ravalent@liii.com (Bob Valentine) wrote:
>I'm looking for a simple circuit to trigger a 12-volt relay at a
>certain temperature, and then turn it off at another pre-set
>temperature, or at at x degrees less than turn-on.

This is not a very difficult task.  The first answer is to use an LM75 
temperature sensor, which has a programmable over-temperature set point 
and a hysteresis set point.  When the device temperature exceeds the 
over-temperature set point, the overtemp shutdown (O.S.) output goes low. 
When the temperature comes back below the hysteresis set point, the O.S. 
output goes back high.  The part has an I2C (I-squared C) interface and 
has resolution to 1/2 degree C.

The other thing is to use a temp sensor, like the LM34 or LM35, with a 
simple comparator:

    ____          |
   |    |      |\ |                 In this circuit, the output will go
   |LM34|------|+ \                 high when the LM34 output goes above
   |____|      |    \_____ Output   the value at the comparator (-)
      |        |    /  |            input, which is found to be:
    __|__     -|- /    |            (V+) * (Req1) / (Req1 + Ri), where
     ___     | |/      |            Req1 = Rs * Rf / (Rs + Rf).  When the
      _      |         |            output from the LM34 goes high, it
       ______|___ Rf __|            will then remain high until the LM34
      |      |                      output falls to a value of:
      Ri     Rs                     (V+) * Rs / (Rs + Req2), where Req2 =
      |      |                      Ri * Rf / (Ri + Rf).
      |    __|__
      V+    ___

I will leave it to you to calculate the needed resistor values.  The LM34 
puts out 10mV per degree Fahrenheit.

Nick Gray
Applications Engineer
National Semiconductor Corporation
National Data Sheets at "http://www.national.com"

Date: Fri, 19 Apr 1996 20:10:48 GMT

Original Subject: Re: Simple Thermostat

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