Subject:
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Re: Resistances of touch sensors and lamps
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 9 Oct 2000 16:37:36 GMT
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Original-From:
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John Barnes <barnes@=NoMoreSpam=sensors.com>
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Viewed:
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599 times
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I have been keeping an eye on this thread and feel compelled to
add my observation and experience;
The touch sensors tend to exhibit quite an extended range of resisitance
values as they are depressed. I have a couple which start as high as 7k on
first contact!
I have experimented with switch mutiplexors trying to find a bullet proof
scheme
and so far the only reliable way seems to be to construct an R-2R-4R-8R
binary system where R is significantly greater than the maximum switch
resistance. Otherwise it is impossible to positively identify one switch
fully pressed from two others partially pressed.
In my case I choose R to be 40k.
+------ SW1 ------ 40k -------+
+------ SW2 ------ 80k -------+
+------ SW3 ------ 160k -------+
+------ SW4 ------ 330k -------+-------- RCX input -------+
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+-----------------------------------------------------------------------+
This provides totally unambiguous readings when the RCX input is set to
passive/raw mode while the readings obtained are well above the 10 bit
digitization noise floor.
(I use 1/8 watt 1% resistors hidden under a 2x8 piece of electric plating with
a short pigtail with Lego 2x2 connector for connection to the RCX.)
Actual available 1% values are 40.2k, 80.6k and 162k.
It is not necessary to put a 320k resistor in the lsb position. That last 10k
just doesn't matter.
Conversion from the readings obtained for each of the 15 switch combinations
to a four bit number representing the switch settings is quite straightforward
using ohms law. In actual use, I build a sensor watch task which continually
reads the input port, converts the value to the four bit setting and places it
in a global location for other tasks to test as if they were individual
sensors.
JB
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Message has 1 Reply:
 | | Re: Resistances of touch sensors and lamps
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| Using the more restrictive criteria that the resistors only come from Radio Shack retail stores, you can't build a bullet proof four switch mux. However, I came up with a similar conclusion about the range of resistor values to use because of the (...) (24 years ago, 15-Oct-00, to lugnet.robotics)
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Message is in Reply To:
| | Resistances of touch sensors and lamps
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| Hi all, I played around with touch sensors and a lamp this weekend, trying to measure their resistance. Since my Ohmmeter proved rather unreliable, the values keep wandering around and it seemed zero was not well adjusted, I just took a look at the (...) (24 years ago, 8-Oct-00, to lugnet.robotics)
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