Dual Temperature Sensor

The Working of this circuit is same as as Temperature sensor Project , The only Difference is that it can detect two different temperatures of your choice instead of one.

Only one Thirmistor is used to detect both the temperatures, but two voltage comparitors are used. For both the comparitors refrance voltage is different, which can be set by the combination of R1 & VR1 for IC1, and R3 & VR3 for IC2.
Now both the comparitors have a different value of fixed voltages on their one pin , while for the 2nd pin of comparitor, voltage level is same as across the thirmistor.
Now by changing the temperature of thirmistor its resistance changes which conciquently changes voltages across thirmistor ie. voltages of 2nd pin of both the comparitors.

Comparitor continueusly compares the voltage levels on its both pins and gives "0" or "1" as per conditions.
Now as both the Comparitors have different referance voltages ( voltage level on its pin "3" applied by the voltage dividers.), so the opperate on different temperatures.
you can check the effects of changing the position of thermistor and changing the pins of comparitor ( pin 2 and pn 3) you will see that it will inverse your results.
For more details about its working please see "Temperature sensor" 

How to Opperate on a specific temperature:

In order to opperate it on a specific temperature ie on 65 degree , you will first have to check that what is the  voltage level accros thirmistor at this specific temperature, Now simply apply this voltage level ( by adjesting the variable resistance ) on comparitor's pin no.3 . And apply the thirmistors voltages on its pin no. 2.

Draw Back:

The main draw back of the circut is that at higher temperature bothe indications turn ON. ie. by heating the thirmistor at first one indication (for relitively lower temperature) turns ON, if now we continue heating the 2nd indication (for relitevely higher temperature) also turns ON. but previous indication (for relitively lower temperature) does not turn off. which logicaly remain should ON. because new teprature is also higher than this.

Solution:

The solution to this problem is very easy , you can use an X-OR gate after the out put of 2nd comparitor. This will turn off the 1st indication when 2nd indication is turn ON.

New modified circuit diagram is shown below.

Keep Smiling:- and Remembor others in your Prayers