Design and development of heater scheme
With the improvement of power supply and the continuous improvement of people's living standards, heaters have gradually entered ordinary households, bringing strong warmth to consumers in the mid-winter season. But most heaters do not have a good human-computer interaction interface and remote control function. This design mainly studies this part, so that the heater can use the LCD display to display real-time working temperature, and use the infrared remote control to remotely control the heater. Make products more competitive in the market and bring more convenience to people.
1. The composition of the heater program system
As shown in Figure 1, the system is composed of five parts: MCU main controller, keyboard and liquid crystal display, infrared input, temperature detection and output controller.
The main modules are as follows: (1) Keyboard and LCD module. This module is responsible for starting and shutting down the system. It also controls the upper limit of the heater's working temperature, sets the heater's working time, and controls the shaking head and the fan. The LCD screen displays the ambient temperature of the system, the set temperature and time, etc. (2) Infrared input module. This module uses the infrared integrated receiving head to decode and analyze instructions through the single-chip microcomputer. (3) Temperature detection module. The module uses thermistor to measure the resistance to calculate the temperature to prevent the system from overheating and damaging the device. (4) Control output module. This module is the relay corresponding to I/O port output corresponding level control.
2. Design scheme of remote control heater
The LCD dot matrix is relatively large. If you directly use the I/O of the single-chip microcomputer to drive through the resistor network bias voltage, it will make the I/O operation of the single-chip microcomputer very cumbersome. Here we choose a commonly used chip HT1621. The HT1621 is a 128-point memory mapping and multifunctional LCD driver. The software configuration feature of the HT1621 makes it suitable for a variety of LCD applications, including LCD modules and display subsystems.
2.1 Display controller
The liquid crystal display controller is the display part of the multifunctional automatic intelligent switch, which can monitor the operation of the power supply line in real time, accurately and online. Once the line has leakage, overload, short circuit, overvoltage, undervoltage and lack of phase, the intelligent safety power switch can immediately start the protection program and display it on the HT1621 LCD controller. The main controller and the HT1621 chip are connected according to the conventional pins, and the oscillator can use the internal oscillator of the HT1621 without external connection. LCD is used to display content, it only needs to write signals and not read signals.
2.2 Temperature detection technology
The heater often works under higher temperature conditions, and the device is very likely to be damaged due to overheating, so the system must be checked at all times. The ambient temperature and device temperature are input to the single-chip microcomputer in the form of numerical values through the I/O port, and the single-chip microcomputer processes these values with special significance and then sends instructions. If the single-chip microcomputer used does not have an A/D converter, the single-chip microcomputer cannot obtain the thermistor resistance value used to measure the operating temperature of the heater. If the system does not require high A/D conversion accuracy, and considering the cost, you can use an RC circuit to measure resistance through the length of its charging time. Among them, the RC circuit is powered by the I/O port of the microcontroller. This method is generally used on single-chip microcomputers with push-pull I/O ports. Close the switch, the power supply charges the capacitor C through R. Use the I/O port of the single-chip microcomputer to charge the standard resistance and the resistance to be measured. When the voltage is the high-level threshold of the I/O port, record their charging time, and the resistance value can be obtained through the corresponding calculation formula, and the charging time It can be obtained by the timer of the microcontroller.
As the main control unit of the system, the single-chip microcomputer controls all the input and output, and adjusts the air supply mode of the heater and the temperature of the heater according to the user key information. The AT89S51 was selected in this design, which is a low-power, high-performance CMOS 8-bit microcontroller. In addition, AT89S51 is designed and configured with an oscillation frequency of 0Hz and power saving mode can be set through software. In idle mode, the CPU suspends work, while the RAM timer counter, serial port, and external interrupt system can continue to work. The power-down mode freezes the oscillator and saves RAM data, and stops other chip functions until external interrupt activation or hardware reset.
2.4 Infrared remote control system
The universal infrared remote control system is composed of two parts: transmitting and receiving. The application/decoding special integrated circuit chip is used for control operation. The transmitting part includes a keyboard matrix, encoding modulation, and LED infrared transmitter; the receiving part includes optical and electrical conversion amplifiers, demodulation, and decoding circuits. The receiving circuit can use an integrated infrared receiver that integrates infrared receiving and amplification. It does not need any external components to complete all the work from infrared receiving to output compatible with TTL level signals, and the size is comparable to ordinary plastic packaging. The triode is the same size, it is suitable for all kinds of infrared remote control and infrared data transmission.
Third, the hardware design of the heater
The overall circuit of the system is divided into: button detection circuit, infrared detection and HT1621 data circuit, and relay control circuit according to actual circuit functions. See Figure 2 heater circuit diagram. Measure the environment and the operating temperature of the device to ensure that the device works under normal operating conditions. In this process, the microcontroller is used to detect, and then the corresponding step is taken when the corresponding condition is triggered.
3.1 Button detection circuit
The key detection circuit includes: 6 contact keys, the connected case is used to adjust the temperature of the heater, one for heating and one for cooling; the connected keys are TIME (display time) and HEAT (heating by electric wire); the keys connected to P64 are The air supply speed, one of which accelerates and the other decelerates; the resistance of the three current-limiting resistors is 100 ohms; the 3V high-level power supply is provided by the system power supply. The single-chip microcomputer uses 3 I/O ports to collect button information, and each port is connected to 2 buttons. Because the microcontroller AT89S51 first sets the I/O port as an internal pull-up input, then the button pulled to the ground can be detected; then set the I/O port as an internal pull-down input, then the button pulled to the power supply can be detected. In order to prevent the two buttons from being pressed at the same time, causing a short circuit, a small resistor is connected in series between the two buttons and the power supply.
3.2 Infrared detection
Infrared detection and HT1621 multiplex the MCU port. Its circuit composition includes: RN, adjustable port resistance, its maximum resistance value is 10k; RF, infrared detection port resistance, resistance value is 10k; AC filter capacitor is 1uF; P60 and P61 ports It is a multiplexed I/O port for infrared detection input and DATA line of HT1621. In order to prevent the data received by infrared from interfering with the communication between the MCU and the HT1621, the infrared input part is connected in series with resistors RF and RN. RN is an adjustable resistance, because P60 is also an external interrupt input port and needs to be adjusted to an appropriate resistance value. In this design, RN can maintain the same resistance value as RF.
3.3 Output control circuit
The output control circuit mainly includes fan switch control, shaking head switch control and heating switch control. This module is the relay corresponding to the corresponding level control of the I/O output.