Subject:
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Re: Micro-Switches, Mux
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Newsgroups:
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lugnet.robotics.rcx
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Date:
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Wed, 28 Feb 2001 06:02:17 GMT
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Viewed:
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1431 times
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"Stef Mientki" <s.mientki@ids.kun.nl> wrote in message
news:3A9C14C9.76834B8@ids.kun.nl...
>
> There was some doubt in another newsgroup (lugnet.robotics) about my
> suggestions, so I've made an overview,
> with drawings om my website (it has also some more pratical values of the
> resistors),
>
>
http://baserv.uci.kun.nl/~smientki/Lego_Knex/Lego_electronica/Switch_multipl
exer.htm
Nice job Stef, that's a really helpful page.
I built a version of TFMs mux
(http://www.akasa.bc.ca/tfm/lego_resistor.html) a while ago but I used
resistor values that are not ideal (10k, 20k, 39k, 82k). Your spreadsheet
confirms this and is *very* accurate (+- 1 raw) when compared to the real
readings!
However, I couldn't help thinking that the 'best' resistor step should be
R-2R-4R-8R (as said by John Barnes http://news.lugnet.com/robotics/?n=12712)
in both the parallel and series arrangement. Correct me if I'm wrong, but
your spreadsheet seems to agree with this, if 'best' means making the lowest
'delta' value as large as possible while keeping a nice interval between all
the other raw values.
I reckon that the best (realistic) values for TFMs mux are as he suggests.
The spreadsheet formula means that my 'best' rating gets better as the
resistance decreases but in practice the internal resistance of the Lego
touch sensors comes into the reckoning.
For Michael Gasperi's arrangement
(http://www.plazaearth.com/usr/gasperi/3rmux.htm) I would say that the
'best' values are 18k, 36k, 72k, 144k. However 20k, 40k, 80k, 160k is just
about as good and these values should be quite easy to get hold of. If you
are using Lego touch sensors then John Barnes' values (see link above) of
40k, 80k, 160k, 330k are a good bet to sort out partially pressed sensors.
One way round this is to modify the Lego touch sensors by sticking a piece
of conductive metal on the internal 'rubber contact' to give a very low
resistance switch. Could this be potentially damaging to the RCX?
As you say, Michael Gasperi's gives better (larger) intervals between
successive readings but for the record I have to say that TFMs was very easy
to build.
Well, you've inspired me! I'm off to the Maplin website (UK equivalent of
RadioShack) to order some resistors. I'm going to build a
Gasperi-style-4-touch-sensor-expander. Now I wonder if I can improve on the
original design.... :-)
Tom. [wishing he knew a lot more about electronics]
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Message is in Reply To:
 | | Re: Micro-Switches, Mux
|
| There was some doubt in another newsgroup (lugnet.robotics) about my suggestions, so I've made an overview, with drawings om my website (it has also some more pratical values of the resistors), (...) met vriendelijke groet, (24 years ago, 27-Feb-01, to lugnet.robotics.rcx)
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