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'Interfacing Futaba RC receiver to PIC'
1998\09\07@153403 by Nicholas Irias

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I have an RC vehicle project with an onboard PIC that needs to read several
servo signals that come from a Futaba receiver.  With my old (~1984)
receiver, the signal wire was able to drive a NEC optocoupler, with a 220
ohm resistor in series.  This allowed me to interface the radio to the
computer without having any direct electrical connections.

But the new Futaba radio that I bought this year has a "weak" signal wire.
If connected directly to a PIC input pin, I can read it with no problem, but
it is incapable of driviing an optocoupler under any circumstances.  It
behaves as though there was a 1M resister wired in series inside the
receiver.

My question is: what is the best way to interface the radio to the PIC?  The
only two choices I have come up with so far are:
1) use the radio's power to amplify the receiver signal, and then feed that
into the optocoupler, or
2) directly connect the receiver signal to the PIC and assume that the
prospects for interference between the systems is minimal.  I have never
observed any interference in bench testing, so I'm not certain its a big
concern.  I just notice that people like Vantec always advertise that their
systems are opto-isolated.

thanks for any suggestions

Nicholas

1998\09\07@162016 by David VanHorn

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>But the new Futaba radio that I bought this year has a "weak" signal wire.
>If connected directly to a PIC input pin, I can read it with no problem,
but
>it is incapable of driviing an optocoupler under any circumstances.  It
>behaves as though there was a 1M resister wired in series inside the
>receiver.


That's a wimpy output for sure.. I think you're stuck with adding an octal
(for eight channels) buffer chip. Something from the 4000 series CMOS would
work best, since they can run from 3V to 15V.  Then you should be able to
drive the optos just fine.

1998\09\08@125443 by Andy Kunz

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>But the new Futaba radio that I bought this year has a "weak" signal wire.

On my products and those of SubTech, I use a 2n4401 with 10K pull-up and
200 ohms on the base.  The pulses are inverted, but I just handle that in
software anyway.  The alternative would be to use a pull-down setup.

Either way, a 4MHz oscillator isn't causing much of a problem.  What are
you using?

Andy


==================================================================
Andy Kunz - Statistical Research, Inc. - Westfield, New Jersey USA
==================================================================

1998\09\08@142810 by Nicholas Irias

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>>But the new Futaba radio that I bought this year has a "weak" signal wire.
>
>On my products and those of SubTech, I use a 2n4401 with 10K pull-up and
>200 ohms on the base.  The pulses are inverted, but I just handle that in
>software anyway.  The alternative would be to use a pull-down setup.

I just tried that today, feeding the servo signal into a small switching
transistor (no model, since it was in my junk box), and then feeding the
amplified signal into the optocoupler.  Seems to work just fine.  The
amplified pulses are a few milliseconds narrower than the original pulses,
shifting the neutral point, but I can fix that in software.

It didnt seem to require a pullup, but I will add a pulldown for good
measure.

>
>Either way, a 4MHz oscillator isn't causing much of a problem.  What are
>you using?

I've got a 20MHz crystal, and have not observed any interference yet, even
when the radio and the PIC share the same 5V source (a Powertrends
regulator) and the servo signal is directly tied to a PIC input pin.  But
Murphy's law dictates that any such interference won't manifest itself until
an expensive model is at risk of crashing.

1998\09\08@145855 by David VanHorn

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>>Either way, a 4MHz oscillator isn't causing much of a problem.  What are
>>you using?
>
>I've got a 20MHz crystal, and have not observed any interference yet, even
>when the radio and the PIC share the same 5V source (a Powertrends
>regulator) and the servo signal is directly tied to a PIC input pin.  But
>Murphy's law dictates that any such interference won't manifest itself
until
>an expensive model is at risk of crashing.


You shouldn't have a problem here, 20, 40, 60, 80, nicely spaced away from
the 72MHz R/C band. I'd want to scan it anyway, just to be sure. Do you have
any ham friends? Anyone with a wideband receiver should be able to scan it
for you. I use an Icom RC-8500 100kHz to 2GHz continuous coverage.  I bought
it instead of renting an expensive spectrum analyzer for a month, and it
does a much better job.

1998\09\09@192104 by paulb

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Nicholas Irias wrote:

> I just tried that today, feeding the servo signal into a small
> switching transistor (no model, since it was in my junk box), and then
> feeding the amplified signal into the optocoupler.  Seems to work just
> fine.  The amplified pulses are a few milliseconds narrower than the
> original pulses, shifting the neutral point, but I can fix that in
> software.

 "Cheapskate" is right!  For not many cents, you could use an HCMOS
gate (74HC14) whcih should introduce very little distortion and have
another five gates in reserve.

> It didnt seem to require a pullup, but I will add a pulldown for good
> measure.

 Unnecessary if you do the above, and the HC14 should drive an
optocoupler if you must (with a resistor).  I'm puzzled about the
enthusiasm for optocouplers here, as I have a funny suspicion you are
not going to be using a separate, isolated power supply anyway.
--
 Cheers,
       Paul B.

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