Development of an economical temperature and humidity monitor controlled by an 8-bit single-chip microcomputer

0 Introduction

Temperature and humidity are important parameters in human daily life, industrial production process, weather forecast, material storage, etc. Therefore, the research on temperature and humidity sensing and its monitoring and control is a very important research content. This paper introduces an economical temperature and humidity monitor controlled by an 8-bit single-chip microcomputer. The monitor can also be used in some occasions where dew point monitoring is required.

1 System Hardware Composition

This system can complete the real-time measurement of temperature and humidity, and can control the temperature and humidity in real time by cooperating with the relevant control parts. It also has the functions of temperature, humidity and dew point display, which can meet the needs of general business environment for temperature, humidity and dew point detection. The system consists of STC89C51RC, RAM, keyboard, temperature sensor, humidity sensor, A/D conversion circuit, D/A conversion circuit, etc. As shown in Figure 1.

The STC89 series is a 51 series single-chip microcomputer with a Flash structure. Therefore, the program memory (ROM) does not need to be expanded externally. In order to memorize the corrected parameters, various correction parameters need to be stored. This system uses a ferroelectric memory with a power-off protection function so that the correction parameters can be stored after correction. The system uses an AD590 integrated temperature sensor as a temperature sensor, which has the characteristics of high sensing accuracy and stable working performance. The humidity sensor uses the HS11OO humidity sensor. The HS11OO capacitive relative sensor adopts a unique capacitor unit design, which is suitable for large batches and low costs. Its characteristics are: rapid recovery after long-term saturation, high reliability and long-term stability, fast response, etc.

This system has two D/A conversion outputs, which can perform feedback control on temperature or humidity. D/A uses two DAC7512, which is a rail-to-rail low-power 12-bit serial input D/A converter that outputs a standard 4-20 mA current signal to meet the needs of temperature and humidity control.

The system also has the function of communicating with the host computer. It adopts RS422 communication mode and can implement long-distance one-to-many point communication. The host computer can query the temperature, humidity and dew point data of up to 64 sampling points.

The system also has a calibration function, which can perform 2-point calibration for temperature and 4-point calibration for humidity. In addition, the system also has a dew point display function to meet the needs of dew point display in different usage occasions.

2 Software Design

The signal output by the temperature sensor needs to be conditioned before A/D conversion. The A/D conversion uses the ADS1110 chip with internal reference voltage and self-calibration function. It uses the I2C bus to communicate with the host and has a 16-bit conversion accuracy. The system samples the temperature and humidity in a timed loop, and displays the sampling results directly on the display after unit conversion. At the same time, the dew point is calculated and the calculation results are displayed on the display. The keyboard input is read by timed scanning of the keyboard, and it is determined whether to perform humidity or temperature calibration. The system workflow is shown in Figure 2.

Follow the steps below to calculate the dew point value:

(1) Measure relative humidity (RH) and ambient temperature (Tα, unit: °C);

(2) Calculate the saturated vapor pressure (Svp, unit: kPa) using the following formula:

log(Svp(T))=(C1T-C2)／(T-C3)(T>0) (1)
or
log(Svp(T))=C4-C5／T(T<0) (2)

(3) Calculate the current water vapor pressure (Vp, unit: kPa) according to the above formula: Vp = RH × Svp / 100

(4) The dew point temperature (Td, unit: °C) is inversely calculated according to formula (1) or formula (2). The temperature T in formula (1) or formula (2) is replaced by the dew point temperature Td, and the saturated vapor pressure Svp is replaced by the current water vapor pressure Vp.

ADS1110 and DAC7512 use I2C bus to communicate with the host, so it is necessary to use I2C bus programming to read A/D sampling results and output phase control signals. In this system, the host uses P3.4 and P3.5 lines to communicate with ADS1100. When communicating, it is necessary to pay attention to the relationship between the sampling frequency, which cannot be too fast, otherwise it will cause "confusion" and make the read data incorrect. Use the expansion port P4.0, P4.1, and P4.2 of STC89 to communicate with DAC7512 and output D/A conversion results, which can be used to control humidity or temperature in real time.

3 Experimental Results

Table 1 shows the measurement results of temperature and dew point using this system under 70% humidity, and Table 2 shows the measurement results of dew point changing with humidity using this system under constant humidity.

It can be seen from the table that when this system is used for dew point monitoring, the maximum error is 0.6℃ and the relative error is less than 1%. Through experiments, the temperature measurement error of this machine is within 1%, and the humidity and dew point errors are within 2%, which can meet the needs of general industrial occasions.

4 Conclusion

The temperature and humidity monitor controlled by an 8-bit single-chip microcomputer has low cost, good reliability, and easy system operation. The measurement accuracy can meet the needs of general household appliances, agricultural greenhouses, office automation and industrial control systems for temperature and humidity control and monitoring. When used to monitor dew point, its error is less than 2%. It can be used in some occasions that need to monitor dew point and can meet the needs of general industrial occasions.