Temperature and humidity monitoring system based on MAX6613 and IH3605 (Fig.)

Introduction
Temperature and humidity control is widely used in people's production and life. People use thermometers and hygrometers to collect temperature and humidity, and control temperature and humidity through manual operation of heating, humidification, ventilation and cooling equipment. This not only controls low and real-time performance, but also increases the labor intensity of operators. Even though some users use semiconductor diodes as temperature sensors, the results are not ideal due to their poor interchangeability. This paper puts forward a method to construct a temperature and humidity monitoring system using integrated temperature sensor MAX6613 and integrated humidity sensor IH3605 as detection elements, combined with ADuC824. The system can easily realize real-time control of temperature and humidity, paperless recording and voice alarm, and can also communicate with PC to realize printing and analysis of temperature and humidity.

System Composition and Working Principle
The hardware principle of temperature and humidity monitoring system is shown in Fig. 1. It is composed of single-chip computer system (including keyboard, LCD display, data memory and communication interface), integrated temperature sensor MAX6613, integrated humidity sensor IH3605, real-time clock, voice system and heating, humidification and ventilation devices. The working principle and functions of each component are analyzed below.

< <Single-chip computer system
ADuC824 is a high performance micro-converter produced by AD Corporation in USA. It integrates two high-channel in a single chip. Δ Functional components such as ADC, programmed gain amplifier PGA, 8-bit MCU, 8KB flash/erase program memory, 640B flash/erase data memory, 256B data RAM and timer/counter are compatible with 8051. Especially two separate sigma- Δ ADC, with the resolution of 24 bits and 16 bits for primary and secondary channels, has programmable self-correction function. There is also a universal UART serial I/O, an I2C-compatible two-line and SPI serial port, a watchdog timer (WDT), and a power monitor (PSM). Its performance can completely meet the requirements of the system.
Keyboard is used to set alarm range and control value of upper and lower limits of temperature and humidity, set sampling interval and adjust system time. LCD display uses an integrated package LCD display module TC1602A, which is used to display the upper and lower limits of temperature and humidity, control values and current values. The model of control driver in the module is HD44780, and it can easily interface with ADuC824.
Because the system needs no paper to record temperature and humidity values, an expanded flash EPPROM with capacity of 4MB AT29C040A does not need programmed high voltage and ultraviolet radiation during writing. It has both hardware and software data protection to prevent accidental rewriting of its contents and is very convenient to use.
The communication interface circuit is mainly composed of two-way RS232 data transceiver MAX232 chips, which only needs +5V power supply to operate. The upper computer (PC) invokes temperature and humidity values through a communication interface for printing or analysis.

< <Integrated temperature and humidity sensor
MAX6613 Output Voltage Vout and MeasurementThe relation of measurement temperature T is: Vout=V0+S.T. In the formula, the output voltage of V0 is at 0 C. S is the sensitivity of the sensor, S=11.23 mV/C; T is the current measured temperature.
Due to the low-pass filter, programmed gain control and A/D converter inside ADuC824, the Vout output of MAX6613 can be directly connected to the AIN1 pin of ADuC824. The specific interface circuit is shown in Figure 2.
IH3605 is an integrated humidity sensor manufactured by HONEYWELL. It has integrated signal conditioning circuit. It has many advantages such as high linearity, good interchangeability and wide output voltage range. Its unique multi-layer structure enables it to resist the erosion of moisture, dust, dirt, oil and other chemicals.
The actual relative humidity of IH3605 at current temperature T is: RH=1613 × (Vout-800)/(52730-108T), Vout unit mV, T is the current actual temperature value. Only connect Vout of IH3605 to AIN2 pin of ADuC824 when using.

< <Real Time Clock
Because the system needs to record the actual time of temperature and humidity measurement, the calendar clock chip DS12C887 produced by Dallas Company is selected to automatically generate time information such as centuries, years, months, days, hours, minutes and seconds.

< <phonetic system
For temperature and humidity exceedance and control value alarm, the system uses voice alarm mode. The voice alarm is controlled by the ISD1400 chip. It has a recording time of 20 seconds and the voice address is provided directly by the port line of ADuC824 via 74LS373. The pre-recorded voice of temperature and humidity is used to judge the measured temperature and humidity by ADuC824. The voice is emitted by 74LS373 in any combination to realize the alarm function of temperature and humidity.

< <Heater drive circuit
The drive circuit of the heater is shown in Fig. 3. It is implemented by photoelectric isolator and bidirectional thyristor, and has the characteristics of strong and weak electrical isolation, long life, safe and reliable operation. Control whether the ADuC824 works through P3.5. The drive control of other humidification and ventilation devices in the system can also be achieved by similar drive circuit.

System Software Design
The design of system monitor is simple, and the main program is executed by sequential query.
After the system is initialized, the temperature and humidity values that need to be controlled and their alarm ranges of upper and lower limits can be set through the keyboard each time. Set the time interval for temperature and humidity sampling; Adjust system time. After sampling the temperature and humidity, a suitable digital filter method can be selected to process the data according to the practical application. The value of temperature and humidity after processing is one.The facet is sent for LCD real-time display and stored in AT29C040A as a history along with the current sampling time. On the other hand, compare with the temperature and humidity values that need to be controlled and their upper and lower alarm ranges to determine whether voice alarm is needed. At the same time, ADuC824 controls the heater, humidifier and ventilator to perform the correct action according to the result of judgment, in order to achieve the purpose of temperature and humidity monitoring. The system monitor flow diagram is shown in Figure 4.

Concluding remarks
The system uses MAX6613 and IH3605 as temperature and humidity sensors to improve the real-time monitoring and control. At the same time, because ADuC824 is selected, it can ensure that the system automatically recovers from the dead state to the normal working state, and the security and reliability of the system are also guaranteed, so it has certain practical value. If you need to do group control, you can use this system as an extension, and the host (PC) can communicate with each extension through the network to achieve the purpose of monitoring.