Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!umich!yale!think!snorkelwacker!apple!well!nagle
From: nagle@well.sf.ca.us (John Nagle)
Newsgroups: comp.robotics
Subject: Re: Cheap servos.
Message-ID: <18170@well.sf.ca.us>
Date: 27 May 90 05:03:59 GMT
References: <56631@bbn.BBN.COM> <8115@brazos.Rice.edu>
Reply-To: nagle@well.sf.ca.us (John Nagle)
Distribution: usa
Organization: Whole Earth 'Lectronic Link, Sausalito, CA
Lines: 28

In article <8115@brazos.Rice.edu> adnan@rice.edu (Sarmad Adnan) writes:
>Hi, 
>
>I need to control RC/Model-Aircraft type pwm servos from
>a computers parallel port. Specifically what I am looking for
>is a circuit to use an 8-bit TTL I/O port to control a few of these 
>servos for a robotic gripper.

      Electrically, R/C servos are really easy to drive.  There are
three wires, usually red, black, and some other color.  Black is
ground, red is power (+5 works fine) and the other color is the
control signal, which is a 5v pulse-width modulated signal.  Any
TTL output will usually work (you may need a pull-up resistor),
and even CMOS outputs will work; I've controlled servos directly
from the outputs of a MC68HC11 CMOS microcontroller.

      The waveform is a square wave of varying duty cycle.  The  
period should be 22ms, and the ON time is varied from 1 to 2 ms
to control the servo.  Note that the ON time is much shorter than
the full cycle; this is for compatability with the way multiple
servo control signals are sent over the R/C link.

      There's a full servomotor control system inside those little
things, with a position-sensing potentiometer, a motor, and appropriate
control electronics.  A wide range of servo sizes and speeds is
available; check your local hobby shop.

					John Nagle