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rctime and lm35?? — Parallax Forums

rctime and lm35??

tercou1tercou1 Posts: 25
edited 2012-01-25 09:05 in BASIC Stamp
is there any schematics that could show me how to connect an lm35 to an rc circuit so that i could connect it to a bs2? do i replace the pot with the temp sensor?? I want the bs2 to alert me if the temp. is approaching 0 degrees celsius. thanks in advance.. anybody with any code regarding this problem would be really appreciated

Comments

  • ercoerco Posts: 20,257
    edited 2012-01-24 16:58
    Neat sensor, but you'll have a hard time RCtiming it directly, its output voltage is too low. It outputs 0 volts at 0 degrees C, then increases 10 mV per degree C. It wouldn't be able to trigger the Stamp's TTL input voltage (~1.4V) until the temp hits 140 degrees C or higher.

    See Tracy Allen's info at http://www.emesystems.com/OL2heat.htm#TF_LM34 and also http://forums.parallax.com/showthread.php?135361-ADC0834-amp-LM34-vs.-Offset&highlight=ds18b20

    You could RCtime a thermistor directly, but you're going to need an op amp or ADC to use that particular sensor.
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2012-01-25 00:20
    Erco is right, the LM35 does not lend itself well to the RCTime method. Is it only the yes/no you are after, above or below freezing, or are you also after quantitative data? T
  • bsnutbsnut Posts: 521
    edited 2012-01-25 00:37
    Are you limited on I/O pins for your application? Why I ask this, is a ADC only needs 3 I/O pins and is able see this sensor small voltage changes.
  • tercou1tercou1 Posts: 25
    edited 2012-01-25 05:02
    hi tracy, it's the yes/no that i'm looking for, I want to be able to tell if the water in a pipe is going to freeze. if it is, then i want the bs2 to alert me via an lcd screen. I suppose i could go the extra mile and have a running temp display on the lcd :) i have been looking at the ds for 0831adc and it seems pretty straight forward.. what do you think?
  • tercou1tercou1 Posts: 25
    edited 2012-01-25 05:27
    Thanks Erco
    erco wrote: »
    Neat sensor, but you'll have a hard time RCtiming it directly, its output voltage is too low. It outputs 0 volts at 0 degrees C, then increases 10 mV per degree C. It wouldn't be able to trigger the Stamp's TTL input voltage (~1.4V) until the temp hits 140 degrees C or higher.

    See Tracy Allen's info at http://www.emesystems.com/OL2heat.htm#TF_LM34 and also http://forums.parallax.com/showthread.php?135361-ADC0834-amp-LM34-vs.-Offset&highlight=ds18b20

    You could RCtime a thermistor directly, but you're going to need an op amp or ADC to use that particular sensor.
  • ercoerco Posts: 20,257
    edited 2012-01-25 09:05
    One last thought just in case your LM35 is loctited in place already. :)

    The voltage output approaching 0 degrees C is very low, but you could run that into a 99-cent comparator (LM339 or equiv), using a trimpot to set a reference voltage. The comparator snaps on when the sensor voltage drops below the reference voltage, and that output can trigger your Stamp or other electronics directly.
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