Arduino-Based Shadow Alarm

The Arduino-Based Shadow Alarm is an electronic security system that detects changes in ambient light to trigger an alert. At the heart of the circuit is a light-dependent resistor (LDR), whose electrical resistance changes depending on the amount of light falling on its surface. When an object passes over the LDR, it casts a shadow, reducing the light intensity and causing a measurable voltage shift at the analog input of the Arduino Uno board. The Arduino continually monitors this analog voltage. When the voltage drops past a pre-programmed threshold—indicating that a shadow has been cast—the microcontroller changes the state of its digital output pin. This output pin controls a BC547 transistor, which acts as a solid-state switch. Because the microcontroller's digital pins cannot supply enough current to directly drive heavier external electromechanical components, the transistor amplifies the control signal to activate a 9V relay. The relay acts as an electromagnetic switch to handle the higher-power circuit driving the piezobuzzer, producing a loud audible alarm. The system can be powered continuously using a 9V DC adapter or made portable with a 9V PP3 battery, while onboard capacitors help stabilize the power supply voltage against sudden fluctuations. A physical power switch allows the user to turn the entire unit on and off conveniently.