Design of household heating bathing device based on single chip microcomputer control

3.1 Data Collection

The DS18B20 chip is used to detect room temperature, heating water temperature, and bathing water temperature signals. DS18B20 is a one-line digital temperature sensor produced by DALLAS, with a 3-pin TO-92 small package; the temperature measurement range is -55℃~+125℃, and it can be programmed to 9-12 bit A/D conversion accuracy, and the temperature measurement resolution can reach 0.0625℃; multiple DS18B20 can be connected in parallel to 3 or 2 lines, and the CPU only needs one port line to communicate with many DS18B20, occupying fewer ports of the microprocessor, which can save a lot of leads and logic circuits, and is more suitable for the system to measure temperature.

The detection of water level uses three probe points in the expansion tank: high, middle and low. The lowest probe point is connected to the working power supply, the middle is the low water level probe point, and the top is the high water level probe point. In order to prevent the probe points from forming scale due to the ionization adsorption of water, in addition to the selection of special materials, the working power supply is powered by AC signals. The detection of the heating and bathing water pressure switch signal uses a switch sensor. When there is no water flowing through the water pressure switch, a high level signal is output; when there is water flowing through the water pressure switch, a low level signal is output.

3.2 Executive Agency

The actuators are all controlled by relays. The execution signals sent by the single-chip microcomputer are amplified by photoelectric isolation and transistor drive to control the current of the relay coil, so that the relay contacts can be actuated to connect or disconnect the power supply of the circulation pump, solenoid valve and each heating body. The sound alarm circuit is composed of a transistor amplifier and a buzzer, and is controlled by the P1 port of the single-chip microcomputer. In order to protect the single-chip microcomputer system and reduce external signal interference, photoelectric couplers are used for signal isolation between each switch sensor, actuator and single-chip microcomputer.

3.3 Keyboard and display circuit

The keyboard circuit has two functions, one is to set the heating system, and the other is to select the bathing water temperature. The setting of the heating system includes setting the system time, working mode, and customizing the heating temperature value of each time period. Setting the system time mainly calls the initialization program of DS12887 again to make the system time consistent with the current calendar. The heating system has five working modes: continuous operation at a constant temperature all day, operation at a user-defined temperature for each time period, operation at the system default temperature for each time period, anti-freeze operation when on a business trip, and heating function stop. The choice of bathing water temperature mainly uses the keyboard to set the power of the bathing heating body to participate in heating to control the outlet water temperature. When taking a shower, you can choose different shower water temperatures according to your personal feeling. The display circuit uses TN-LCD pen segment liquid crystal, and the display screen is pre-customized. The LCD display can display calendar information, temperature information, working mode information, over-temperature, water shortage, leakage and other fault information. The display module integrates the driver, display RAM, oscillator, etc., and uses the serial port for data communication, which is easy to interface with the single-chip computer.

3.4 Other circuits

The time calendar chip uses DS12887/DS12C887 produced by DALLAS, which has complete clock, alarm and calendar functions until 2100, and has 114 bytes of low-power RAM for power-off protection. The chip encapsulates the crystal oscillator, oscillation circuit, charging circuit and rechargeable lithium battery on the top of the chip to form a thickened integrated circuit module.

DS12887/DS12C887 has a dedicated interface circuit inside, which makes the timing requirements of the external circuit very simple. No peripheral circuit components are required when using it, and the interface with the microcontroller is greatly simplified. In the heating and bathing control system, in addition to providing calendar information and second interrupt signals, the RAM and lithium battery of DS12887/DS12C887 can well protect the user's customized settings.

4 System control software design

The main software design flow charts of the system are shown in Figures 3 and 4.

The second interrupt of DS12887 is connected to the external interrupt 0 of the microcontroller, and the main control function of the system is completed in the interrupt service program of INT0. In order to ensure that the user's power load is not too large, the heating and bathing functions are controlled by interlocking, and bathing is given priority. When the system detects that the bathing water pressure switch is turned on, the heating body is turned off, and only the original working state of the circulation pump is retained; the water outlet temperature is selected by the panel keyboard, and the maximum water temperature is controlled at 65℃ in the bathing function. Only when the bathing water pressure switch is turned off, the bathing heating body is powered off, and the heating body can be powered on. When the heating function is working, the water level is first detected. When the water level is too low, the solenoid valve is opened to supply water; after reaching the high water level, the solenoid valve is closed and the circulation pump is turned on. If it is found that the heating water pressure switch is not turned on, it means that the water circulation system is faulty. After the water level and water circulation are normal, the water temperature and room temperature are detected. According to the working mode setting and the current clock, the saved heating temperature setting value is read, compared with the measured room temperature value, and the corresponding control signal is given; when the room temperature is 2°C lower than the set temperature, the three groups of heating bodies are heated at the same time, until the room temperature is 2°C higher than the set temperature, only one group of heating bodies is left for insulation heating; the three groups of heating bodies are rotated as insulation heating bodies to extend the service life of the heating bodies. In the heating function, the maximum heating water temperature is controlled at 85°C, and heating is stopped and an alarm is issued when the temperature exceeds the set temperature.

The author's innovation points:

The nanometer material far infrared thin film electric heating tube is used, which is small in size and high in thermal efficiency. The single chip microcomputer is used to detect parameters such as water temperature, water level, and room temperature, and it has protections such as over-temperature, water shortage, and leakage, which improves the reliability and practicality of the system. After debugging and inspection, the whole system has the characteristics of low investment, strong functions, and high efficiency, and has broad application prospects.