you want the product to roll within the drum rather than hug the wall. Out there on the internet you'll find a suitable equation for optimum speed versus drum size. An example of which is given here http://www.pauloabbe.com/productLines/millingEquipment/principlesofgrinding.html

If magnetic sensors might be a potential problem you could always go for a reflective optical system. They use a twin sensor head (transmitter + receiver) and pieces of reflective tape on the drum. You can have as many pieces as you can fit to increase resolution.

Jed,

I suspect the vinyl sheets are "magic" stickers like those pizza ads and else that are to be stuck on a fridge door. They contain periodic poles NSNSNS... so not very good for counting pulses. Just take such a sticker and cut off a strip, use the end of the strip to go around the rest of the sticker, the strip end jumps up and down to indicate periodic poles. A physics professor once showed this in a seminar I attended, pretty cool trick.

Why not read the drum speed by optical means? This is what I would do. Just put some reflective dots to the drum and count the number of reflexes.

justjed: ... But with 8 of them around the perimeter of a 5" diameter drum, will the flux density at the midpoint between 2 of them be low enough for it to turn off?

My recommendation is to use a Hall sensor that turns on when it sees one polarity of the magnetic field, and off when it sees the other. Such as the US1881. It makes for a more reliable system, and doesn't cause false tacho pulses if the wheel wobbles back and forth slightly when the vehicle is stationary. Of course, this means you need twice as many magnets (with alternating polarities) to get the same frequency. But you can mitigate this by counting edges rather than pulses, thereby doubling the frequency.

Again, thanks all for the input.

@jackrae: Nice link. Also, significantly different from what else I’ve read in re. media load, and what influences speed. Yes, I’m 99% sure I will be running more slowly. It will take some trial & error, but fortunately for me, I don’t think that beyond a certain point, particle size will be of much importance.

@liudr: It’s justjed … Baaaaaahahahahahahaah! I’ve been waiting for the chance to say that! ]:smiley: Interesting about those stickers. That + what Grumpy_Mike said seems to rule them out with certainty. (Not that I thought anything uncertain about advice from GM.)

@Udo Klein & jackrae: I hadn’t considered optical, simply because I am concerned about needing to design to eliminate stray light readings. No doubt this can be done successfully, but a magnetic system seems to me to be more tolerant of play in positioning. Well, really, I hadn’t given it much thought at all, in part because typically, in tach applications, I see hall sensors used.

@dc42: I like that part. Less expensive than others I have looked at, and it will make no difference to me if I have to do alternating poles. If I use an ISR in mode CHANGE, I won’t even care which way it flipped.

If I use an ISR in mode CHANGE, I won't even care which way it flipped.

I think you are missing something. Some hall effect sensors will give a high when they see say a north pole, they continue to give high when the field is removed and only give a low with a south pole.

justjed,

I'll remember to make a video about these magic stickers.

Grumpy_Mike:

If I use an ISR in mode CHANGE, I won't even care which way it flipped.

I think you are missing something. Some hall effect sensors will give a high when they see say a north pole, they continue to give high when the field is removed and only give a low with a south pole.

Oh, I'm certainly missing a lot of things. :~ But I think I do understand this. I did notice, when browsing the sensors, that a variety of modes is possible. If I use the US1881 -- a latch -- that dc42 suggested, and do an even number of magnets, alternating poles, then a change ISR would trip on both poles, alternating H/L. I won't care if it's going hi-low, or low-hi. If I'm off here, I do hope for enlightenment.

liudr: I'll remember to make a video about these magic stickers.

That will be interesting to see.

Now I just need to think of some stuff to buy along with hall sensors and magnets, to make me feel like shipping charges are worthwhile. :)

justjed:

liudr: I'll remember to make a video about these magic stickers.

That will be interesting to see.

Now I just need to think of some stuff to buy along with hall sensors and magnets, to make me feel like shipping charges are worthwhile. :)

I always buy too much when shipping is high. Good/evil business model, like super size soda.

liudr: I always buy too much when shipping is high. Good/evil business model, like super size soda.

I might give Tayda a try. Folks here seem to like them. I checked Terry's store, but no hall sensors. :(

justjed: typically, in tach applications, I see hall sensors used.

Well, I'm not sure where you have looked, but this is generally not the case. I see them used where magnets are employed for other reasons, but the by far the most common approach is optical. Optical is cheap, has huge bandwidth, and is easy to implement. You might want to consider it.

BillO:

justjed: typically, in tach applications, I see hall sensors used.

Well, I'm not sure where you have looked, but this is generally not the case. I see them used where magnets are employed for other reasons, but the by far the most common approach is optical. Optical is cheap, has huge bandwidth, and is easy to implement. You might want to consider it.

I think it depends on situation. For outdoors environment where infrared is plenty (from the sun) and dust and dirt are unavoidable, magnetic sensing is best fit, like for car speed. For indoor well-controlled environment lots of infrared optical sensors are used, like old-school computer mouse with a big ball inside, most rotary encoders for precision motion stages, laser light choppers etc.

BillO:

justjed: typically, in tach applications, I see hall sensors used.

Well, I'm not sure where you have looked,

Well, e.g., under the hood of my decrepit Volvo (R.I.P.).

Optical is cheap, has huge bandwidth, and is easy to implement. You might want to consider it.

A hall-effect sensor is ~$1 (cheaper in various places, but not where I can also find other stuff I want to put on the same order), and the magnets are cheap as well. I'm sure optical can work too, but magnetic will be, I think, more robust in this application.

One place where magnetic sensing is very useful is in dirty applications. If the area you're getting the reading from is at all exposed to the elements, then magnetic definitely is the way to go. This is probably the case for your application, and high RPM is definitely not a factor.

For sensing wheel rpm on my dirt bikes I use little neodymium button magnets. They produce amazingly dense fields and certainly work well with a hall effect devices, even at a fair distance. I get them at trailtech.net/magnets.html, but they are probably available on ebay for less.