Alt-BEAM Archive

Message #00516



To: Chiu-Yuan Fang chiumanfu@home.com, beam beam@corp.sgi.com
From: Sean Rigter rigter@cafe.net
Date: Tue, 16 Feb 1999 21:19:57 -0800
Subject: [alt-beam] Re: 240-based 'reverser' query



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Nah Justin

Chiu-Yuan Fang wrote:

> Here's a qeustion for Ian. Are the 47Kohm resistors for implex or some other
> reason?

Hi Chiu

You didn't recognize this circuit? Not exactly textbook but I've used it
for years in my designs. This design reverses 4 stages at once and is
only useful for some applications. Check out my attached 241 reverser
solution which includes the NuPNC (continuously looks for and corrects
core "saturation").

Anyway no, the resistors are each in parallel with an inverter stage.
Inversion is controlled as follows: 1. tristate the inverter and the
signal bypasses the inverting buffer (through the resistor) non-inverted
2. Un-tristate and the signal at the "output" is inverted by the active
buffer.

I believe the diagram is incomplete : it should show the "outputs"
connected to the adjacent "unlabled inputs" and the associated
"unlabled buffered outputs" are capable of driving a small motor load.
The sensor on the left is the reverse Nu and the sensor on the right the
"OFF/STOP" switch (used also during PNC)

enjoy

wilf

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517 Tue, 16 Feb 1999 21:51:51 -0800 [alt-beam] Re: 240-based 'reverser' query Justin , beam Sean Rigter You are absolutely right Justin: this is not a good diagram

1. No doubling = less current is the price you pay for this reverser
2. Always use 74ACxxx for higher current (typ ~50mA per output)
3. The diagram is wrong: the bottom output/input labels are swapped
4. Missing connections everywhere!
5. Where did this diagram come from?

Want me to correct it?

enjoy

wilf

Justin wrote:
>
> 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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