The power supply unit provides two DC voltages (5V and 12V ) for different uses. The image below shows a simple circuit to get 5v and 12v power from one circuit. The AC mains will be stepped down by a step-down transformer T1, rectified by a bridge rectifier B1, and filtered by a smoothing capacitor C1 to obtain a stable DC voltage level. IC 7805 voltage regulator for getting 5V DC. IC 7812 voltage regulator is used to obtain 12V DC.

In this system, the microcontroller is programmed in such a way that it always searches for input from sensors. If the microcontroller is interrupted by a signal from any sensor, a command will be sent to the user mobile station via the GSM MODAM by sending an SMS. The LCD is connected to the microcontroller and displays the state of the MODEM, the state of the sensor. For example, if the microcontroller initializes the MODEM, if any, the MODEM initialization fails and the user doesn't know what's going on in the system, so we use the LCD to display the status.
We use GSM / GPRS 900 a-ic/" title="SIM900A product parameters, documentation and supply information" target="_blank">SIM900A modem works on the frequency 900 / 1800MHz. The modem has an RS232 interface and can be connected to PC and microcontroller with RS232 chip ( MAX232 ). The baud rate is configured by AT commands from 9600-115200. It has an onboard regulated power supply. Using this MODEM, we can make voice calls by simple AT commands, Text messages, read text messages, answer incoming calls and surf the Internet, etc.
Dual frequency GSM/GPRS 900/1800MHz. Configurable baud rate. SIM card holder. Built-in network status LED. Built-in powerful TCP/IP protocol stack for internet data transmission via GPRS.
The important role of GSM modems in GSM-based industrial automation security systems: Today everything will be automated through smart MODEM technology, with which we can access any device remotely. The GSM MODEM is using AT commands to communicate with the microcontroller, for example, if we want to send an SMS to the A number 98xxxxxxxx, the command we want to send is AT+CMGS="<98xxxxxxxxxx>",,,.
The RS-232 is a communication cable typically used to transmit and receive serial data between two devices. In this project, it provides communication between MODEM and microcontroller. A smoke detector device is a smoke sensing device that receives an indication of a fire. Smoke detectors are useful in homes, offices, schools and industry. The MQ-2 sensor detects combustible gases and smoke at concentrations between 300ppm and 10,000ppm.
This can be easily connected to a microcontroller. Whenever the smoke detects any smoke/fire, it sends a signal to the microcontroller and the MC then sends a text message to the user's phone. An LDR is a device with a variable resistance that changes with the intensity of light falling on it. This feature allows them to be used in light sensing applications. In this project, we can use this sensor for the purpose of switching lights on and off.