In today’s industrial and agricultural production, more and more occasions need to collect temperature and humidity, so it is very important to measure the temperature accurately and conveniently. The traditional wired temperature measurement method has some problems, such as complex wiring, easy aging of the line, difficult troubleshooting of the line, re wiring of equipment layout and so on. Especially in the case that the cable network is not unobstructed or it is inconvenient to set up the line due to the limitation of the site environmental factors, it brings great trouble to the data collection of temperature and humidity. In order to monitor the real-time temperature and humidity data, it is necessary to collect, send and receive the data by wireless transmission, and process the data collected by wireless through the host computer, so as to control and monitor the operation of the equipment, and reduce the unnecessary expenditure of line equipment.
Design of temperature and humidity acquisition circuit
DHT11 is a temperature and humidity composite sensor with calibrated digital signal output. The sensor uses special digital module acquisition technology and temperature and humidity sensing technology, with high reliability and excellent long-term stability. Figure 2 shows the temperature acquisition circuit. DHT11 sensor consists of a resistive humidity sensor and a NTC temperature sensor, which can be connected with high-performance 8-bit MCU. Calibration coefficients are stored in OTP memory in the form of program, which can be called in the process of signal detection inside the sensor. The single line serial interface makes the system integration simple and fast, and the signal transmission distance can reach more than 20m. When the length of connecting line is less than 20m, 5K Ω pull-up resistance shall be used; when it is more than 20m, appropriate pull-up resistance shall be used according to the situation.
Design of wireless transmitting and receiving circuit
NRF24L01 is a wireless communication chip produced by Nordic company. It adopts FSK modulation mode and integrates Nordic’s own enhanced short burst protocol. Can achieve point-to-point or 1 to 6 wireless communication. The wireless communication speed can reach 2 Mb / s. The circuit schematic diagram of Nordic wireless transmitting and receiving chip nRF24L01 is shown in Figure 3.
NRF24L01 is a single RF transceiver chip, which works in 2 . four ~ two . The frequency synthesizer, power amplifier, crystal oscillator and modulator are built in the chip. The output power and communication channel can be configured by program. The energy consumption of nRF24L01 is very low – When the power of 5 DBM is transmitted, the working current is only 10 . 5 mA, the working current is only 18 Ma when receiving, it has a variety of low-power working modes, energy saving and environmental protection, convenient design. The pin functions of nRF24L01 wireless transceiver module are listed in Table 1, and the circuit diagram of nRF24L01 connected with MCU is shown in Figure 4.