Inductive communication

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Hi.
I am thinking of trying to establish a bit of communication between two AVRs, using some form of inductive coupling. For starters I will be satisfied with only one-way (simplex) data.
I'd like the receiver to be as small as possible. If the receiving coil can be kept within 2-3 cm in diameter, I'll be very happy. The transmitter can be larger if needed. I only want to transfer data, not energy to power the AVR. I need 2-5 bytes data, and the inductive reach should be 1-2 cm. The data transfer speed can be very low.

I tried to Google quite a bit for inspiration on how to make the analogue frontend of the receiver, but usually end up with patent applications or other things for something irrelevant. Do you have any good pointers?
I'd like to keep it as simple as possible. Maybe even down to the pick-up coil being part of a tuned LC circuit, and then using a diode as AM demodulator. I'd like to experiment with FSK (instead of AM) if the demodulator can be made simple enough.

I am planning to use a XMega E5 in the receiver, so it may be possible to integrate some of the decoder/demodulator functionality in its custom logic module, that in turn can interface to the USART. But that can always come later. For starters I'd just like a bit of inspiration on how to make the coils and surrounding circuitry, and what range of frequencies that are suitable for this type of application. Any ideas?

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Inductive coupled communications is quite possible. It is "just" a transformer with a very low coupling coefficient.

You need to be aware that the coupling depends very highly on relative orientation of the two coils. It is highest when they are parallel and have the same center line.

In the receiving coil, the voltage is proportional to the rate of change of the field strength. In the primary, the generated field is proportional to the current. The primary current grows exponentially with time (like a capacitor charging voltage)..

I would suggest either pulse-width modulate to transmit your data or send a burst of pulses for one logic state, and no pulses for the other. A few KHz, in either case, ought to work.

At these frequencies, an op-amp can function as the receiver/detector.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Two thoughts,

Google "telephone pickup coil" (Images) for both an example coil and several schematics, (which I didn't review, but which might give you a starting point).

The telephone pickup coils are great when one needs a small coil inside a case with a cable attached. You might end up with a different coil, but getting a couple of these to start experimenting with would be reasonable.

Suction cup stick them to opposite sides of a small piece of Plexiglas for starters.

Alignment will be important.

I assume when Jim says use PWM he is referring to a "pulse" of 4 KHz sin wave, for example, and not a logical pulse.

Either would be doable. But I think detecting the sin wave would be easier than detecting the impulse from a single logic level transition at each end of a logic PWM signal.

Your original suggestion of FSK is also a good approach.

As with all wireless communications there will be noise and signal detection errors. Adding a "Start Bit or Byte" and a simple CheckSum to the string of data, and sending the data packet several times, will be helpful in reliably detecting the transmitted information.

JC

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Thank you for the suggestions. I will look into them.
As for the alignment, that is not a problem. They will always be parallel.

My idea is to build a receiver into each of the cars of a "Faller Car System":
http://www.faller.de/App/WebObjects/XSeMIPS.woa/cms/page/pid.14.17.109/lg.en/Car-System.html
As it is today, they are small cars that just run at constant speed (battery driven), and a small magnet under the car follows an iron wire hidden in the road. You can make the cars stop (rather abrupt) by triggering a reed switch in the car with an electromagnet in the road.

I want to remove the reed switch, and replace it with a coil, so I can communicate different commands to the cars. I will have some areas where I want the cars to do something, like slow down, flash its turn signal or whatever. In the areas where I want something to happen, I will put a transmitting coil under the roadbed, that continuously transmits a signal (a byte or two) that tell the cars what to do.

That is why I need it small and light at the receiver end. If possible, I can see the benefit of two-way communication for car detection and identification. But that is a task for another day.

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pace makers have inductive comm links. The pick-up loop is about 8 in. in diameter. Inside the pace maker, there is some inductor that's quite small. You can google for what they use.

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See https://www.avrfreaks.net/index.p...
It's Martin Ossmann's webinar.

Sounds good, Svuppe, have fun :)

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Thought about that when I still had my pond, home and family. Put a cable around the pond, connect the wires in series to make a coil and remotely control a toy boat.

These days, with RF modules going for less than 2 bucks, that seems the hard way.

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http://cappels.org/dproj/minmassrf/Min_Mass_Wireless_Coupler.html

He has several projects that use this technique.

Rick

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Wohoo. Thanks a lot for that link. That was exactly the kind of information I was hoping to get a pointer to. That will get me kick-started.
I haven't seen the webinar yet, but I will one of the coming days.