Re: Hardware question: Interfacing a servo PPM to a 3.3v micro...

On Mar 17, 12:08 pm, Tim Wescott <t...@xxxxxxxxxxxxxxxx> wrote:
frenchy wrote:
Hello,
I am bringing an RC servo signal into the input capture module of the
PIC24FJ64GA002. The RC receiver outputs a voltage pulse that has an
amplitude which approximately tracks the battery level of the
receiver. Therefore the servo pulse-high voltage can be up to 8volts
and can sag down to 4 volts or so. Which of these options is the best
design practice?...
1) Put a 10k in series and hook the servo pulse up directly to the
input capture pin. Any injected current will be very small (<0.5mA).
2) A resistor divider seems like a bad idea because as the battery
voltage sags, the input to the input capture will not be in a reliable
range because it will sag to.
3) Use an NPN transistor with the emitter tied to GND. The 4-7v
pulses will go to the base through a ???ohm resistor and the collecter
will go through a ???ohm resistor to the +3.3v rail. The collector is
now input into the input capture. This will invert the signal, but it
will also force it to be nicely in the 0 to 3.3v digital range.

Being that this is my first PIC24F project AND my first dealings with
Radio Controlled hobby applications, I am dealing with the system as
it is. My friend's system that I am working with uses the motor
batteries through the motor controllers (Traxxas XL-1) to "backfeed"
power to the receiver (Futaba FP-R7H). The receiver therefore does
not have its own independent power, it runs off the motor batteries
(PowerMax 2400mAH Peak Racing Pack) and varies from approx 7v to 4v as
they discharge. Should I actually prefer to run the receiver off from
its own power and use the resistor divider scheme? I was thinking
that the inverting transistor scheme (#3 above) would be more
versatile because it provides a digital signal that is always between
3.3v and GND, no matter which battery and no matter how discharged.

Thanks for any input.
respectfully,
frenchy

Please check Wikipedia for the definition of "crossposting", and compare
it to multiple posting.

Cross posting should be done with care, multiple posting not at all.

So please stop multiple posting!

--

Tim Wescott
Wescott Design Serviceshttp://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details athttp://www.wescottdesign.com/actfes/actfes.html

I just researched it and found that I will never multi-post again!!
"Multi-posting is a waste of bandwidth, money, and people's time, with
no advantages whatever, and should never be indulged in."
sorry,
frenchy
.

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