Alt-BEAM Archive

Message #04770



To: Bruce Robinson Bruce_Robinson@bc.sympatico.ca, beam@sgiblab.sgi.com
From: "Ben Hitchcock" beh01@uow.edu.au
Date: Tue, 22 Jun 1999 19:50:02 +0000
Subject: [alt-beam] Re: Heads: reinventing the wheel.


Well done! Wish I'd thought of that.

And I think the Bruce to Ben work ratio is probably about 50 to one (I
thought of the idea and posted it in under ten minutes, then forgot about it
when wilf said it wasn't possible) whereas you redesigned it until it
worked. Well done.

The 'Bruce' head?

Does anyone remember a monty python sketch about Australians named Bruce?

Anyway, doesn't matter,
Ben

>While sitting around waiting for walker parts to arrive, I decided to
>play around with head circuits for the first time. I came up with what
>seemed like a good idea: use some kind of pulse generator to drive two
>Nv's in parallel. Use some kind of photodevice (photo-resistors in my
>case) to change the delay. The outputs of the Nv's go to opposite inputs
>of the head motor (or driver).
>
>Theory of operation: when both photocells receive the same amount of
>light, the motor receives inputs on both sides, and so doesn't move. If
>one photocell receives more light, its resistance is lower, so the Nv
>it's attached to will time out first; since the othe Nv is still active,
>this will turn the motor slightly. Made sense to me, so I tried it --
>dismal failure. It worked reasonably well in dim light, but not in
>bright light.
>
>Now I save all the messages on BEAM circuits, even if I don't understand
>them, so I went to my circuit folder to look up the recent discussion on
>heads (which I didn't follow, but saved). And guess what I found? A
>suggestion from Ben Hitchcock for pretty much the same circuit -- he
>used photodiodes and an Nv oscillator, I used photocells and an
>unbalanced bicore to create pulses. The principle was the same. I also
>found a comment on this circuit from Wilf:
>
>> ... The problem with this circuit is that the output pulse width becomes
>> progressively narrower as the light level increases reducing output
>> dutycycle to approach zero.
>
>Yeah, well I'd just figured that out the hard way. But I'm a stubborn
>cuss and it seemed to me there ought to be some way to make this work.
>Besides, I didn't have the parts used in the other head circuits. I
>thought about using the light-sensitive optics from modern welding
>helmets to make a lens that would darken in bright light (too
>expensive!), and I thought about some kind of primitive iris driven by a
>tiny motor (too complicated).
>
>And then I went back to basics. This circuit is like a race: two runners
>going like mad to reach the finish line. But we're not interested in the
>winner's time; all we're interested in is the time between when the
>winner finishes and the loser crosses the finish line. We could care
>less about the time while they're both running. So what we really want
>to do here is once the winner crosses the finish line, get the loser to
>slow down to a walk -- after all, he/she can't win, so why hurry.
>
>In other words, what I wanted to do was have a "normal" delay through a
>typical (1 meg, say) resistor, but use the photocell in parallel to
>shorten the delay. But when one Nv timed out, I wanted to cut the other
>photocell out of the circuit. After several complicated and unworkable
>designs, I realized the answer was staring me in the face. Remarkably
>simple, too.
>
>Use a large resistor to ground as on a typical Nv. Hook up a photocell
>in parallel, but instead of grounding it, run it to the output of the
>other Nv, with an isolating diode. When the two Nv's are triggered, the
>outputs both go low. This allows the photocells to charge the
>capacitors quickly, until one of the Nv's times out, sending the output
>high. That cuts the photocell to the still-active Nv out of the circuit
>(thanks to the diode), allowing the active Nv to slow down and finish at
>a leisurely pace.
>
>And having learned the hard way, I tested this circuit before
>broadcasting it, and it works very well indeed. You DO need to have well
>matched photocells, however.
>
>So, with all due credit to Ben, who thought of it first (even if I
>didn't copy him) ...
>
>Enjoy,
>Bruce
>


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