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From: nagle@netcom.com (John Nagle)
Subject: Re: Trouble making SERVO driver
Message-ID: <nagleD4BC9r.FK7@netcom.com>
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
References: <D49sDL.DE4@undergrad.math.uwaterloo.ca>
Date: Mon, 20 Feb 1995 19:17:51 GMT
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rbairos@undergrad.math.uwaterloo.ca (Rob Bairos) writes:
>I'm starting a project that requires the positioning of mirrors or
>prisms to position a beam of light over 2 axis.

>I thought servos might do the trick but I've never worked with
>any before.  I picked up a couple and am having trouble driving them
>correctly.

     What do you mean by "servos"?  Servomotors generally?  Model
radio-control servos?  Servoed mirror galvonometers (the usual
devices for light beam driving for laser light shows and such)?

>My driver is just a 556 timer outputting a 100 hz pulse with a hi period 
>of 1-5 ms.

     You're trying to drive R/C servos, right?  The signal should be
on for 0.5 to 1.5ms; your pulses are too long.  Frequency isn't that
critical, but use around 50-60Hz.

>My analog skills are basic but the timers output is severly
>affected when the servo is attached, though it moves it forward/backward
>luckily enough.

     For R/C servos, red is power, black is ground, and the other wire is 
control.  Use +5 for both power and control, but put a big filter cap 
between power and ground at the servo.  The control input draws so
little power compared to the output drive capability of a 556 that
if the servo is affecting the timer, you must have something hooked
up wrong.

>My question is: does anyone have any experience building a servo
>driver?  are there any online specs or documentation?  An
>application circuit would be great if it existed.

      Most people just drive these things straight off the timer port
of a microcontroller, with a buffer in between for electrical oscillation.
No analog parts are necessary at all.  I've driven these off a 68HC11
microprocessor.  

      What are you trying to do, anyway?

					John Nagle
