Subject:
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calculating sensor resistance (Ohm's Law)
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 6 Apr 2001 21:21:58 GMT
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Viewed:
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458 times
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I've been helping my little sister with her RCX-intensive science
project, but we just finished all the calculations and her results seem
screwy. Does anyone who understands RCX internals and/or Ohm's Law have
some time to look this over and make sure I've got all everything right?
This is what we've done: We first made a couple of pressure sensors;
each one has a force sensitive resistor (FSR; same functionality as a
pot) in series with a 1K resistor.
With these sensors (one plugged into each RCX input), we were able to
record raw values 0-1023 in the datalog. We can convert these values to
voltages just by scaling 0-1023 to 0-5, but what I need to figure out is
the resistance of the FSR that corresponds to each of these voltage
values.
This is an Ohm's Law question. The problem is that I do not know what
the inside of the RCX brick looks like. With the information I could
find on the web, I can only guess that the whole setup looks like this:
inside the lego -----+ my sensor
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+5v 10k | 1k
+-----/\/\/\/\/--+-- input --/\/\/\-+
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--- +-+ |
- |V| |
--- +-+ |
- | |
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+----------------+-- input --/\{}/\-+
ground | FSR
| (pot)
inside the lego -----+
In which case it's a series resistance and I can find the resistance of
the FSR with the following equation:
V 1+pot
--- = --------
5 10+1+pot
But the values I'm getting don't really make sense. I just wanted to
check, is this the correct formula? And is my picture of the inside of
the RCX correct?
Thanks so much.
Lauren
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Message has 2 Replies:
 | | Re: calculating sensor resistance (Ohm's Law)
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| hi Lauren, you're quiet right, the inside of the RCX (in touch-sensor mode) is ok, and Ohm's law is also. So the problem lies probably in the sensor used. The cheap resistive pressure sensors are totally non-linear, more expensive sensors are full (...) (23 years ago, 6-Apr-01, to lugnet.robotics)
| | | Re: calculating sensor resistance (Ohm's Law)
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| Lauren, Your diagrahm is correct. Your voltage assumption is not. On RCX the full scale is 2 volts to 5 volts, not 0-5 like you have assumed. Try re-calculating with a 3 volt span. - pete. (...) -- Pete Sevcik sevcik@flash.net Techno-stuff Robotics (...) (23 years ago, 7-Apr-01, to lugnet.robotics)
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