Path: utzoo!utgpu!watmath!clyde!att!rutgers!rochester!pt.cs.cmu.edu!cadre!pitt!darth!liber
From: liber@darth.UUCP (Eric Liber)
Newsgroups: sci.electronics
Subject: Re: A Challenging PWM Design Problem
Message-ID: <412@darth.UUCP>
Date: 7 Dec 88 19:17:20 GMT
References: <1663@maccs.McMaster.CA>
Distribution: rec.ham-radio, sci.electronics
Organization: Darth Software, Allison Park, Pa.
Lines: 67

> 
> I have an interesting Pulse Width Modulation design problem;
> 
> Given the following PWM waveforms:
> 
>   Minimum pulse width:
> 
>     <----------------- 18 mS ------------------->
>     .--.                                        .--.
> ____|  |________________________________________|  |_____
> 
>   -->  <--
>     1.0mS
> 
> 
>   Maximum pulse width:
>     <----------------- 18 mS ------------------->
>     .----.                                      .----.
> ____|    |______________________________________|    |___
> 
>   -->    <-- 
>     1.9mS


This looks just like a "standard" Model Radio Control signal.  By standard
I mean from the type of equipment that Futaba, Kraft and others make not 
what you find at Radio Shack (toys mostly).  

Anyway there are several "chips" that will do what you want with varying 
degrees of simplicity.  Natioal Semi makes a chip - NE554 that is a servo
amplifier chip for model radio control servos.  I suspect that with only
minor component changes from the "normal" values (and maby the addition of
and OP amp for output buffering) it would do very nicely.  These are available
from digikey and specifications are in the National Linear Data book.  If you
can't get the data give me a call and i will xerox and send to you. (412-
367 7308 after 7:00 pm).

Other possibilities are the receiver chips from National and TI that are 
complete Radio control Receivers.  These would probably be more difficult to 
use since they have much more circuitry than you need.  I don't remember
the numbers off hand but the linear data books should have them.  Exar used
to make a chip similar to the National chip (2264 i think) but i don't know
if it is still in production.  

Last, several radio control equipment companies make digital proportional
to motor driver converters.  These units allow for input such as you specify
and output a varying dc signal to drive high current motors in boats and 
cars (and electric airplanes!).  The units for airplanes are uni-polarity
(don't need back up for an airplane :-) ) and are usually bi-polarity for
cars and boats.  The major disadvantage of these units is that they are
typically designed for high current loads (10-200 amps!) and are not 
especially cheap.  

One more thought .. I have seen modelers take the electronics from a radio
control servo (for an airplane let's say) and modify it to drive a small 
motor in very small boat models (less than 12" long).  These servos are 
available from various mail order sources in the $9 - $15 range.  Get a copy
of Radio Control Modeler Magazine to get an idea of sources.  Be aware that
name brand "quality" servos can run from $30 - $45 and more so if you go this
route you want "no name" cheapies.  Note that the circuit boards for the 
electronics in these beasties are small usually less than 1" square (something)
like a postage stamp so you are not going to be burdened with lots of real
estate.  

good luck .. if i can help some more please ask.

Eric (the red) Liber