Required:- A more sensitive Hall Effect sensor than A3144

Hi Guys

Can anyone recommend a Hall effect transistor (non-latching preferably) that is more sensitive to magnet strength than an A3144? I'm using it to switch functions on an Arduino based project and because of other factors, the operating magnet has to be no thicker than 0.5mm. The existing range of neodymium magnets I have of that size operate the switch with a gap of up to 3mm, but I have to operate it at 4-5mm distance through 3mm thick resin. With the magnet up tight with the resin surface, operation of the switch is patchy and not reliable. Obviously the choices I have are to increase the transistor sensitivity or increase the magnet strength. The magnet strength option has proved to be a bust as I can't source anything more powerful than I have without increasing the magnet thickness. The second option is why I'm asking - is there a hall effect transistor that will operate at less than 35 to 70 gauss (the A3144 parameters).

Thanks

What configuration do you have the magnet and hall sensor in? Are they oriented the best way? Can the magnet be longer or wider (since it cannot be thicker)?

Hi MarkT

The sensor is located with its sensor face orientated parallel with the magnet pole face. The magnet in use is currently 10mm diameter by 0.5mm thick. I could get another that is 15mm diameter, but the worry then becomes the fragile nature of a magnet that thin (0,5mm).

I can get the gap I need if I double up the magnets, pole to pole which is the same as using a magnet 1mm thick, but placing them side by side to increase the surface area has no effect on the range.

Basically the magnet has to be thin enough to locate within a business card without adding to its bulk, and the sensor is glued to the inner surface of a 3mm thick resin 'wall'. Unfortunately the resin thickness is a fixed parameter and can't be drilled, filed or have its thickness reduced.

Is it possible to put something ferromagnetic (i.e. steel or ferrite or ideally soft iron) behind the sensor to concentrate the flux from the magnet? Possibly even a small steel washer or two?

http://www.allegromicro.com/en/Design-Center/Technical-Documents/Hall-Effect-Sensor-IC-Publications/Hall-Effect-IC-Application-Guide.aspx#Q44

http://www.allegromicro.com/en/Design-Center/Technical-Documents/Hall-Effect-Sensor-IC-Publications/Hall-Effect-IC-Application-Guide.aspx

Or a second (similar) magnet spaced behind the sensor to bias the flux at the sensor towards the operating point?

There are a few a bit more sensitive here:- http://allegromicro.com/fr-FR/Products/Magnetic-Digital-Position-Sensor-ICs/Hall-Effect-Unipolar-Switches.aspx

@ cld_1, thanks for the great suggestions.

I tried both, and found that using a magnet either cancelled the effect completely or made no measurable difference (depending on which pole was facing the sensor). Adding a piece of steel behind the sensor did make a difference, albeit only increasing the operating distance by 1mm, but that is an increase so I may experiment with various sizes of steel (whatever washers/nuts etc I can find in my workshop!!) as I've got a bag full of A3144s.

@Grumpy_Mike - bingo!

Aha, the A1120 there seems to be exactly what I'm looking for and they're available from RS at a decent price. I'll order some and try them out.

Just noticed that you both picked up on Allegromicro - itmust be my age but can't help thinking square steering wheels. ( To anyone younger than me (most of you I expect) and those outside the UK, the Allegro was a British Leyland car of the 70's that deservedly received a great deal of scorn, derision and dislike! It also had a weird shaped steering wheel due to the bad seat position!)

Thanks to both you guys and please accept some Karma!

Ooops, just a heads up for people wanting to buy A1120s in the UK, RS are out of stock until an estimated June this year (2014), but Farnells do have them at 88p plus VAT.

it must be my age but can't help thinking square steering wheels.

Well I remember the car but cars were never my thing so I don't know about the steering wheel.

( To anyone younger than me (most of you I expect)

There are a lot of us wrinkles on this forum. :)

STDummy: Adding a piece of steel behind the sensor did make a difference, albeit only increasing the operating distance by 1mm, but that is an increase so I may experiment with various sizes of steel (whatever washers/nuts etc I can find in my workshop!!) as I've got a bag full of A3144s.

A washer without a hole in it -- if that isn't a contradiction in terms -- might be better if you can think of something!

I'm slightly older than you (and living in Cheltenham...).

and living in Cheltenham

My son lives there, his partner works in "The Ale House".

Hi, have you tried placing a magnet BESIDE the hall effect device and a steel strip in the card, the strip then completes a magnetic circuit of sort. The hall effect device will always be in some field, but it should change significantly with the steel strip connecting them.

(Hey only an idea, first one of the day, at 7:45am in the morning, 25DegC outside ,looking get to 34DegC today. Today work day, inside aircon, yes yes yes..)

Tom...... 8)

Hall slices only respond to the component of the magnetic field normal to the slice, so getting the orientation right is important. It's handy sometimes (3 analog hall sensors mutually perpendicular can measure the B-field vector directly).

STDummy: Just noticed that you both picked up on Allegromicro - itmust be my age but can't help thinking square steering wheels. ( To anyone younger than me (most of you I expect) and those outside the UK, the Allegro was a British Leyland car of the 70's that deservedly received a great deal of scorn, derision and dislike! It also had a weird shaped steering wheel due to the bad seat position!)

Known as the Austin Agro to this Londoner.

STDummy: I ... found that using a magnet either cancelled the effect completely or made no measurable difference (depending on which pole was facing the sensor) ...

The idea with using a magnet behind the sensor is to increase the field at the sensor by about a third or half: enough to ensure the sensor switches when required without switching it on unintentionally. As the front magnet is 3mm plus in front of the sensor the second magnet would probably need to be placed about 4 to 6mm behind it -- any closer would affect the operation of the switch.

cld_1:

STDummy: I ... found that using a magnet either cancelled the effect completely or made no measurable difference (depending on which pole was facing the sensor) ...

The idea with using a magnet behind the sensor is to increase the field at the sensor by about a third or half: enough to ensure the sensor switches when required without switching it on unintentionally. As the front magnet is 3mm plus in front of the sensor the second magnet would probably need to be placed about 4 to 6mm behind it -- any closer would affect the operation of the switch.

How about winding a small coil and place it behind your Hall device. You've got +ve and Gnd going to the device so powering the coil shouldn't be a problem. You will have to experiment with the number of turns and the placement of the coil. If the coil gives too much magnetic flux, put a resistor in series with it.

hai ** the a3503 or a1302 - both linear the a3144 - has a scmitt trigger as output - high or low - nowt between

Sensitivity said to be 2.5 mV / gauss - midpoint of 2.5 volt at zero gauss thus detects N OR S

bye for now - D

Buy a HMC5883L