Alt-BEAM Archive

Message #00759



To: beam@corp.sgi.com
From: Justin jaf60@student.canterbury.ac.nz
Date: Mon, 22 Feb 1999 21:25:34 +1300
Subject: [alt-beam] Re: It works, but I don't know how... new diagram


> 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.

No, I think you're probably right. Further experimentation has yielded
results that infer this... :)
>
> 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.

That's ripped directly from the SE part of the Alf circuit. Like the
SuneaterII, it only checks the voltage a small percent of the time,
which is where the pulse generator comes in. Given the apparent losses
of the full circuit though, I'm not sure such efficiency is going to
count for anything :-)

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.

I actually tried that, but this was before I thought of adding a
resistor, and so it didn't work. I've just tried it now - thanks - you
just sliced two components off the design! :-)

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.

I don't quite follow - since the motors are often running in reverse,
wouldn't a reversed diode become a short, depending on which way the
motor is running? or have I misunderstood what you mean?
It sounds like a useful addition though. I suppose the amount of energy
you get back depends a lot on the motors. (Currently it just has "TPM"
motors, which don't have much inertia or inductance.)

BTW, is there a better name than "TPM - Tiny Pager Motor"? It's the only
one I've come accross, but to most people it would apply to any pager
motor...)

> 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.

I've got a full schematic up now at
http://www.geocities.com/SoHo/Lofts/1121/beam/bivore.gif
so you can see. It's two bicores, making a reversable photovore.
But now I'll have to update it to remove those two componments :-)

Thanks

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