1) The Detection of Air Humidity and Temperature.
Firstly, we explain how to acquire the temperature and the humidity data of the vegetable crop from DHT22/AM2302 sensor. The circuit for wiring the DHT22/AM2302 sensor is shown in Figure 2. In case of sending a request for information, MCU send the start signal to the DHT22 for changing the AM2302 standby status to running status. After MCU has finished sent the start signal, the AM2302 will reply a response signal of 40-bit data that contains humidity and temperature data back to the MCU. the MCU must send the start signal to the AM2302, Otherwise AM2302 will not give the response signal to the MCU. However, the AM2302 will change running status to standby status when transmission has finished or if it does not receive the start signal from the MCU again. The communication process is shown in Figure 3. The second part, we present how the MCU retrieve data from the DHT22 with a single cable. The MCU use a single cable in order to adjust voltage HIGH (5 Vdc) and LOW (0 Vdc) with time step. When the DHT22 status is in standby, the state of the signal data pin is HIGH with Pull-Up. When MCU begin to read, It assign data pin as output pin, and then make start bit by send voltage LOW to sensor at least 800 ȝsec (Microsecond). Actually it should be 1 ms (1 Millisecond), and afterward the MCU changes the signal to HIGH voltage for 20 ȝsec (Microsecond) and waiting for a reply.