The heart of the circuit is a LM324 IC. The IC contains four separate operational amplifiers and all the four are used in the circuit. All the op amps are working in comparator mode and the output of each op amp is adjusted with the 10K resistor connect to it. For example if you want to set the first LED (LED1) to activate on desired temperature then adjust the VR1, for second LED adjust VR2, for third LED adjust VR3 and for Forth LED adjust VR4.  A 10K thermistor is used to sense the temperature. The operating voltage of the circuit is 5V to 12V DC. 
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This article describes a very good quality four level temperature sensor using a LM324 IC. The circuit will show visual indication by activating LEDs on four different preset levels of temperature. These four different levels can be achieved by adjusting each 10K variable resistor.
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Four Level Temperature Sensor Using LM324