# Sending data over a magnetic field

I need to find a way to trigger this Hall Effect Sensor with a non-permanent magnet. Does anyone know if an inductor's magnetic field would be strong enough? Or am I going to have to build an electromagnet? If it helps to know I am trying to send data wirelessly by turning a magnetic field on and off and tripping the sensor on the other side to pulse out RS232 data.

EDIT: Current: Needs to be no more than 30mA. Distance: A 4mm air gap. Minimum data rate: Around 256Bits/second. Size: Needs to be as small as possible.(Must fit into 5x10x5mm spot) Cost: Looking to keep it under $1.50 EDIT2: It is Unidirectional - 30mA transmit - for now I don't really care how much it costs. I just need to get it working EDIT3: Okay. I have decided to do it optically. Does anyone known where I can get an IR detector in the 1.5micron range?(Should I move this to a new question?) Actual data sheet here • BTW the range of the magnetic field only needs to be about 4mm. Also the sensor is triggered at 55Gauss. – Axis Apr 30, 2012 at 4:54 • Building an electromagnet that could trigger it would be fairly simple. Are you against this route? Apr 30, 2012 at 5:30 • No. I was just wondering if there was some kind of pre-made part I could use. – Axis Apr 30, 2012 at 5:37 • Perhaps you could ask a more general question related to your goal, too... Apr 30, 2012 at 7:27 • Why not optically? Apr 30, 2012 at 15:37 ## 2 Answers Update from OP. Current: Needs to be no more than 30mA. Distance: A 4mm air gap. Minimum data rate: Around 256Bits/second. Size: Needs to be as small as possible.(Must fit into 5x10x5mm spot) Cost: Looking to keep it under$1.50

Is that 30 mA receive or 30 mA transmit.
Unidirectional?
Cost of \$1.50 covers what? TX & RX, just one (which?),Hall cell in that price.
How many? 1 10? 100? 100,000?
MUCH more information allows us to provide a single instant answer without playing death of 1000 cuts / iterations.

The Hall cell chosen is completely unsuitable for this task.
This is because it is a sampling type which sleeps for most of the time and wakes to take a reading occasionally.
Th data sheets hows that it has a 0.1% on time and 99.9% off time.
Cycle time is 45 to 90 ms and on time is 45 to 90 uS.
So you can only signal at most at 1 bit per on time if you are careful or at about 10 bps max and probably less.

There are many Hall cells available which are not the sampling type and low enough current.

[This is Digikeys cheapest at about 58c/1.]http://www.semicon.toshiba.co.jp/docs/datasheet/en/Sensor/TCS20DPR_en_datasheet_110207.pdf)
This has 4.4 mT sensitivity worst case.
Mutiply T by 10,000 to get Gaus.
4.4 mT x 10,000 = 44 Gauss = about te same as before.

Doable at range and size specified. Implementation details depend on all answers not yet known.

More when more known ...

This question is eminently answerable but rather than giving you a single "this will work" answer, having more information will lead to a much better answer. What range do you want to work over from the face of the Hall cell to the face of the inductor?
Is there anything in the way obstructing, spinning, cutting ...?
Is it in seawater, embedded in a block of steel or a lava field, ...?
What maximum data rate do you require?

Be as specific as possible re constraints on cost, size, and anything else you can hink of. DO NOT have us say xxx meets your needs and then say "Oh, but it must be British Racing Green and work at 2000 feet underwater" or whatever :-)

Don't let the following worry you. The answer is a piece of ferrite and some wire - but this is "what lies underneath":

IF there is a need to wind a coil and activate the sensor at a distance, then it may come down to formulae like this:

Relating to an inductor like this:

Or it's big brother which has finite thickness, from here

## BUT probably not.

Adding a core increases the field by the permeability of the core - but, we'll come to that.

FWIW those formulae are about the nicest I've seen for a common problem that usually get's a horrendously complex answer. This is mainly geometry. Most analyses are for the field INSIDE the solenoid and few deal with it beyond the ends.

• @JasonS - You can 'favorite' or star the question and be notified of any changes to any of the answers. Check just below the voting arrows on the question. There is currently no way to favorite an answer. You could weigh in on this feature request if you think that's important. Apr 30, 2012 at 14:24
• The Hall sensor you link has an operating frequency of 25 Hz. I'm not sure if it is sampling at that frequency or just autozeroing itself. I think Hall might the wrong technology altogether and the OP would be better served with a pair of tuned coils. I examine such a device here: markrages.tumblr.com/post/21967136457 Apr 30, 2012 at 18:57

"Does anyone know if an inductor's magnetic field would be strong enough?"

This is can't be answered as long as

• the type of construction of the inductor is unknown
• the current going through the inductor is unknown.

Consider that an inductor probably is built in a way that its external field is as low as possible because stray filed are not wanted.
A toroidal inductor would be a particular bad choice as most of its field is kept inside.

Maybe you can reuse a solenoid of a relay.

EDIT: If you just want to have it magnetic, as you said, you can also use another coil to pick up the change of the magnetic field. Such devices are called transformers.