The resistors in the circuit always give us the feeling that it is always dull and boring, making people feel that the resistors are unpredictable. However, it is the resistors that are constantly transforming our lives. We are no longer sighing the magic of the resistors all the time. Sometimes we feel the fun of it without knowing it. 
 
Water heaters are already the electrical products commonly used in our homes. It brings convenience to people's lives. Many often use but don't know how it works.

 
R1 is the temperature-regulating resistor and R2 is the thermistor. The two inputs of the AND circuit are connected to a water level sensor and a water temperature sensor. When the water tank is full of water, point B is high voltage (that is, the input is logic 1), and R2 is the thermistor. The lower the temperature, the larger the resistance value of the thermistor, the larger the partial pressure at both ends, when the voltage reaches a certain level. When the point A is high voltage (that is, the input is logic 1), when both A and B are high voltage, the output of the AND gate is high voltage, and the heating switch of the electric water heater is turned on, in the water tank. The water is heated. When the temperature is higher than a certain temperature, the resistance of R2 becomes smaller, the voltage across R2 becomes smaller, and the heater stops heating, so the water heater is kept warm. The heater will not heat up when the tank is not full or when the temperature is high. 


 
Through the above understanding, we can clearly know that the thermistor plays an important role in this link. Without the thermistor, our electric water heater can't provide us with hot water. It can be seen how much convenience resistors bring in our lives, making our lives simpler and more comfortable.