Circuit for regulated power supply required for the project is shown in Fig. 2. It is built around 12V-0-12V, 750mA step-down centre-tapped transformer, full-wave rectifier comprising two 1N4007 diodes (D1 and D2), 5V regulator 7805 (IC1), and two electrolytic capacitors: 1000µF, 25V (C1) and 100μF, 25V (C2).
The main circuit, shown in Fig. 3, is built around soil moisture sensor module DHT11, STM32F103 board known as Blue Pill, LM016L LCD, ESP8266 board, and a few other components. The power requirement of this project is 5V for STM32 and 3.3V 3.6V for ESP8266 Wi-Fi module.
5V is converted to 3.6V through the potential divider circuit formed by silicon diodes D3 and D4 and the 1k resistor connected in series with them. Each silicon diode's cut in voltage is 0.7V. So, the two diodes in series drop the voltage by 1.4V and we get 5V-1.4V = 3.6V.
As shown in Fig. 3, the DHT11 sensor is used to measure temperature and humidity. The sensor comes with a dedicated in-built NTC thermistor to measure temperature. It has an 8-bit microcontroller on-board to output the values of temperature and humidity as serial data through one-wire protocol. The sensor has only one data pin through which both temperature and humidity values can be read, thus saving pins on the microcontroller side. The sensor is factory calibrated and easily interfaces with a microcontroller. Pin details of DHT11 are shown in Fig. 4 and the STM32F103C8T6 (Blue Pill) is shown in Fig. 5.
Voltage regulator for STM32 can be replace by 5V battery or 5V DC adaptor. The STM microcontroller is used for getting the sensor data and displaying it on LCD and also sending the data to the Wi-Fi board. The ESP receives the data and uploads it to ThingSpeak cloud. But before that we need to set up the ThingSpeak and get the API token to incorporate it in the code.
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