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

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?


TIA

"thomas" <thomas@yahoo.com> wrote in message
news:26f28$4774190a$d8447b12$25112@FUSE.NET...
> 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.
> Is there a type that can detect the coming and going of just a S field
> (the field will never be totally absent)?
>
> 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?
>
>
> TIA

Why worry about a ferrous target in the damp - it doesn't worry many car &
motorcycle makers, you could also apply protective lacquer. In any case
ferrous alloys used for magnetic applications tend to form a thin film of
rust and then stop corroding unless fully exposed to weather, note also that
hall sensors are favoured for their immunity to environments that would foul
up opto sensors.

thomas 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.
> Is there a type that can detect the coming and going of just a S field (the
> field will never be totally absent)?
>
> 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?
>
>
> TIA
>
>
I think the gear tooth sensor will do just fine.
You can also use any small tranformer,from which you can
remove the iron on one side of the coil.
Unless of course your speed can go to zero, in which case
you need the hall type tooth sensor.
You could also try a reed relay, they have a long lifetime
as long as they only have to switch a small current.

On Dec 27, 1:26=A0pm, "thomas" <tho...@yahoo.com> wrote:
> I'd like to use a Hall Effect sensor to detect 6 magnets on the outside of=
a
> 2.5" rotating cylinder. =A0It rotates at 20 RPM resulting in 120 pulses pe=
r
> minute (I'll calculate speed from this). =A0This will be a damp environmen=
t 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. =A0I have NOT found small inexpen=
sive
> ring magnets that are magnetized diametrically (N and S are on the same
> face).
>
> There are many many types of Hall Effect sensors. =A0Most 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 ... n27705.pdf

> Is there a type that can detect the coming and going of just a S field (th=
e
> 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. =A0Can I ju=
st
> 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.

On Thu, 27 Dec 2007 16:26:46 -0500, "thomas" <thomas@yahoo.com> 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.
>Is there a type that can detect the coming and going of just a S field (the
>field will never be totally absent)?
>
>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?
>

As others have mentioned, a gear tooth sensor should be just fine.
The standard "reluctance" sensor is just a magnet plus a coil of
wire around a ferrous core. The coil senses changes in the
magnetic field caused by the gear teeth concentrating the flux
momentarily as they pass by.

But if you really want to mount rotating magnets, then all you
need for a sensor is a coil of wire around a core. The whole
thing can be potted in plastic if you are worried about moisture.

If you have a junkyard nearby, you might cannibalize an old
automotive distributor from the days before electronic timing.
(But which still used electronic ignition.)

The GM design used an 8-tooth "gear" that rotated inside a
ring with 8 teeth pointing inward so that they came very close to
the "gear" teeth. That gave much higher output than the
conventional single geartooth sensor (used by Ford and others).

Best regards,


Bob Masta

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