Alt-BEAM Archive
Message #08699
To: beam@sgiblab.sgi.com
From: Bob Shannon bshannon@tiac.net
Date: Thu, 30 Dec 1999 19:02:40 -0500
Subject: [alt-beam] Re: FLED Photovore
"Phillip A. Ryals" wrote:
> >but one of the pins on the 1381 is ground, isn't it?
Yes, but...
>
> That's what I was wondering... Could it be just a matter of connecting the
> FLED on the other two points?
>
> Hate to drive everyone nuts with attachments, but would it be like this?
First off, I have not tested this, but...
I doubt this would work either.
The problem here is that the eye circuit taken from the photopopper charges a
small
capacitor, usually a .22 uf cap, with a current through the photodiode. So this
bitty
little .22 uf cap charges up (slowly) through the photodiode, and as it
approaches
the turn on point of the FLED, the FLED starts to draw current. This is the
only point
where the 1381 is 'better' than the FLED, near the turn-on point.
(and because the a solar engine normally spends little of its time near the
turn-on point
there is not as much difference between a FLED SE and a 1381 based SE in real
world performance!)
This current draw will probably depelete the voltage stored in that bitty .22 uf
cap, and
never trigger the SE. This can be a problem with some FLED solar engines even
when
the capacitor being depleted is as large as 2,200 uf, with the solar cell
directly across
the cap. Clearly the poor .22 uf cap is going to have a problem. Simply
raising the caps
value is not the right solution either.
The classic FLED photovore uses 2 solar cells, and 2 storage caps, basically 2
totally
seperate solar engines. This produces a very different sort of behavior than
the more
modern circuits with a single solar cell and cap.
With a single cell and cap, the time between each step is nearly constant,
pop...pop...pop.
But the 'classic' FLED 'vore is very different in its behavior, there is only
chaotic coupling
between the two solar engines, it may go
pop..pop.....pop.....pop..pop.......pop... Its much more common to get both
motors to fire together with the dual FLED SE 'vore than with the photopopper
circuit.
When you place a classic FLED 'vore next to a photopopper, the difference in
behavior is
quite striking. There are other differences, like having the whole solar cell
surface measuring
the ambient lighting, rather than a single (wide angle and all) photodiode.
See, you can get very
different behaviors from the photopopper just based on where the photodiode eyes
are placed.
With the 'classic' FLED 'vore, you angle the 2 solar cells for a similar effect,
but the greater sensitive area does make a difference.
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