Design of temperature and humidity control system based on LTM8901

introduction

Nowadays, intelligence has become a new development trend in the field of automation, and has promoted the rapid expansion of intelligent functions to instruments, meters, equipment and other industries. The measurement and control of temperature and humidity are widely used in industrial and agricultural production, meteorological observation, constant temperature and humidity air-conditioned rooms, scientific research and daily life. This design is based on the intelligent environmental temperature and humidity control system of the temperature and humidity sensor LTM8901, with AT89S52 as the controller. Through simulation experiments, the detection and control of the ambient greenhouse temperature and humidity can be realized.

System hardware design

The system detects the temperature and humidity of the environment through the integrated digital temperature and humidity sensor, converts the temperature and humidity of the environment into digital quantities, and transmits the results to the single-chip microcomputer, which is displayed through the digital tube. The single-chip microcomputer uses the keyboard to set the maximum and minimum values ​​of the ambient temperature and humidity, and compares the current value detected with the preset maximum and minimum values. According to the comparison result, the single-chip microcomputer makes corresponding judgments to control the working state of the output relay and perform corresponding control operations, thereby realizing the detection and control of the ambient temperature and humidity. The system block diagram is shown in Figure 1.

Temperature and humidity detection circuit

The system uses Changying Technology's LTM8901 as the environmental temperature and humidity sensor. LTM8901 is a digital temperature and humidity probe specially provided for users to design their own products. It can be used at a single point or in a multi-point network with other ITU equipment. Its humidity measurement range is 1% to 99%RH, the resolution is 0.5%RH, and the measurement accuracy is ±3.0%RH (typical value); the temperature measurement range is -25℃ to +60℃, the resolution is 0.0625℃, and the measurement accuracy is ±0.5℃; the typical response time is 5 seconds; the operating voltage range is 4.5V to 5.5V. LTM8901 directly outputs the measurement results as digital signals, which are serially transmitted to the microcontroller through the "one-line bus". At the same time, the check code can be transmitted, and it has strong anti-interference and error correction capabilities. Its circuit connection is shown in Figure 2.

Keyboard input and digital display circuit

The main function of the keyboard input circuit is to allow users to set the temperature and humidity reference values, including the maximum and minimum values. The keyboard input circuit consists of 4×4 small keyboards, with a total of 16 function keys, including 0~9 numeric keys, temperature setting keys, humidity setting keys, maximum value keys, minimum value keys, confirmation keys and clear keys. When the temperature setting key is valid, the maximum and minimum values ​​of the preset temperature can be changed. When the temperature setting key is valid, the maximum and minimum values ​​of the preset humidity can be changed. The maximum value key is used to indicate that the input is the maximum limit, and the minimum value key is used to indicate that the input is the minimum limit. It can be used in conjunction with the confirmation key to set the preset limit of temperature and humidity. The clear key is used to clear the current setting value.

The digital display circuit consists of 6 digital tubes, which are divided into two groups, one for displaying temperature value and the other for displaying humidity value. When the user sets the temperature and humidity reference value, the maximum or minimum value is displayed; when the system is in monitoring control, the currently detected temperature and humidity value is displayed.

Output Circuit

The system uses relay output to control the ambient temperature and humidity by changing the working state of the temperature control system or humidity control system. The system temperature and humidity control equipment mainly includes heating devices, ventilation devices, dehumidification devices and humidification devices. ULN2803 is an 8-fold Darlington inverting driver, whose input end is connected to the output port of the microcontroller, and the output end is connected to the input end of the relay. The relay uses the domestic AC solid-state relay (AC-SSR) H220D15, which uses photoelectric isolation inside to effectively avoid electromagnetic interference. When the microcontroller detects that the temperature value or humidity value exceeds the preset range, the I/O port of the microcontroller outputs a control signal to drive the relay, start the electrical appliance, and realize the control of the temperature control system or humidity control system.

System software design

The system software design adopts a modular structure, and each functional sub-block is independent, which is convenient for debugging. The main flow chart of the system is shown in Figure 3. As can be seen from the figure, the entire software is structurally divided into the main program, keyboard scanning program, LTM8901 control program, display program and output control program. The main program implements the initialization of each module and then enters the interrupt waiting. The main task of the keyboard scanning program is to determine which key is pressed, so as to execute the corresponding functional subroutine. The LTM8901 control program mainly detects the temperature and humidity values ​​of the environment and transmits the detection results to the microcontroller. The display program is used to display the user preset value or the detection value. The output control program mainly compares the detection value with the user preset value, and sets or resets the corresponding output port according to the comparison result, thereby controlling the working state of the relay.

The control program of LTM8901 mainly includes the following steps: send a start conversion command (negative pulse of 250~350μs); delay 850~1000ms for LTM01 conversion; send a host computer read command. When the microcontroller sends a read command 00H, read the humidity value, then delay 4.5~5 ms, send a read command 01H, and read the temperature value.

Conclusion

LTM8901 is a monolithic integrated digital temperature and humidity sensor. All signal processing is completed internally. It uses a "one-line bus" for communication and completes data transmission in digital form, saving the I/O port of the microcontroller and the A/D device, reducing costs. The system has a simple circuit structure, accurate detection, and good stability, and can achieve measurement and control of ambient temperature and humidity.