Alt-BEAM Archive
Message #01156
To: Sean Rigter rigter@cafe.net
From: Steven Bolt sbolt@xs4all.nl
Date: Tue, 2 Mar 1999 08:46:20 +0100 (CET)
Subject: [alt-beam] Re: New photovore circuit
On Mon, 1 Mar 1999, Sean Rigter wrote:
> > But the use of clocks/watches and the way my photovores
> > back up without reversing their motors *really* is mine - I hope :)
>
> A very "beamish" design solution (my highest accolade)!
:)
> You have to overcome the hysteresis of your Schmitt circuit which
> increases when the "effective" resistance of the photodiodes goes up so
> you need "expensive" sensitive photo diodes.
The schmitt-trigger was done that way to get a reasonable - not
excessively high - input resistance. The feelers are also
connected to that input, and I didn't want the circuit to turn into
a square wave generator every time the feelers touched something.
Given a `mere' few megs of input resistance, the diodes are needed
to match, but even without the feelers I would have wanted to keep
the input resistance lower than the >20 meg you're probably looking
at.
Those led sensors need to be put on a real mobile `bot to see what
happens IRL, I think.
> > Finally, the midpoint does not get noticeably out of balance until
> > the light is already too strong for the BPW41/resistor combination.
>
> This problem could be fixed if you use 2 resistors in a symetric
> arrangement 8^)
What I was trying to say is that it just isn't a problem. There
originally were two resistors, but I got the same performance using
only one.
---8<---
> > Assuming you do try, I don't think that pot in the center will have
> > much effect. My advice would be to first measure the current
> > running through those reversed LEDs. It may not be measurable :)
>
> Yep, two fixed resistors are fine too.
Fixed resistors may not change the situation either.
Did you try to measure the current?
> I use a FET opamp buffer for the scope probes to measure the bias point
> voltage during development. FET opamps could also be part of my beam
> designs but they are too "analog" for my taste 8^)
Unbuffered cmos inverters can be used as `opamps' :)
> (Ik kom toch van Amsterdam maar velen jaaren geleeden)
Just 15 miles from where I live. And your Dutch isn't bad!
On Mon, 1 Mar 1999, Wilf Rigter wrote:
> The 2 photodiodes are in series between the supply lines and so
> are yours. But my "Open loop" Oscillator (OlO) is quite a
> different animal compared to your schmitt trigger. The OlO is
> similar to the BiCore oscillator but also resembles the textbook
> CMOS oscillator. The OlO is a gated oscillator since it only
> oscillates if the dc of the bias point is in the linear input
> range, at which point it oscillates with a TC proportional to
> RC/2 (where R is the effective resistance of one photo diode).
Intriguing, though I wonder if it will corner well? Time to put
your circuit on a real mobile `bot! Btw: What frequencies are you
seeing? What influence does light intensity have? What's the current
consumption (excluding motors)?
> The bottom line is this OlO is a low power design compared to an
> simple Schmitt trigger or linear comparator but probably not as
> efficient as your "sampling" comparator.
The efficiency of SunEater_II/III is mostly a matter of having the
motor(s) work in their best voltage range, by being able to adjust the
switch-on/-off levels in a range that matches the photovore-type
application pretty well. And the output really switches all available
power to the motor. Quiescent current consumption is good but not
special: typically 20uA or less.
> However if anyone designed the same OlO circuit before I did (and
> I am sure someone did) the credit of discovery naturally goes to
> them.
Any discovery you make on your own is as yours as it can be, I
think!
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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