Transferring data over induced coupling

[ardhuru]

I built a propeller clock (one of those POV things) a few years back, and am always looking for ways to improve the design.

My version uses a brushless fan motor, with a hand wound pair of coils to form an air-core transformer to transfer power to the rotating clock PCB.

The 'primary' is driven by a 555 oscillating at about 32 Khz; this I found to be somewhere near the optimal value. The pic on the PCB does everything, including driving the display, reading the RTC etc.

Now, I'd like to move the RTC to the base unit, to make the rotating PCB significantly lighter (the RTC chip, battery, holder). So, I need a way to transfer the time information to the rotating PCB; one way of course, would be to use IR.

But, I was wondering if I could inject this information into the transformer coil itself? Suppose I use another pic in the base to read the RTC, and then output the time serially, how could I incorporate that in the 32 KHz signal required by the transformer primary? Would it make sense to interrupt the modulation from the 555 to inject the time signal every few 100s of ms? I suspect the power transfer can tolerate this. My worry is, how well would the raw serial output travel over the induced coupling? And also, how easy would it be to filter out that signal from the rotating 'secondary'?

Yet another idea would be to get rid of the 555, and use the base pic itself to generate the 32 Khz, and periodically stop that to give out the time serially.

Do hope this doesnt sound too confusing.

Any suggestions, folks?

Regards,

Anand

[mackrackit]

I have not tried sending data over a coupling like that, but are you sure this will help?
You will still have hardware on the arm for the receiving PIC and you probably need/want that for a counter balance.

The easy way to cut down on the weight would be surface mounts.

But the induction idea is intriguing. Of the top I wonder if the same or similar protocol used for IR would work...

[Bruce]

You definitely can communicate with coils. Look at the data sheet for the MCP2030. You
can use the MCP2030 with any PIC type.

Or use the PIC16F639 with the built-in MCP2030. I used the 16F639 to build the ID badge
in the attached photo. The board on the right is the initiator, which communicates with the
ID badge up to about 6 foot away with 125kHz pulsed communications.

Look for AN1024 PKE System Design Using the PIC16F639 for schematics.

I built this for a customer, so I can't share all the code, but I can give you enough to get
started if you need it.

Here's some more good info: http://ww1.microchip.com/downloads/e...Doc/51637a.pdf

[ardhuru]

You will still have hardware on the arm for the receiving PIC and you probably need/want that for a counter balance.

The easy way to cut down on the weight would be surface mounts.

But the induction idea is intriguing. Of the top I wonder if the same or similar protocol used for IR would work...

Thanks for the insight.

Getting rid of the CR2032+its holder itself would reduce the weight significantly. Of course, looking at the RTC consumption I could use a much smaller battery with solder tabs. I agree, I would need to redesign the PCB, but a slimmer, longer board with the pic right at the very opposite end should be enough of a balance, I think.

SMDs? Sure, but I just havent mustered the guts to make the transition yet

And of course, IR would be the easiest way to go, but there would be issues with it interfering with the index pulses. Also, if I dont use IR, I could also get rid of the 3 pin module I am currently using. Plus, I find the concept of being able to achieve this thru' the existing hardware (nothing extra specifically for that) too tempting!

Regards,

Anand

[mackrackit]

SMDs are not that bad. One or two now and then a loupe and a "pointy" iron will do. More? A toaster oven works fine.

But I am going to look at Bruce's idea. Six feet! WOW! That would be great for many things.

[ardhuru]

Bruce, thanks for the pointer; it made for some very interesting reading.

Like Dave says SIX FEET? Wow!

My requirement is to transmit just a few mm, so I'm hoping I could get away without using any extra hardware at all. The question I'm wondering about, considering the *very* modest range requirement, would I need to modulate the Debugout signal, or just feed it to the MOSFET driving the coil every few 100s of ms (the rest of the time the coil wouldkeep getting its 32 KHz for power transfer).

BTW, I had no idea of the existence of the 16F639. Sure opens up loads of possibilities!

Regards,

Anand

[Bruce]

It took a bit of experimentation to get the 6+' distance. It's all about placement of the
small RFID LF antennas on the receiving unit. Once you get that dialed-in you get a
whopping range. The first pictures were the initial prototypes. These had around 3+'
range.

This picture is the final version that works at 6+' range. If you compare placement of the
small RFID antennas on this one to the one above you'll know how/where to put them for
excellent range. You can walk up to the initiator with the ID badge in your back pocket, and
it still gets solid comms.

The final ID board is about 1/3 the size of a credit card, and modified to fit into a keychain
type plastic enclosure.

SMD caps & resistors are all 0402 size. Boy were those fun to hand-solder on the last
prototype. Phew...

would I need to modulate the Debugout signal

I used a TC4422A high-speed MOSFET driver to drive the large LF antenna, and PWM to send
data like this;

ID_Init:   
    T1CON.0 = 0  ' Shut-down Timer1 during wake-up routine
    ' Wakeup sequence. 4mS ON, 500uS gap, 2mS ON, 2mS OFF. Then
    ' follow up with header & data.
    CCP1CON = 12   ' PWM ON
    PAUSEUS AGC_STABLE
    CCP1CON = 0    ' PWM OFF
    PAUSEUS GAP
    CCP1CON = 12
    PAUSEUS 2210   ' 2mS ON
    CCP1CON = 0
    PAUSEUS 2210   ' 2mS OFF
    ' Wakeup sequence complete
    
    CCP1CON = 12   ' Send header after initial wakeup
    PAUSE 1        ' 1mS high on receiver LFDATA pin
    CCP1CON = 0
    PAUSEUS 500    ' header complete
    
    ' Transponder should now be waiting for data payload
    FOR LOOPS = 0 TO 7 ' loop for all 8-bits
    CCP1CON = 12
    IF DAT.0[LOOPS] THEN
       PAUSEUS 500
    ELSE
       PAUSEUS 250
    ENDIF
    CCP1CON = 0
    PAUSEUS 500    ' this pause allows 500uS for ID badge to respond
    NEXT LOOPS     ' after challenge code is sent.
    CCP1CON = 0    ' turn off PWM
    ' Data payload complete
    PAUSE 5        ' Pause a bit so interrupt flag-bit can be set when ID badge responds
    TMR1L = 0
    TMR1H = 0      ' Clear Timer1 count before re-enabling
    T1CON.0 = 1    ' Re-enable Timer1 on exit
    RETURN
    
    ' Notes: After wakeup the receivers 16F639 AFE inactivity
    ' timer will expire after 16mS. Any gap in transmision that
    ' exceeds 16mS will require another wakeup sequence before data
    ' can be sent.

The receiving end just uses PULSIN to grab the inbound data.