Re: Hall effect sensor

On Dec 27, 1:26 pm, "thomas" <tho...@xxxxxxxxx> wrote:
I'd like to use a Hall Effect sensor to detect 6 magnets on the outside of a
2.5" rotating cylinder.  It rotates at 20 RPM resulting in 120 pulses per
minute (I'll calculate speed from this).  This will be a damp environment so
a ferrous 'target' is not an option.

I have found small inexpensive ring magnets (with the S pole facing out)
that I can easily mount on the cylinder.  I have NOT found small inexpensive
ring magnets that are magnetized diametrically (N and S are on the same
face).

There are many many types of Hall Effect sensors.  Most need alternating N
and S fields: either with 2 opposite poled magnets, or with a single N/S
ring magnet.

Here's a link that helps explain the types of hall sensors:
http://www.allegromicro.com/en/Products/Design/an/an27705.pdf

Is there a type that can detect the coming and going of just a S field (the
field will never be totally absent)?

That's called a unipolar sensor; sensitive only to one magnetic
polarity, typically south.
Example: Honeywell's SR13C-A1. I used that sensor to convert a
motorcycle distributor from points to solid state. I installed
quarter inch diameter neo magnets in a fixed aluminum disc above the
base plate, one hall sensor on the base plate directly below each
magnet, and had a steel shutter disc rotating between the magnets and
the sensors, to break the magnetic field. Some ancillary circuitry to
drive the ignition coil. It worked the first time I turned over the
engine.

I found a Gear Tooth Sensor type that senses a ferrous target.  Can I just
use the S facing ring magnet instead of the ferrous target?

Well, I've never used that kind of sensor, so I can't say it would
work. But I would encourage you not to give up on the hall sensors.
I had an easy time with them. If you use a unipolar sensor and get it
installed the right distance from the magnets you shouldn't have any
problem. This is as much a mechanical job as an electronic one.
.

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