Alt-BEAM Archive

Message #07930



To: beam@sgiblab.sgi.com
From: "Maynard" cybug@home.net
Date: Tue, 23 Nov 1999 08:45:57 -0700
Subject: [alt-beam] Re: Cholroplast thingy


Hi Wilf:

I can see a lot of strengths to your design! I'm anxious to try it out
ASAP.

Thanks

Craig

> Hi Craig,
>
> The "small" caps in my schematic also have a long time constant because I
> use very low current green LEDs for photovoltaic cells. The small caps are
a
> lot cheaper than 1uf monolithic caps and much lower leakage current than
> 1uf electrolytics. In your simple "feeler/tank" turning arrangement, the
> turning time constant depends on the light level which may actually be
> desirable since it tracks the solar cell charging rate. This only works if
> you use small solar main storage cap ie 4700uf, where each SE trigger
> results in a relatively small forward motion but which is repeated
> frequently. If you use large super caps, the bot could turn a few circles
> before the LED cap charges up again. (g)
>
> It is interesting to observe how placing the caps in parallel or in the
> positive feedback path creates different behaviour. Putting the caps in
the
> feedback path is somewhat desirable since it forces clean switching of the
> motor driver. I noticed that with the large parallel caps there can be a
lag
> in response and when light is equal on both PDs, both drivers come on.
With
> the smaller feedback caps, the bot is always waggling, and the motors are
> pulsewidth modulated when the light is nearly equal. Attached is yet
> another version of the PhotoChlorSE circuit which works very well, using
the
> venerable 2N3904s, good for 100ma motors. The total voltage drop on the
> 2N3904 transistor is 120mV instead of the 2V drop of two series connected
> MPSA12 darlington transistors which should give the motors a little more
> Zap. Note the 0.6V clamped voltage feedback from the base of the
transistors
> was needed to avoid feedback from the sagging voltage on the opamp output
as
> the main cap discharges. I have, in the meantime, added a simple
> feeler/turning circuit with a time out independent of light level which I
> will show you next time.
>
> <>
>
>

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