LDR Plus GSM Based Security System

The LDR Plus GSM Based Security System operates by monitoring changes in ambient light levels inside an enclosed space, such as a drawer, safe box, or secure cabinet. At the heart of the circuit is an Arduino Uno board, which continuously reads the analog voltage output from a Light Dependent Resistor (LDR) divider network. When the secure enclosure is closed, the interior remains dark, causing the resistance of the LDR to increase significantly and resulting in a specific voltage reading across its terminals. If an unauthorized person opens the drawer or cabinet, ambient room light floods the interior, striking the surface of the LDR. This causes its resistance to drop sharply, altering the voltage signal sent to the Arduino's analog input pin. Upon detecting this threshold voltage change, the microcontroller executes its embedded program logic. It simultaneously triggers an onboard buzzer to emit an audible alarm and commands a SIM800L GSM module via serial communication. Powered by a compatible battery or external USB source, the SIM800L module utilizes an inserted 2G SIM card to immediately transmit an SMS text message alert to a pre-programmed mobile number. The Adafruit FONA library handles the necessary serial AT commands required to interface smoothly with the cellular network, ensuring reliable remote notification of the security breach.