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I know that data can be sent over power lines through various different types of modulation - that is as far as my knowledge ends.

I have a small D.I.Y. project (a board game) that requires some form of wireless data transfer.

The range is insignificant, depending on implementation it will be either a maximum of 2cm or 30cm.

The problem is not only do I have to transmit data to the pieces (this would be simply achieved with some nrf24l01+ RF transceivers otherwise), but the data has to be modified depending on the pieces location on the board.

I intend to power the pieces via inductive charging loops. But due to the amount of power required in the pieces (5v, ~150mA (24 individually addressable RGB LED's running at a low intensity and an ATTiny)), there won't be much space left for NFC/RFID tags or possibly an IR transmitter.

As I only need to send a couple of bytes of data at a time, I was thinking I could either use an NFC/RFID module and somehow boost the power it induces.

Or more likely, to use the induction could that I plan to power the pieces with and send the data at the same time (and in the same direction as the power) with some form of modulation.

My knowledge of circuits and this sort of stuff is extremely lacking, but I'm willing to try things.

Are there any off the shelf chips or low component count circuits that will do this?

This caught my eye, but I don't have any idea whether it would work once the now modulated 5v DC signal had been converted to AC by the induction coils and then rectified back to DC at the receiving end. Also, I'm not sure whether the receiving circuitry would be significantly different or just reversed.

http://www.audioscientific.com/45_Circuit_makes_simple_FSK_modulator.jpg

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  • \$\begingroup\$ it seems to me that youur chosen microcontroller is capable of what you ask given the right firmware, PSK,OOK, or FSK of the power signal should work \$\endgroup\$ – Jasen Jan 5 '16 at 23:21
  • \$\begingroup\$ I think that, right there, shows how little I know about all this. With that in mind then, does the conversion from DC to AC and back affect the modulation? Assuming I'm just using 3 pins, e.g. PWR, GND & DAT, I presume that the DAT output of the sending chip would be a constant modulated +v matching PWR, and would be connected to the transmit coil. On the receiving side, add the coil is also powering the device I presume then that once rectified back to DC I would connect the +v to both PWR and DAT? \$\endgroup\$ – Escribblings Jan 5 '16 at 23:31
  • \$\begingroup\$ I would run the power at 125KHZ and connect one end of the coil to a microcontroller pin the (possilby through a resistor) then I would be able to observe the phase and duration of each peak. maybe run the microcontroller at 10Mhz and use the input capture feature of one of the timers to detect the modulation. \$\endgroup\$ – Jasen Jan 5 '16 at 23:48
  • \$\begingroup\$ is this a gome board for Reversi? having the pieces change colour automatically woule be really neat! \$\endgroup\$ – Jasen Jan 5 '16 at 23:49
  • \$\begingroup\$ So your saying that the DC-AC-DC conversion won't affect the modulation? Does it matter what sort of rectifier is used? And no, it's not for Reversi/Othello - but there's no reason it couldn't be. \$\endgroup\$ – Escribblings Jan 6 '16 at 0:04
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If your data rate is low, a very simple possible solution is send the data serially by modulating the power transmitter off and on. You will need to have a capacitor on the receiver that is large enough to hold up the operation of the receiver for the duration of the longest "off" bit, but you get to pick how long this is and so you can make it relatively short (on the order of milliseconds would work if your carrier frequency is 125KHz). You also get to pick the spacing between the bits to ensure that the duty cycle is high enough to power the receiver continuously even when sending lots of off bits.

The data/power receiver circuit could be as simple as....

schematic

simulate this circuit – Schematic created using CircuitLab

There are lots of strategies to detct the incoming bits on the MCU, averaging lots of samples over time spans on the order of the size of the 0 bits (which should be very long compared to the timespans of the carrier frequency).

Notes:

  1. For a robust design, you will probably want some kind of voltage regulation just after the rectifier, or you can just be very careful about the coil alignment and geometry so that you can physically never have an over voltage.

  2. If your power requirements are large you can use full wave rectification to double output power at the cost of 2 more diodes.

  3. For simpler code, you might want to implement a serial protocol with 2-bit symbols so a single off "blink" might mean a 1 bit and an off "blink" followed by another a specified time later would be a 0 bit. This could make decoding the stream as simple as "wait for a blink, then delay 1 time period, then look at the data level to get the received bit".

  4. Depending on the frequencies, coil sizes, geometry, and transmitter design, you might want to add a capacitor in parallel with the receiver loop to bring it into resonance for better efficiency.

  5. If you are bold, you might want to consider capacitive power transfer! It can be simpler and cheaper depending on your design (especially good if you have pieces that are tall relative to the size of the base, and potentially less alignment sensitive than coils so the pieces do not need to be so carefully aligned).

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  • \$\begingroup\$ Thank you. I will look at this in more detail once I've woken up, especially point #5. Could you expand on point #3 a little? I'm not sure I fully understand. \$\endgroup\$ – Escribblings Jan 6 '16 at 6:18
  • \$\begingroup\$ I am just suggesting that it might be easier to use a low duty cycle, self clocking way of sending the bits rather than something like normal serial since serial typically has relatively equal amounts of on and off time, and also requires longer term accurate timing. With off-off/off-on type of signaling, the only timing you need to get right is the single period between the first off and the (potential) second, and you start over again on each bit. That is just one way, there are lots of simple ways to do self clocking and low duty cycle bit streams. \$\endgroup\$ – bigjosh Jan 6 '16 at 14:41
  • \$\begingroup\$ Thank you. I will look in to "self clocking". Also I have found calculators to work out the inductance of resonant coils, but am struggling to A) use that to calculate a voltage/current output and B) Work out the inductance of the capacitative method you mentioned. You also mentioned height regarding the latter - how does height factor in? \$\endgroup\$ – Escribblings Jan 6 '16 at 16:52
  • \$\begingroup\$ Height is helpful in a capacitive system because the farther away from the bottom you get, the weaker the electric field from the power source (assuming power is coming from the bottom). Some info on the Murata site here. \$\endgroup\$ – bigjosh Jan 7 '16 at 3:57

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