In many application like automotive the temperature needs to be monitored to prevent further damage to system. The circuit uses a PTC (Positive Temperature Coefficient) thermistor and is used to indicate tow different temperature trip point by turning on LEDs when the pre defined temperature is reached.

The circuit is constructed around the LM2904 dual operational amplifier from the 12 volt battery system. The thermistor in series with 10K resistor form ground to the positive 9.1V point. The top of the thermistor is tied to both non-inverting inputs of the LM2904. The voltage at these inputs will change as the thermistor resistance changes with temperature. Each inverting input terminals on the LM294 has a reference, or threshold trip point set by 10K resistor and a 2K potentiometer in series across the 9.1 V regulated voltage. When this threshold is exceeded on the non inverting input of LM2904, the LED lights. The tow trip points can be recalibrated or set to trip at different temperatures by adjusting the 2K potentiometer in each section. In addition to being used as warning lights as shown in circuit and can be added to turn on the fan motor or active a relay.

In the circuit if hysteresis band is required then a additional resistor of 120K can be added in the feedback path of the operational amplifier.

   Schematic

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