very simple thermistor circuit help.

Discussion in 'The Projects Forum' started by danton133, Mar 12, 2010.

  1. danton133

    Thread Starter New Member

    Mar 12, 2010
    1
    0
    I have a thermistor varying between 4.5k and 5k ohms for the range of temperature I want to measure. Using a potential divider I have the voltage varying between 5.36v and 5.64v. I want to power an LED only when the temperature is inside this range. How do I do this? Is this voltage range too small? I have tried amplifying it with a 741 amp which gave me output varying from 11.26 to 11.38 this still seems too small. I have looked at this circuit I found online: http://www.camotruck.net/rollins/electronics/thermo-tpcd.png
    It seems to compare a voltage set by the pot to a voltage caused by the change in the thermistor resistance. Aside from the fact I would not know how to configure this I think the voltage range I have is too small for this to work. will i be better of increasing the temperature range i want to measure? Please help.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You need what is called a "window comparator" circuit.

    Rob Paisley has a pretty decent page on comparators here:
    http://home.cogeco.ca/~rpaisley4/Comparators.html

    A voltage window comparator is described partway down the page. However, if you are just beginning with comparators, this is a good page to read all the way through, and study the circuits. Build some of them, and see how they work.
     
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