Alt-BEAM Archive
Message #00511
To: beam@corp.sgi.com
From: Justin jaf60@student.canterbury.ac.nz
Date: Wed, 17 Feb 1999 14:51:54 +1300
Subject: [alt-beam] 240-based 'reverser' query
I was going to send this to Ian Bernstein, but thought the list might be
a better place.
http://www.beam-online.com/Robots/Circuits/reverser.gif
This reverser circuit on Beam-Online is the dream circuit I'd been
looking for for a long time. Comparing it to the bicore design there,
the reverser lacks the doubling-up-for-higher-current (forgive my [lack
of] technical terminology :) which led me to assume it was designed for
use with motor drivers. However the mantis circuit;
http://www.beam-online.com/India_'98/November5th/MantisSchem.gif
does not double the inputs/outputs, yet drives a motor directly, so I
assume my assumption was mistaken :)
My current assumption is that the reverser does not need drivers for a
small efficient motor, but you would want to either double it up a bit
or add drivers for larger or less efficient motors.
My question is, can someone put me in the ballpark as to what is
(resonably) "safe" here - I'm guessing that the reverser cicuit "as-is"
will drive a Namiki pager motor (or better) without problems, but for
something larger I might want to use those extra inverters.
Have I got this right? What kind of current seems OK before changes need
to be made?
Second question - a troubleshooter.
I've breadboarded the reverser circuit, and have the outputs connected
to two LED's of opposed polarity (ie a low-current indicator of the
polarity of the output). I have the circuit powered by 3 AA's in series,
and the battery pack is also connected to the inputs.
When I turn the circuit on, the first LED lights up so dimly that it
took me a long time to establish that it lit up at all. When I reverse
the circuit, the LED goes out, and the second LED turns on, bright and
true.
When the cap runs out and it reverses back, the second LED goes out, the
first starts again, but is still barely running.
My understanding is that the first LED should light up properly when the
circuit is not reversed (like the second one does). Instead, it might be
getting no signal at all and just running via the dregs of the input
that makes it through the 47K resistors.
I've tried shifting the inputs/outputs to different inverters, doubling
them up, and so on, but the results are the same. I think I might be
missing something obvious. Can someone point out my blind spot?
(I'm trying to integrate the reverser into a photovore, and the easiest
way to do this would be for the cap discharge to power the reverser as
well as the appropriate inputs, hence the battery arrangement on my
breadboard)
One last question, the reverser circuit shows both sets of inputs on the
left side of the 74hc240 and the outputs on the right, yet AFAICS, the
output of the inverters is on the left side for the second set of
inputs/outputs. Does it not make a difference which end you use, or is
the circuit wrong, or have I missed something obvious?
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