REED SWITCH & MAGNET INTERACTION

Introduction
Purpose›Explore the technology of how magnets interact with reed switches
producing a reed sensor

Objectives›››

Define key terms of magnets and reed switches
Describe the different positions of the magnet relative to the reed switch
and its effects in two dimensions and three dimensions
Describe the sensing function

Key Terms
Permanent Magnet›

Steady magnetic field›

Has a north and south pole›

Available in various sizes›

Available in different strengths›

Magnetic strength is measured in gauss or milliTesla››

Magnets come in different materials – rare earth, Alnico, andferriteRare earth are the strongest magnets

Key Terms
Reed Switch›

Two ferromagnetic leads›

Hermetically sealed in a glass cylinder›

The two reeds are separated internally forming a gap›

Sensitivity to closure is measured in milli-Tesla (mT) or ampere turns (AT)›Come in various cylindrical sizes

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When the magnet is brought in close enough to the reed switch thecontacts will close. This is called the pull-in point. (PI)
When the magnet is removed from the proximity of the reed switch thecontacts will open. This is called the drop-out point. (DO)Hysteresis is the ratio of the pull-in divided by the drop-out (PI/DO)

Reed Switch Sensors›

Reed switch sensors are comprised of a reed switch and a magnet›

Reed switches are packaged in many ways depending on the application›

The magnet is usually packaged or attached to a moving part››

The magnet is brought into the sphere of influence of the reed switch
closing the contacts and carrying out the sensing application
The key to a successful application is the proper usage of the magnet andthe reed switch as they interact in this sphere of influence

Click to Animate

TWO DIMENSIONAL MAGNETIC FIELDSPARALLEL MAGNET PLACEMENT

Magnet Parallel to the Reed Switch and Centered
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When the magnet is parallel tothe reed switch 3 lobes are
generated with magnetic
sensitivity arrays
The solid lines represent pull-in
points (PI)The dotted lines show the dropout points (DO)Reversing the polarity of themagnet rotating it 180° will haveno effect on the generated
magnetic sensitivity lobes

Magnetic Lines of Flux›››

The lines of magnetic flux are
shown leaving the north pole ofthe magnet and entering thesouth pole of the magnet.
Bringing the magnet closer tothe reed switch will create theinteraction between themagnetic flux and the magnetic
sensitivity lobes
The two fields aresuperimposed upon one
another creating the interactionthat closes the contacts andupon withdrawal opens thecontacts

Magnet Parallel to the Reed Switch and Centered
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When the magnet is parallel tothe reed switch 3 lobes arepresent showing the generated
arrays of magnetic sensitivity.
The magnet will move into thecenter lobe
The solid lines represent pull-in
points (PI)The dotted lines show thedrop-out points (DO)Once the magnet crosses theselines, the contacts will change
their state

Click to Animate

Magnet Parallel to the Reed Switch
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When the magnet is parallel the 3 lobes are present as below showing thegenerated arrays of magnetic sensitivity.
In this case the magnet is coming from the side but still offset from theaxis of the reed switch
The solid lines represent pull-in points and dotted lines the drop-out
pointsOnce the magnet crosses these lines, the contacts will change their state
Click to Animate

Magnet Parallel to the Reed Switch

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The magnet is parallel so the 3 lobes are still present
In this case the magnet is coming from the side along the axisof the reed switch
With magnet again moving left to right it will move into thefirst lobe on the left closing the contacts
When the magnet is withdrawn, moving left, the contacts will
open
Click to Animate

Magnet Parallel to the Reed Switch
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Again the magnet is parallel to the reed switch so the 3 lobes are stillpresent
In this case the magnet is again coming from the side along the axis of thereed switch
With magnet again moving left to right it will pass through to the otherend of the switch
With this approach the contacts will close and open three times
Click to Animate

TWO DIMENSIONAL MAGNETIC FIELDSPERPENDICULAR MAGNET PLACEMENT

Magnet Perpendicular to the Reed Switch›

When the magnet is turned perpendicular to the reed switch the magnetic
sensitivity lobes change dramatically.

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The lines of flux of the magnet remain the same always›

The magnetic sensitivity lobes are now only on the ends›

The sensitivity lobes are offset from the center of the axis of the reed switch›

Bringing the magnet into the upper left lobe and then withdrawing it will closeand open the contacts

Magnet Perpendicular to the Reed Switch››

When the magnet is turned perpendicular to the reed switch
four magnetic sensitivity lobes are present.
Bringing the magnet in offset to the axis of the switch andthrough to the other end of the switch the contacts will closeand open two times
Click to Animate

Magnet Perpendicular to the Reed Switch››

Four magnetic sensitivity lobes are present when the magnet
is perpendicular to the reed switch
Bringing the magnet from left to right along the axis of theswitch and through to the other end of the reed switch thecontacts will never close
Click to Animate

Magnet Perpendicular to the Reed Switch››

Again the magnet isperpendicular to thereed switch
Bringing the magnet
from top downward
into the upper left
lobe the contacts willclose and then open
when the magnet iswithdrawn upwards

Click to Animate

Magnet Perpendicular to the Reed Switch

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Magnet is perpendicular
to the reed switch
Bringing the magnet
downward into theupper left lobe of thecontacts and continue
through the lower left
lobe.
In this case, the contacts
will close and open whenthe magnet goes througheach lobe.

Click to Animate

Magnet Perpendicular to the Reed Switch

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Magnet isperpendicular to thereed switch
Bringing the magnet
downward throughthe center of the reedswitchThe contacts will
never close.

Magnet Parallel to the Reed Switch & Rotated›››When the magnet is parallel to the reed switch the 3 lobes are generated
with magnetic sensitivity arrays.
In this case the magnet is placed inside the pull-in lobe so the reed
contacts are closed.
Now we rotate the magnet 90° about its axis.

Magnet Parallel & Rotated 90°›››

The magnet is now rotated 90° making it
perpendicular to the reed switch
The magnetic sensitivity lobes now change.
The contacts will now open.

Magnet Parallel to the Reed Switch & Centered

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With the magnet parallel to the reed switch the typical 3
lobes are present
Here the magnet is positioned inside the pull-in lobe.
In this case, a magnetic shunt is positioned between themagnet and the reed switch
The contacts remain open even though the magnet is within
the pull-in lobe

Magnet Parallel to the Reed Switch & Centered

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With the magnet parallel to the reed switch the typical 3
lobes are present
Here the magnet is positioned inside the pull-in lobe.
Now the magnetic shunt is removed from its position
between the magnet and the reed switch
The reed switch contacts now close

Click to Animate

THREE DIMENSIONAL MAGNETIC FIELDS
3-D MAGNETIC FIELD ARRAYS

Three Dimensional Fields

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The magnetic flux lines emanatingfrom the magnet looks like a beerbarrel keg in three dimensions
With the magnet parallel to thereed switch, the center magnetic
lobe array looks like a donut
The two outside lobes look more
like a boxer’s punching bag – thekind hung from the ceiling.
The magnetic flux and lobe arrays
are symmetricalA View of a Three Dimensional Field Array
Click to see this video on
our YouTube Channel

Summary
Reed switch sensors are comprised of a reed switch and a magnet

The reed switch is packaged appropriately for its application
The magnet is usually attached to a moving mechanism that allows themagnet to be brought into the reed switch’s sphere of influence
The sensitivity lobe arrays are generated by the influence between themagnet’s magnetic flux in association with the reed switch
Understanding their influence with each other is key to a successful
reed sensor application

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