(a) Heating chamber 12 V R₁ R₂ w Heater R4 Heater/ Thermistor Thermistor 120°C 0.5°C 11.12 Electronic thermostat. A small chamber must be maintained at a constant temperature of 80°C ± 0.5°C. The circuit in Figure 11.75 is used to do so. The thermistor serves as the temperature sensor and together with the comparator it forms a thermostat. The transistor is added to allow the large current needed for the heating element. The NTC thermistor has a material constant of 3500 K and a resistance of 10 km at 20°C. Assuming that in the range given the material constant does not vary with temperature, select the resistors R1, R2, and R3 to satisfy the requirements of the thermostat. Assume that the simple model of the thermistor is sufficiently accurate and, for best performance, that the switching on-off occurs with the reference voltage set at 6 V.