Alt-BEAM Archive
Message #01069
To: Jake842@aol.com
From: Steven Bolt sbolt@xs4all.nl
Date: Sun, 28 Feb 1999 10:24:56 +0100 (CET)
Subject: [alt-beam] Re: Symet
On Sat, 27 Feb 1999 Jake842@aol.com wrote:
> I just finished my first 1381 Symet. It has 6 1000uf Capacitors and a
> solarbotics solar panel. In bright light it triggers about once a second, but
> the bursts are barily enough to move it anywhere. So I put a LED in between
> the 3rd leg of the 2.7 1381
> and th negative of the capacitors. Nothing happened, i then reversed the
> polarity of the Led, that didn't work so i tried a regular diode, that didn't
> work either. How can I make the bursts more powerful? Any help would be
> appreciated.
2.7V and 6000uF combined with the usual pager or small recorder
motor equate to a very short, high power burst. If you are using
an unsuitable transistor to drive the motor - like a 2N3904 - most
of that power is likely to warm up the transistor a little bit. If
the transistor is able to handle the current, then depending on the
inertia of the motor and `bot, most power will be wasted heating
the motor or spinning the shaft without moving the `bot.
A possible remedy:
- Make the `bot as easy to accelerate as you can. In the case of
`direct drive', use just a thin bit of rubber tubing on the
shaft; keep the diameter small. And of course, add lightness :)
- Establish the minimum initial voltage necessary to do the job,
by allowing the cap to charge with a voltmeter connected, and
starting the motor at various voltages by manually shorting wires
using for instance a microswitch. Use long enough, thin wires for
the test, so as not to impede the motion.
- Also establish the best capacitor value using similar means as
above, and repeat steps 2 and 3 a couple of times.
- Change the circuit to trigger at the voltage found. Make sure
that the transistor which switches current to the motor is up to
the job.
A lower voltage decreases the initial power peak to something
which can be more efficiently converted to acceleration. Since that
acceleration will take some time, the storage cap needs to have a
certain size; a longer burst requires a larger cap. If you dislike
what you get out of 6000uF, make it 10000uF, or even 47000uF.
But note that motion per unit of time depends on the size of the
solar panel and the quality of the motor. The cap mostly determines
the frequency. A large cap gives you few but long bursts, with long
charge cycles in between. A small cap shortens the burst, but also
the charge time, often making the `bot more interesting to the
public.
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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