Author Topic: Discrete transistor circuit to generate variable PPM signal?  (Read 5633 times)

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Offline ade

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Re: Discrete transistor circuit to generate variable PPM signal?
« Reply #25 on: February 01, 2016, 12:15:15 pm »
The PPM timing is standardized.  This page explains it pretty nicely:
http://skymixer.net/electronics/84-rc-receivers/78-rc-ppm-signal
RC PPM signals are not standardized and often differ between RC manufacturers.  I.e., they may have different timings, polarity, voltages, channel order, etc.  PPM signals from JR vs. Futaba vs. Walkera will all be different.

(Which is another reason why using a microcontroller may be preferable.)
 

Online Mechatrommer

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  • reassessing directives...
Re: Discrete transistor circuit to generate variable PPM signal?
« Reply #26 on: February 01, 2016, 04:45:46 pm »
RC PPM signals are not standardized and often differ between RC manufacturers.
i can confirm that through conjecture. the protocol maybe standardized, but the timing is not. my FlySky 8 channels RC PPM is 50Hz = 20ms frequency, all 8 PPMs are in that time frame, each PPM is 2ms pulse max making 8 channels requires 16ms to generate, still fit in the 20ms period with 4ms relaxed time for the receiver... thinking about how 12 channels is produced, it mus'nt be 20ms period... yeah, take that in discretes for the challenge,not to mention that within the 5-10% eqv PWM pulse you must have a decent resolution for your servo to work smoothly...
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline rfeecs

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Re: Discrete transistor circuit to generate variable PPM signal?
« Reply #27 on: February 01, 2016, 05:54:03 pm »
Yes, a little common sense.  The servo pulses are 1mS-2mS, so that sets almost all the timing.  That leaves the frame period.  Like the reference I linked to says:
Quote
(1) PPM frame period duration is not critical and depends on RC Transmitter type & Manufacturer.
So it's a bit loosey goosey.  But I think we all get the idea.
 

Offline Refrigerator

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Re: Discrete transistor circuit to generate variable PPM signal?
« Reply #28 on: February 03, 2016, 02:40:21 am »
Thanks, i guess i went a bit to the wrong direction, your approach seems much better, the signal there looks like my PPM decoder ( also discrete transistors ) could handle it, although the decoder i have now is only 2CH and the signal there looks like 3CH so i'll have to make another one, but that's no problem.
If all this fails i'll give the Heathkit transmitter circuit a try since it's guaranteed to work.

I built it with three pulses because I though two channels = three pulses. With only two pulses you need one less transistor  ;D . Since I was in a hurry I did the or gate with mosfets; a bipolar one would be much more old-style. Anyway, thanks for the interesting discussion! If you get a working prototype let me know.
Thanks, i'll put this together once i find some spare time.
RC PPM signals are not standardized and often differ between RC manufacturers.
i can confirm that through conjecture. the protocol maybe standardized, but the timing is not. my FlySky 8 channels RC PPM is 50Hz = 20ms frequency, all 8 PPMs are in that time frame, each PPM is 2ms pulse max making 8 channels requires 16ms to generate, still fit in the 20ms period with 4ms relaxed time for the receiver... thinking about how 12 channels is produced, it mus'nt be 20ms period... yeah, take that in discretes for the challenge,not to mention that within the 5-10% eqv PWM pulse you must have a decent resolution for your servo to work smoothly...
I'm not trying to reach the moon with my circuit and 12 channels is just ridiculous, even 8 CH is too much IMO. :)

Seems like these days people are just afraid of challenges, i guess tapping the keyboard a few times is just easier.
Just started a blog at http://brimmingideas.blogspot.com/ . Not much in it as of now but more is sure to come :)
 


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