Question

4. A cars thermostat system is designed to trigger a warning light to signal the driver if the temperature of the engine coolant reaches above 100 C. This warning light should turn on when it receives a 5V signal output voltage from a comparator. Under normal operating conditions, the coolant temperature fluctuates by 5C. Assume that the thermostat electronics system has a transfer function of 25 mV/C. Design a comparator that will warn the driver if the engine temperature is too high but will not trigger on normal fluctuations of the temperature of the coolant.

Answer 1

The total voltage change from the thermostat system when temperature reaches 100 degrees is,

The small fluctuations create a voltage of,

Hence the reference voltage of the comparator should be 0.25 V, above which the comparator should give an output of +5V signal (Vcc is ground) . Below that the comparator will give an output of 0V (-Vcc is connected to ground). The circuit will be as follows,

We have to select R1 and R2 such that Vref is 0.25 V. We know,

Any set of R1 and R2 such that this ratio is 19 can be selected for the above circuit. Let us assume R1 = 1 Kilo-Ohm and R2 as 19 Kilo-Ohm.