Measuring 15-150 ohm resistances

Random

First time using this forum thing. Looks pretty helpful.

I'm trying to use a BS2E to measure resistances in the 15-150 ohm range.· I've built the circuit from "what's a microcontroller" for evaluating different resistors, then did the one with the 10K pot.· I've went on to trying a 15-150 ohm resistor but can't get it to work for the lower 85% of the range (about 15-125).

Anybody know the formula for·rc time constant·in parallel circuit?··I've found a reference·to be a "pot" command that can be scaled but I don't think it applies to a Basic Stamp 2.· Perhaps some additional A to D device?

·

Jon Williams

Yes, I think a 12-bit A2D would be a cleaner, more accurate way to go than using RCTIME -- no cap tolerance issues to corrupt your result.

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Jon Williams
Applications Engineer, Parallax
Dallas Office

Aristides Alvarez

You're right about POT. It's a BASIC Stamp 1 command.
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For the BASIC Stamp 2 family you have the RCTIME command, which can be useful in certain situations.
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All the equations and information you need to use this command can be found in the Help File that Jon did for the BASIC Stamp Editor (rightmost icon on the BS Editor's tool bar).
There you'll find sample circuits and equations to adjust component values optimizing your readings.
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In the case the resolution of the RCTIME command is not enough for your application, I recommend you to try our Basic Analog and Digital book:
http://www.parallax.com/detail.asp?product_id=28129
(You can download it for free at that link).
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All this material could be easily applied to your BS2e because it has the same time unit than the BS2 (2 microseconds).



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Aristides Alvarez
Education Manager
aalvarez@parallax.com
Parallax, Inc. www.parallax.com
California, USA

Bean

I think as a rule with RCTIME with lower the resistance the larger value cap you would want to use.

Terry