Alt-BEAM Archive
Message #00750
To: Justin jaf60@student.canterbury.ac.nz, beam beam@corp.sgi.com
From: Sean Rigter rigter@cafe.net
Date: Sun, 21 Feb 1999 19:23:23 -0800
Subject: [alt-beam] Re: It works, but I don't know how... new diagram
Hi Justin,
Nice schematic!
But I'm amazed it works at all.
Here's my guess why it first it didn't work and then worked when you
added the LED.
Looks like you were missing a pull-up resistor on 245 pin 19 so the
leakage through the "off" transistor was enough to turn on the 245 at
low voltage never allowing the V+ voltage to rise above about 1.2V.
Adding all that other stuff was not enough to make a difference.
When you added the series LED ~ 1.5V forward potential and maybe some
"photo voltaic" effects, it set the voltage at pin 19 at about 1.5V
Since the logic 0 threshold of HC logic is proportional to Vcc, this
requires the voltage level of Vcc to rise above ~ 3V before the HC low
threshold rises above the 1.5V at pin 19 and is enough to turn on the
245.
Still, with so many uncontrolled variables it's hard to figure out
what's happening! Don't bother measuring voltages at pin 19 of this
"working" circuit. It will probably stop working because of the
extremely low currents involved.
Anyway here are some questions and suggestions for improvements for you.
What's the narrow pulse generator for? Looks like any pulse above 1.8V
is latched anyway. Pulse gen just a delay? Can't you just connect the 3
diodes to +V? The 2 feedback diodes hold the latched state until V+
drops below 1.2V. The transistor inverts the latch output but the first
inverter in the latch has the inverted output available. Dump the
transistor and connect pin 19 direct to the inverted latch output.
Probably still needs a (1M) pull-up resistor on pin 19 to hold the 245
in tristate during start-up because if pin 19 is left floating it may
turn on the 245 at V+=1.2V and the motor current would prevent the V+
from rising any higher. Consider using reverse biased (Ge?) diodes from
the motor terminals to 0V and +V. In theory, when the 245 turns off,
some of the stored (magnetic and kinetic) energy is dumped back into the
cap through those diodes.
What's in the "sensors,control circuits" box? Presumably photo diode
stuff to figure out which motor(s) get fired next provide full turning
and reverse drive capability. Shouldn't need any additional logic
though.
regards
wilf
Justin wrote:
> > I don't quite understand why the LED made all the difference (this is a
> > case of orang-utan electonics :-), and am interested to find out, as the
> > circuit is now _technically_ doing what I wanted it to do, but seems to
> > be losing a fair bit of power to the drivers while not engaged, among
> > other things (my assumption is still that I'm going about this in very
> > much the wrong way).
> > If I replace the LED with anything else (diode, photodiode, short,
> > resistor, etc) it won't work. The amount of light falling on the LED
> > affects things however (seems to change duration or cutoff of drive),
> > which might turn out to be useful, but at the moment is not.
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