A Digital Valve for Controlling Water Temperature Based on Microcontroller PIC16C71

The digital valve adopts an NTC thermistor as a temperature sensor, forms a simple voltage divider circuit with a fixed resistor as a water temperature measurement circuit, and uses the built-in 8-bit A/D converter of the PIC16C71 microcontroller to convert the analog voltage on the thermistor into digital voltage. The PIC16C71 microcontroller controls the DC motor to drive the water mixing valve to adjust the mixing ratio of cold and hot water, achieving water temperature adjustment. The control circuit diagram is given, and the parameter selection and temperature measurement accuracy of the water temperature measurement circuit is discussed in detail. Experiments and analysis show that choosing an NTC thermistor and voltage divider with a larger resistance value can better solve the problem of thermal breakdown caused by the larger power consumption of the thermistor.

With the development of society, various water heaters and hot water pipes have entered thousands of households. People have various requirements for the water temperature on different occasions. It is often necessary to mix hot and cold water to the required temperature.

As a valuable technology, the intelligent digital valve can control the water temperature of various water heaters and pipeline hot water and can quickly and accurately adjust the required temperature of hot water. It is able to be used in showers, washes, and other places requiring hot water with a constant temperature. This design is in line with this need, with the PIC16C71 microcontroller as the core, this digital valve is able to control the mechanical part to automatically adjust the mixing ratio of cold water and hot water, to realize the automatic control of the water temperature, and to solve the problems of water temperature fluctuating cold and hot caused by fluctuations in water pressure and changes in water temperature or changes in water output. This way has a more obvious saving effect than manual adjustment of water temperature.

1. System structure and working principle

Hot water and tap water are used as the two inputs of the control valve, and the mixing valve is used to control the input amount and proportion of cold and hot water, and the mixed water flows out through the water outlet for use by users. The temperature sensor installed at the water outlet senses the temperature at the water outlet and transmits it to the microcontroller through the temperature measurement circuit. The composition of the digital valve is shown in the figure below. When the water outlet switch is turned on, the microcontroller compares the temperature at the water outlet measured by the temperature sensor with the set temperature. When necessary, the microcontroller PIC16C71 controls the DC motor to drive the water mixing valve to adjust the ratio of cold and hot water entering the water valve. Thereby, the temperature of the outlet water is controlled. When the motor turns to the end, the single-chip microcomputer gets the corresponding signal to stop the motor from continuing to rotate in the same direction.

Set the water temperature within the range of 25 to 50°C through the temperature rise or temperature drop button, and the LED digital tube displays the set water temperature value. If the set temperature does not match the temperature detected by the sensor, according to the temperature difference between the two, the single-chip microcomputer outputs pulse voltage signals of different widths to control the DC motor to rotate at different speeds and drives the cold and hot water mixing valve through the transmission mechanism to change the inflow ratio of cold and hot water. When the external conditions change again, such as the water pressure decreases or increases, there is a temperature difference between the water temperature of the outlet pipe and the set temperature. At this time, the single-chip microcomputer controls the rotation of the motor again and automatically adjusts the water temperature, so that the water temperature of the water outlet is automatically adjusted to make the water temperature of the water outlet automatically consistent with the set water temperature.

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