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PICList Thread
'PIC and RF link (FRID?)'
1999\04\22@104113 by Paul A. Brown

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I have an application where I would like to use an 8 pin PIC and some type
of RF data link.  Here are the requirements:

1.  The device will be disposable.
2.  Very low power consumption.  The PIC will spend nearly all of its time
sleeping.  It will only have one set of batteries in it's lifetime.  The
power consumption will determine the usuable lifetime.  I would like a
lifetime of 1-2 years.  Passive RF components would be a definite plus.
3.  Durable packaging.  The environment is dirty, corrosive, and wet.
Impact from falling (10' or so) is likely as is the possibility of being
run over by equipment.  I plan to encase the entire device (including
batteries) in epoxy inside a durable plastic housing.  This will force the
device to be disposable.
4.  Low cost in low quantities.  The quantity is about 50 initially and 25
per year to replace those that have died, have been lost, or have been
broken.  If this works for this customer, there is an additional market,
but it is still very small.
5.  RF type of data link.  The dirty environment and low power consumption
requirements will prevent an IR or other optical data link.  Electrical
contact for data download is not feasible.  Electrical interference is
unlikely.
6.  Data transfer rate.  The actual data rate can be as low as 10 bytes per
second.
7.  Data quantity.  There are two possibilities here.  I would prefer to be
able to transmit 256 to 512 bytes of data.  If such a device is not
available, I could get by with as little as 10 bytes.  Although the system
would not be quite as effective.
8.  I would like a range of two to three feet; a longer range would not
hurt.

At first glance, I thought that I might be able to use a passive RFID tag
as the data link (for the 10 byte version).  However, I could not find any
that allowed the PIC to change the data that the tag would be uploading.
Does anyone have any ideas?  I am not hung up on RF.  I am open to other
concepts so long as they are low cost, durable, dirt resistant, and low
power.  Even something that turns on once every few minutes to transmit
data would be acceptable if it did not use a great deal of power.

Paul Brown

1999\04\22@111224 by Sean Breheny

face picon face
Hi,

At your very low data rate and very short required range, I think that
simply pulsing a PIC pin at RF frequencies might work. This would have
the advantage of a VERY low parts count. However, it would have the
disadvantage of requiring you to run your PIC at frequencies which are
much higher than usual for such a low-power circuit.

Another alternative would be to make a single transistor oscillator
with sorta fast startup time, whose power was suppplied by a PIC pin. You
could then use On-Off Keying modulation. A circuit like this can be made
with VERY few components as well (IIRC, just an FET, two resistors, a
crystal, and two capacitors). I have a circuit which is somewhat similar
to this (although it was designed for FSK modulation and could be greatly
simplified). You can find it at

http://www.people.cornell.edu/pages/shb7/schemx.jpg

I also think that it may not be so desireable to make the data rate as
low as 10 bits/sec. This is because your transmitting power consumption
will be much higher than your required average power consumption so you
have to spend almost all your time sleeping with the transmitter off. I
have had good luck with simple transmitters and receivers sending data
between PICs reliably at 150 bps,and that number was arbitrary, I'm sure
I could have acheived higher.

It WOULD be good,however,to send the same data multiple times in the one
burst so that it would almost be guaranteed that one of the repetitions
would get through error-free.

Unfortunately, I am not all that familiar with RFID technology,so I can't
help you there. I think that in many cases, RFID tags are OTP,so that may
be your problem in trying to find run-time resettable ones.

Good luck,

Sean




On Thu, 22 Apr 1999, Paul A. Brown wrote:

> I have an application where I would like to use an 8 pin PIC and some type
> of RF data link.  Here are the requirements:

[SNIP]

1999\04\22@122357 by Julian Fine

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-----Original Message-----
From: Paul A. Brown <spam_OUTpabrownTakeThisOuTspamELLIOTT-TURBO.COM>
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU <PICLISTspamKILLspamMITVMA.MIT.EDU>
Date: 22 April 1999 04:41
Subject: PIC and RF link (FRID?)


{Quote hidden}

Hi Paul,
We are at present testing a pic 12c509 running at 32 khz based transmitter
that wakes up and transmits data once every 10 seconds.
So far the unit has lasted 2 months on a 25ma capacity battery.
The transmission range is 30 meters but can be cut down and the data rate is
1200 bytes per second.
Depending on the battery the whole unit can be epoxied.
Total cost is about 3 dollars.

************* Julian Fine ***********
********** .....julianKILLspamspam.....fine.co.za ********
******* http://www.fine.co.za *******
** http://www.eagle-wireless.co.za **

1999\04\22@132024 by wwl

picon face
On Thu, 22 Apr 1999 11:10:21 -0400, you wrote:

{Quote hidden}

Somewhere a while ago I saw a really neat idea for an ultra-simple RF
beacon - you feed the PIC's xtal clock into a freq. multiplier (I
think they just clipped it & filtered off a harmonic), and used
sleep/wdt wakeups to generate the RF bursts, i.e. whenever the PIC ran
it transmitted, 'on' time determined by a s/w loop, 'off' time by the
sleep time.  
Alternatively, you could probably generate low-power RF bursts at
least up to 10's of MHz by filtering a harmonic of a signal genereted
off a port pin - I'm sure an RF wizard could do it with one or two
transistors at most.

The ultra-low power combined with a regular wakeup is difficult - the
WDT probably takes too much to use this method. A while ago I designed
an extremely tiny wake-up cct that drew about 100nA - seems to work
pretty well but needs >5v supply (i.e. 2 lithium cells, not one).
You could maybe use the wake-up on pin change facility with an
external RF burst or maybe a magnet applied to a reed switch.

1999\04\22@135755 by Zack Cilliers

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Hi All

Have a look at http://www.rfsolutions.co.uk/

Select rf modules from the Radio Products select box, then
have a look at AM transmitter module AM-TX1.
Easy to use and cheap.

Zack

E-Mail: EraseMEspazzmanspam_OUTspamTakeThisOuTiname.com

One regrets more the things that you
did not do than the things you did do.

1999\04\22@142252 by Rich Clemens

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> Select rf modules from the Radio Products select box, then
> have a look at AM transmitter module AM-TX1.
> Easy to use and cheap.

I have two PIC units connected with the AM modules and find it works but not
well.  I have not had time to investigate fully but I would guess the signal
is not very clean partially due to the PIC running.  I might suggest the FM
modules for better success.
--
Richard Clemens
Associate Professor
Computer Science Department
West Virginia Wesleyan College
Buckhannon, West Virginia  26201
clemensspamspam_OUTwvwc.edu
304.473.8421

1999\04\22@183345 by Lynx {Glenn Jones}

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face
I dont know if this is helpfull, but instead of RF, you might connect the
pic output to an inductor, then have the reciever be another inductor
(amplified of course) but  maybe this would take more current, or would
not have good enough range. just a thought.

------------------------------------------------------------------------------
A member of the PI-100 Club:
3.1415926535897932384626433832795028841971693993751
058209749445923078164062862089986280348253421170679

On Thu, 22 Apr 1999, Paul A. Brown wrote:

{Quote hidden}

1999\04\22@185638 by Dave VanHorn

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----- Original Message -----
From: Lynx {Glenn Jones} <@spam@jones_glKILLspamspamEFN.ORG>
To: <KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Thursday, April 22, 1999 5:33 PM
Subject: Re: PIC and RF link (FRID?)


> I dont know if this is helpfull, but instead of RF, you might connect the
> pic output to an inductor, then have the reciever be another inductor
> (amplified of course) but  maybe this would take more current, or would
> not have good enough range. just a thought.

This is more likely going to toss your PIC into cmos latchup.

This is NOT viable. What you will do is create a fair bit of EMI on every
odd harmonic, and others if the waveform isn't symmetrical. By the time you
get done filtering it so the harmonics are all -60dBc, you'll have a real
mess of componentry.

Motorola and Maxim both make nearly transcievers on a chip.

It IS possible to key an oscillator directly with a PIC output, and generate
CW, and hams have done so, working over distances of thousands of miles.
The conditions have to be right, the antennas are large, and the receivers
are expensive.

1999\04\22@190305 by paulb

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Sean Breheny wrote:

> I have a circuit which is somewhat similar to this (although it was
> designed for FSK modulation and could be greatly simplified).
> http://www.people.cornell.edu/pages/shb7/schemx.jpg

 Yes.  I can't for the life of me figure why the two SGP transistors
are needed (at 5 volts - I can see a potential problem at less)?

> I also think that it may not be so desireable to make the data rate as
> low as 10 bits/sec.  This is because your transmitting power
> consumption will be much higher than your required average power
> consumption so you have to spend almost all your time sleeping with
> the transmitter off.

 I'm with you.  The longer you take to send the data, the *more* likely
you are to encounter a noise burst.  There is a popular misconception in
(amateur) radio circles that there's something "good" or "magical" about
narrow bandwidth.

 Even in those (*very* restricted) circumstances where narrow bandwidth
is useful to limit noise, it requires a narrower filter in the receiver.
Read: "Much more expensive"!

> It WOULD be good,however,to send the same data multiple times in the
> one burst so that it would almost be guaranteed that one of the
> repetitions would get through error-free.

 Definitely.  With checksum of course!
--
 Cheers,
       Paul B.

1999\04\22@193913 by Sean Breheny

face picon face
Hi Paul,

At 09:02 AM 4/23/99 +1000, you wrote:
>  Yes.  I can't for the life of me figure why the two SGP transistors
>are needed (at 5 volts - I can see a potential problem at less)?

You must go easy here :), I designed that circuit more than two years
ago,back when I knew considerably less about electronics. At the time, I
didn't realize that I could power the circuit from a PIC pin, or bias the
diode (acting as a varactor) from a PIC pin. Of course,given the right
passive components I could have.

In any event, the circuit can be simpler yet since in this application,it
can be converted to OOK insread of FSK.

[SNIP]

>> It WOULD be good,however,to send the same data multiple times in the
>> one burst so that it would almost be guaranteed that one of the
>> repetitions would get through error-free.
>
>  Definitely.  With checksum of course!

Yes, I agree.

>--
>  Cheers,
>        Paul B.
>


Sean

|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
RemoveMEshb7TakeThisOuTspamcornell.edu ICQ #: 3329174

1999\04\23@024840 by Lynx {Glenn Jones}

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face
I didnt mean to create RF energy, which im sure will be a byproduct, but
rather a magnetic feild which then induces a current in a nearby inductor.

------------------------------------------------------------------------------
A member of the PI-100 Club:
3.1415926535897932384626433832795028841971693993751
058209749445923078164062862089986280348253421170679

On Thu, 22 Apr 1999, Dave VanHorn wrote:

> {Original Message removed}

1999\04\24@152344 by Paul A. Brown

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This sounds like an interesting possibility.  Is the circuit proprietary?
Is it something you could share?  What are you using as a receiver?  Is
this something your company will sell in this form?  How big is it?  I
would like to be able to squeeze everything except the battery into a space
1/4" X 1/2" X 1."

Thanks,

Paul

1999\04\24@153851 by Paul A. Brown

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>At your very low data rate and very short required range, I think that
>simply pulsing a PIC pin at RF frequencies might work. This would have
>the advantage of a VERY low parts count. However, it would have the
>disadvantage of requiring you to run your PIC at frequencies which are
>much higher than usual for such a low-power circuit.

I like the low parts count idea.  What would it take for an antenna?  Would
the PIC be able to drive it directly, or would it need some type of
amplifier?  Remember, long range is not necessary.  (RF is not my
specialty.)  What would be a simple receiver circuit, assuming I was using
a PIC there as well?

In the "Designing for Dollars Challenge" book by Microchip, one circuit was
given where a PIC  modulated it's own oscillator frequency.  It had a
capacitor between a PIC input pin and one side of the oscillator.  As the
input pin was switched from high (input) to low (output) impedence, the
capacitor was effectively switched in and out of the oscillator circuit.
The writer claimed that up to a 5 kHz shift on a 20 MHz clock could be
achieved.  Does this seem like a good technique?  Again, what would be
necessary to get this signal on the air (amplifier, antenna)?  What would
be a good receiver?


Thanks,

Paul

1999\04\25@190009 by Sean Breheny

face picon face
Hi Paul,

At 03:41 PM 4/24/99 -0400, you wrote:
>I like the low parts count idea.  What would it take for an antenna?  Would
>the PIC be able to drive it directly, or would it need some type of
>amplifier?  Remember, long range is not necessary.  (RF is not my
>specialty.)  What would be a simple receiver circuit, assuming I was using
>a PIC there as well?

What you use for the antenna is dictated by the size constraints you have.
The idea behind an antenna (as I understand it - I am still learning), is
to make a resonant circuit which is also comparable in size to the
wavelength of the signal being transmitted. Normally this is done with a
pair of quarter-wave long pieces of wire (a dipole antenna) or with a
single quarter-wave long piece above a bunch of radials or a piece of metal.

In your case, I think probably the best you could do would be to have a
coil of wire(between the PIC pin and ground. You would also have a
capacitor in series to prevent a large DC current). I am not sure what the
dimensions should be (and right now I don't have the time to try to work it
out,sorry :( ). Someone posted a URL a while ago which gave info on making
small antennas. It was on the site of one of the small transmitter
manufacturers.

The biggest challenge in your case is that you want to operate at
relatively low frequencies (only in the HF region, about 20MHz) so that the
PIC can pulse fast enough. However, as you go lower in frequency, it
becomes more and more difficult to make a small antenna which radiates well.

For your range needs, I think the PIC pin could drive it directly,no
amplifier.

As for the receiver, probably a super-regenerative or regenerative would
work fine. Again, I would guess that there are URL's containing info on
these,too. Unfortunately, I am not familiar enough with their design to
give you a schematic. The basic idea is to have an amplifier with feedback.
In the regen., you limit the feedback to just below the amount which would
cause oscillation. In the superregen., you allow oscillation to build
up,but you have an internal signal which "quenches" the oscillation
periodically so it doesn't have much of a chance to build up fully. Then,
when a signal is received, it throws it into oscillation quicker and you
can detect this by looking at the supply current to the circuit.

In my circuit, I used an NE602 IC to produce a direct-conversion superhet
receiver,but this was an inefficient way to do it (required a large number
of components and wasn't all that sensitive). A rough schematic of this is
also on my site,but it is NOT a correct schematic. I lost my original
schematic and so I had to make quick guesses at much of the stuff. It can
give you a rough idea of what I did,though. It is at

http://www.people.cornell.edu/pages/shb7/schemr2.jpg


{Quote hidden}

Well, it is an interesting technique, but for your application,it isn't
necessary to use FSK (frequency shift keying),I think OOK (on-off keying)
would be simpler and would work just fine for such short range communication.

>
>Thanks,
>
>Paul
>


Good luck,

Sean

|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
spamBeGoneshb7spamBeGonespamcornell.edu ICQ #: 3329174

1999\04\25@191840 by Dave VanHorn

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face
How much range are we talking here?

There's other ways, like spoiling the Q of a coil, that don't require the
processor to operate at a terahertz, and don't create harmonic radiation
from DC to daylight.

1999\04\25@192643 by Sean Breheny

face picon face
I believe he only wants to go about 3 feet or so, but on as small an amount
of power consumption as possible and a very small size.

I am sure there are better ways than what I said,and I myself would be
curious to hear about them. "Spoiling the Q of a coil" sounds especially
interesting to me,because I don't understand why that would help to you
radiate,unless you are talking about trying to get direct magnetic coupling
between two coils. I wasn't sure if that was practical at three feet away
in such a small size.


Sean


At 06:17 PM 4/25/99 -0500, you wrote:
>How much range are we talking here?
>
>There's other ways, like spoiling the Q of a coil, that don't require the
>processor to operate at a terahertz, and don't create harmonic radiation
>from DC to daylight.
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
TakeThisOuTshb7EraseMEspamspam_OUTcornell.edu ICQ #: 3329174

1999\04\25@193507 by Dave VanHorn

flavicon
face
> I am sure there are better ways than what I said,and I myself would be
> curious to hear about them. "Spoiling the Q of a coil" sounds especially
> interesting to me,because I don't understand why that would help to you
> radiate,unless you are talking about trying to get direct magnetic
coupling
> between two coils. I wasn't sure if that was practical at three feet away
> in such a small size.


Checkmate anti-theft systems work this way.
The tag is a paralell resonant circuit.
The base scans around their resonant point, and each time it scans past, the
tag (when present) causes a dip.  If a microprocessor was connected, in such
a way to either spoil the Q of the resonant tag, or to shift it's resonant
frequency, then you could send data, without sending any signal from the tag
at all. (!)

1999\04\25@194335 by Sean Breheny

face picon face
Wow! That DOES sound like an interesting idea. However, how much output
power is required by the receiver unit? I seem to remember seeing heatsinks
and large upright "antenna" coils on such units. I don't know if Paul is
willing to put up with such large receiver equipment.

Sean

At 06:32 PM 4/25/99 -0500, you wrote:
>Checkmate anti-theft systems work this way.
>The tag is a paralell resonant circuit.
>The base scans around their resonant point, and each time it scans past, the
>tag (when present) causes a dip.  If a microprocessor was connected, in such
>a way to either spoil the Q of the resonant tag, or to shift it's resonant
>frequency, then you could send data, without sending any signal from the tag
>at all. (!)
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
RemoveMEshb7spamTakeThisOuTcornell.edu ICQ #: 3329174

1999\04\25@201458 by Dave VanHorn

flavicon
face
> Wow! That DOES sound like an interesting idea. However, how much output
> power is required by the receiver unit? I seem to remember seeing
heatsinks
> and large upright "antenna" coils on such units. I don't know if Paul is
> willing to put up with such large receiver equipment.

The heatsinks may be decorative.
I'm not aware of it requiring a lot of power, what you need is a loop
antenna that will span the area that the "transmitter which does not
transmit" is in.

The checkmate system passes part 15, so it can't be radiating much.


'PIC and RF link (FRID?)'
1999\05\30@184446 by Marc
flavicon
face
> At your very low data rate and very short required range, I think that
> simply pulsing a PIC pin at RF frequencies might work. This would have
> the advantage of a VERY low parts count. However, it would have the
> disadvantage of requiring you to run your PIC at frequencies which are
> much higher than usual for such a low-power circuit.

A really crazy idea:

Clock the PIC AT your RF frequency! Make the CLOCK PCB traces LONG
and use THEM as antenna!!!  Put the PIC asleep for a 0 (18ms), or
stay running a loop to transmit a 1.

Using RC clock, the startup time is very short.

1999\05\30@222310 by Harold Hallikainen

picon face
On Sun, 30 May 1999 12:39:01 +0000 Marc <marcEraseMEspam.....AARGH.FRANKEN.DE> writes:
>> At your very low data rate and very short required range, I think
>that
>> simply pulsing a PIC pin at RF frequencies might work. This would
>have
>> the advantage of a VERY low parts count. However, it would have the
>> disadvantage of requiring you to run your PIC at frequencies which
>are
>> much higher than usual for such a low-power circuit.
>
>A really crazy idea:
>
>Clock the PIC AT your RF frequency! Make the CLOCK PCB traces LONG
>and use THEM as antenna!!!  Put the PIC asleep for a 0 (18ms), or
>stay running a loop to transmit a 1.
>
>Using RC clock, the startup time is very short.

       Interesting idea...   And what would you do for a receiver?  I also
wonder about having a loop (an inductor) and a capacitor in series
between an output pin and ground.  Changing state should result in a
damped oscillation at the resonant frequency of the LC.  The timing of
these RF pulses could be varied to send useful information.  Again, I
don't know what would be a good receiver.  Also, I seem to recall an FCC
prohibition of "damped wave" transmissions...  I'm away from my web
access right now, but I think there's such a prohibition in part 15
somewhere.

Harold



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