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Hi All,
 I've done some searching around and can find specifications. but, which way should a Hall effect sensor be orientated for best efficiency?
There is a 'wide' side' and a 'narrow' side, (truncated triangle, forget the correct term  smiley-roll-sweat)
 I'm assuming the 'wide' side is 'closest' to the sensor (having seen some diagrams of construction)
Is this correct or doesn't it matter? (or, just use a stronger/larger magnet for switching?)

Thanks
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Do you have a datasheet - they show the positioning of the sensor within
the package.
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Sorry, no data sheet I'll try looking up the part number now you've mentioned it (I have a x10 jewelers loupe for reading tiny printing  smiley-grin)
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I could only find E44 printed on sensor, but, found a pretty good article from Honeywell
http://sensing.honeywell.com/index.php?ci_id=47847

Looks like the 'small' face is the sensor face
It's the first time I've seen set-up examples with magnet in drawings
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If it is a three pin trapezoidal profile sensor, then 'most likely' the small face is the side you want to sense with.  If it is a flat package, three or four pin, then the sensing face will usually have circle moulded on it.  You could always hook it up to a test circuit and test with a magnet and voltmeter, But, how are you going to know how to hook it up without a datasheet?
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Keep in mind that hall effect sensors come in many flavors and which you are working with must be known to figure out how to use it. Some kinds I have seen.

Digital output, non-latching, magnetic polarity unsensitive
Digital output latching based on magnetic polarity and value above a given switching point
Analog output voltage proportional to Gauss strength of a certain range/sensitivity.

Many of the digital ones require you supply a pull-up resistor to work (open collector output)

So without a true part number/datasheet you can still get tripped up trying to use it properly.

Lefty

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I've hooked it up and it is 'normally ON' (getting 4.85v out)
When I put a magnet on it output drops to zero.
I was using the 'wide' side of trapezoid (thanks for reminding me  smiley-wink )
I'll try putting magnet on the 'narrow' side and see if I can get it to swich with a gap instead of touching
At present I'm using a 3/16"x 1/16" thick neodymium magnet but really think I need a larger size to give 0.040" airgap instead of contact
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I've hooked it up and it is 'normally ON' (getting 4.85v out)
When I put a magnet on it output drops to zero.
I was using the 'wide' side of trapezoid (thanks for reminding me  smiley-wink )
I'll try putting magnet on the 'narrow' side and see if I can get it to swich with a gap instead of touching
At present I'm using a 3/16"x 1/16" thick neodymium magnet but really think I need a larger size to give 0.040" airgap instead of contact

Try the magnet both ways round. Also, some sensors need to see one pole of the magnet to turn on, and the other pole to turn off.
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I believe it's non-latching as it doesn't need magnet reversed to turn back on (is that a correct assumption for latching and non-latching?)
It will work with magnet 'upside down'
 I used a compass to find out which is North pole of magnet, I had been using South pole
Sensor works with either but does seem more effective on North
I have an freind/aquaintance who is an electrical engineer (one of his several degrees  smiley-roll-sweat)
Saw him this evening and asked if it actually matters if you switch Hall effect sensor with North or South pole of magnet.
Liked the question but didn't have an answer (not sure if it was because of spending all afternoon at a brewery?  smiley-grin )
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Some digital hall sensors need alternating poles, some are Unipolar (sensitive to one particular pole), and a few are ominpolar (equally sensitive to either pole). A unipolar device may sense the "incorrect" pole if it is presented somewhat off centre.
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Hall effect sensors detect whether a magnet is near. Useful for non-contact/waterproof type switches, position sensors, rotary/shaft encoders. I tried dozens of different hall effect sensors to see which one would work best for the SpokePOV kit (to determine the wheel location) and this one came out on top!
Runs at 3.5V up to 24V. To use connect power to pin 1 (all the way to the left), ground to pin 2 (middle) and then a 10K pull up resistor from pin 3 to power. Then listen on pin 3, when the south pole of a magnet is near the front of the sensor, pin 3 will go down to 0V. Otherwise it will stay at whatever the pullup resistor is connected to. Nothing occurs if a magnet's north pole is nearby (unipolar).
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