Temperature and humidity monitoring system based on MAX6613 and IH3605 (Figure)

Introduction <br> Temperature and humidity control are widely used in people's production and life. People use thermometers and hygrometers to collect temperature and humidity. Temperature and humidity are controlled by manual operation of heating, humidifying, ventilation and cooling equipment, so that not only control accuracy Low, low real-time, and the labor intensity of the operator. Even though some users use semiconductor diodes as temperature sensors, their effects are not satisfactory due to poor interchangeability. This paper proposes a method for constructing a temperature and humidity monitoring system using the integrated temperature sensor MAX6613 and the integrated humidity sensor IH3605 as the detection component, combined with the ADuC824. The system can easily realize real-time temperature and humidity control, paperless recording and voice alarm. Communicate with the host computer (PC) to realize functions such as printing and analysis of temperature and humidity.

System composition and working principle
The hardware principle of the temperature and humidity monitoring system is shown in Figure 1. It consists of a single-chip computer system (including keyboard, liquid crystal display, data memory and communication interface), integrated temperature sensor MAX6613, integrated humidity sensor IH3605, real-time clock, voice system and heating, humidification, The ventilation device is composed of several parts, and the working principle and function of each component are analyzed below.

● Single-chip system ADuC824 is a high-performance micro-converter produced by American AD Company. It integrates dual high-precision ∑-Δ ADC, programmable gain amplifier PGA, 8-bit MCU, 8KB flash/electrical erase program memory in a single chip. 640B flash / electrical erase data memory, 256B data RAM and timer / counter and other features, the command system is compatible with the 8051. In particular, two independent sigma-delta ADCs have resolutions of 24 and 16 bits for the primary and secondary channels, respectively, with programmable self-correction. In addition, there is a general-purpose UART serial I/O, an I2C-compatible two-wire serial port and SPI serial port, a watchdog timer (WDT), and a power monitor (PSM). Its performance can fully meet the needs of the system.
The keyboard is used to set the upper and lower alarm ranges and control values ​​of temperature and humidity, set the sampling time interval, and adjust the system time. The liquid crystal display adopts the integrated package liquid crystal display module TC1602A, which is used to display the upper and lower limits of temperature and humidity, control value and current value. The control driver model in the module is HD44780, which can be easily interfaced with ADuC824.
Because the system needs paperless recording temperature and humidity value, the external expansion of the AT29C040A Flash EPPROM with a capacity of 4MB does not require programming high voltage and ultraviolet radiation during the writing process. It has both hardware and software data protection to prevent its content from being exempted. It was accidentally rewritten and it was very convenient to use.
The communication interface circuit is mainly composed of a two-way RS232 data transceiver MAX232 chip, which can work only with a +5V power supply. The host computer (PC) calls the temperature and humidity values ​​through the communication interface for printing or analysis.

● Integrated temperature and humidity sensor MAX6613 output voltage Vout and measurement The relationship between the quantity temperature T is: Vout = V0 + S · T, where V0 is the output voltage at 0 ° C; S is the sensitivity of the sensor, S = 11.23 mV / ° C; T is the current measured temperature value.
Because the ADuC824 has low-pass filtering, programmable gain control and A/D converter, the Vout output of the MAX6613 can be directly connected to the AIN1 pin of the ADuC824. The specific interface circuit is shown in Figure 2.
IH3605 is an integrated humidity sensor produced by HONEYWELL. It integrates signal conditioning circuit with high precision, good linearity, strong interchangeability and large output voltage range. Its unique multilayer structure makes it resistant to moisture. , erosion of dust, dirt, oil and other chemicals.
The actual relative humidity of the IH3605 at the current temperature T is: RH=1613×(Vout-800)/(52730-108T), Vout is mV, and T is the current actual temperature value. When using, just connect the Vout of IH3605 to the AIN2 pin of ADuC824.

● Real-time clock <br> Since the system needs to record the actual time for measuring temperature and humidity, the calendar clock chip DS12C887 manufactured by Dallas is used to automatically generate time information such as century, year, month, day, hour, minute and second.

● Voice system <br> For the temperature and humidity overrun and control value alarm, the system uses voice alarm mode. The voice alarm is controlled by the ISD1400 chip. It has a 20-second recording and playback time. The voice address is directly provided by the ADuC824's port line via 74LS373. Pre-recorded temperature and humidity voice, the measured temperature and humidity are judged by ADuC824, and they are combined and released by 74LS373 to realize the temperature and humidity alarm function.

● Heater drive circuit <br> The heater drive circuit is shown in Figure 3. It is realized by opto-isolator and bidirectional thyristor. It has strong and weak electrical isolation, long life and safe and reliable operation. Control whether it works by P3.5 of the ADuC824. The drive control of other humidification and ventilation devices in the system can also be achieved by a similar drive circuit.

System software design
The design of the system monitor is relatively simple, and the main program is executed in a sequential query manner.
After the system is initialized, each time you can use the keyboard to set the temperature and humidity values ​​to be controlled and their upper and lower alarm ranges; set the time interval for temperature and humidity sampling; adjust the system time. After sampling the temperature and humidity, according to the actual application, select the appropriate digital filtering method for data processing, and the temperature and humidity values ​​after processing The aspect is sent to the LCD for real-time display, and is stored in the AT29C040A as a history together with the current sampling time. On the other hand, compared with the temperature and humidity values ​​that need to be controlled and their upper and lower alarm ranges, it is judged whether a voice alarm is required. At the same time, the ADuC824 controls the heater, humidification, and ventilation device to perform correct actions according to the judgment result. Achieve the purpose of monitoring temperature and humidity. The block diagram of the system monitor program is shown in Figure 4.

Conclusion <br> The system uses the MAX6613 and IH3605 as temperature and humidity sensors to improve the real-time performance and control accuracy of the system. At the same time, because the ADuC824 is selected, it can ensure that the system automatically recovers from the crash state to the normal working state. The safety and reliability of the system are also guaranteed, so it has certain practical value. If group control is required, the system can be used as an extension, and the host (PC) can communicate with each extension through the network to achieve the purpose of monitoring.