Subject:
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Re: slightly off topic: two questions (one of them for EE types)
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 25 Jan 1999 16:38:18 GMT
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Original-From:
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rajackson@qnx*stopspammers*.com
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Viewed:
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949 times
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Previously, you (Ben Erwin) wrote:
> I hate batteries. I have a lego bubble-blower in my office that I use
> as a demo when people tour our center, and it is really annoying to have
> the battery-pack batteries die at inopportune moments. It seems silly
> to have the $250 serial interface box hooked up to it just for a measley
> 9 volts. (Of course, hooking it up to the interface box for only those
> moments when I need to turn it on is a possibility, but that would be
> too easy. Besides, what I want would also be useful for the middle
> school that I teach at, and the interface box 'on' port doesn't allow
> you to vary speed.)
>
> Having said that, I wanted a cheap AC-adapter-to-lego thing that would
> allow me to change polarity and voltage (direction and speed), kind of
> like the functionality of the controller to the lego train. So my first
> question is this:
>
> 1) Does lego sell the train controller separately, and for how much?
>
> Next...
> I drilled two holes in the battery pack and added a 100K pot and coaxial
> DC power jack. It works pretty well, but I noticed something. The lego
> motor goes from full power to totally stopped in only 30 or 40 degrees
> of rotation of the pot (out of 300 degrees).
> So...
>
> 2) Is something wrong? When I look at the voltage it looks like a
> pretty linear decrease for the full rotation, and I was under the
> impression that the standard lego speeds (1-8) corresponded to equal
> decrements of voltage out of 9 volts. I guess not(?)
Two things... there are two styles of pots avalible one is an "audio"
pot and the other is linear. The audio pots are usually marked with a
'A'. Since the audio pots are primarily designed for adjusting the
volume level, they actually have a logrithmic response to rotating
the shaft. This may partialy explain what you are seeing.
The second effect is that small DC motors do not lend them selves
to speed control by simply voltage control (esp at low sppeds).
For this reason pulse width modulation is a prefered method.
A small pulse with modulator could be built and incorporated into
the modified battery box that you currently have. A simple 555 timer
chip and a 2N3055 power transistor would suffice. Radio-Shack
carries these parts and also handbooks that describe the application
of thses chips.
> That's all...
> Thanks for help in advance.
>
> -Ben
> --
> Did you check the web site first?: http://www.crynwr.com/lego-robotics
--
Did you check the web site first?: http://www.crynwr.com/lego-robotics
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