interferencenfc
I want to add NFC tag to hearing aid device for a child to monitor on smartphone (or smatwatch – either carried by children) if the tag is in the range – if not it means that child lost it and I can remember GPS position to know where to find it.
Should I expect NFC to interfere with the device?
RFID is a form of NFC (see ISO 14443 and 15693). They are on different frequency bands, so you you should be fine.
This idea might be patented, so it might not be suitable for a commercial project, but you can actually measure the position and orientation of one electronic device relative to another, with reasonable accuracy, using magnetic fields. This is how Polhemus and Ascension trackers work. They are used in VR motion tracking, and in surgery for tracking the position of surgical instruments during operations.
The basic concept is to have one set of coils transmitting, and another receiving. The transmitter coils emit audio frequency alternating magnetic fields, and the receiver coils then measure the amplitude of the fields in the three receiver coils.
There is some code available online for doing these calculations. You might also take a look at the guy's project page: Open source electromagnetic trackers using OpenIGTLink.
This may not be quite the system you're looking for, as it's fairly complex, and is giving you much more info than you wanted. However, a simpler algorithm could be used which just gave you distance.
A company called Sixense make a gaming controller with a 6DOF sensor in it. I don't know how easy it would be to integrate this technology into your project though.
Now that I know what your application is, I have been thinking of a very similar application. My suggestion would be this:
Use the three orthogonal coils approach. Both the mother and child have a set of coils. The child would be the transmitter. Every few seconds, the child module would transmit an acoustic frequency magnetic field on each coil in turn. The mother module would measure the amplitude of the voltage induced in its coils. If the amplitude was too low, or if no signal was heard for more than a few seconds, then the alarm sounds.
Best Answer
The range of NFC is a couple of centimetres at best. This does not give enough range to be able to check from a wristwatch if the hearing aid is still present in an ear.
Since NFC works by the active party introducing a strong electromagnetic field to communicate over, it also typically requires a relatively large inductor/antenna, making it unpractical for hearing aids or even small wristwatches. A smaller inductor will work in some cases, but will require more power from the active device. It is quite likely that the strong electric currents would interfere with the hearing aids electronics, coupling into the signal from the microphone.
I would look into Bluetooth Smart, aka Bluetooth low energy for your use case. You could use a low TX power on the hearing aid side to reduce power consumption and many smart watches have support for Bluetooth Smart.
Combining audio and digital signals, esp RF signals always requires careful design to avoid parasitic coupling. The smaller the device the more difficult the task. Start by designing solid power plains and keeping the analog audio signals far away from noisy digital- and RF-signals.