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From: strohm@mksol.dseg.ti.com (john r strohm)
Subject: Re: best way to control rc servo using joystick?
Message-ID: <1994Sep20.181126.20953@mksol.dseg.ti.com>
Organization: Texas Instruments, Inc
References: <35kebv$blg@news.bu.edu> <sarge-1909941440570001@ohno.ppp.verdix.com> <35kp4m$g47@pandora.sdsu.edu>
Date: Tue, 20 Sep 1994 18:11:26 GMT
Lines: 26
Xref: glinda.oz.cs.cmu.edu sci.electronics:101375 comp.robotics:13710

In article <35kp4m$g47@pandora.sdsu.edu> fugate@ucssun1.sdsu.edu (fugate) writes:
>>Build (or buy) the modulator. In the RC world there are alot of "Servo
>>Drivers" on the market. But all you really need to do is slap together a
>>555 timer and the correct caps. and the pot from the Joystick to vary the
>>width of the pulse. Can anybody give the particulars on the pulse
>>durations?
>
>
>Why not use a 556 timer?  Have the first stage be an astable oscillating 
>at about 50 Hz.  Feed this into a monostable as the second stage.  Have 
>the pot in the joystick control the on period of the second stage between 1 
>and 2 milliseconds.  Output of the second stage goes to servo (along with 
>+5V and  ground) and should be a variable width (controlled by joystick) 
>pulse at a freq of 50 Hz.

Mainly because a 556 is overkill.  It is utterly trivial to build a 555
circuit that gives you 1-2 msec pulse widths in a frame that is NOMINALLY
20 msec.  Yes, the frame width will vary with the pulse width (2x the pulse
width, actually), but the servo doesn't care about that slight variation.

I have done this with a Royal Titan standard servo.  The big thing to worry
about is stability of the 1-2 msec control pulse.  The place where you are
most likely to get burned is power supply noise coming back from the servo.
I had to use 250 uF across the servo and 0.1 uF across the 555 to get things
to behave; I measured motor current draw at 250 mA with the motor in motion,
and 500 mA surges with the servo "buzzing" because the controller was unstable.
