DIY Arduino-Based Night Security Light System

The DIY Arduino-Based Night Security Light System operates by combining environmental light sensing with passive infrared motion detection to automate a high-voltage output stage. At the core of the circuit, an Arduino Uno continuously monitors inputs from two primary sensors: a Light Dependent Resistor (LDR) voltage divider circuit and a PIR motion sensor. The LDR acts as a photodetector, changing its resistance inversely with ambient light levels, which allows the Arduino to determine whether it is currently daytime or nighttime. Simultaneously, the PIR motion sensor detects changes in infrared radiation emitted by moving bodies within its field of view. When ambient light drops below a programmed threshold (indicating nighttime) and the PIR sensor registers motion, the Arduino changes its digital output state to trigger the actuation stage. Because the low-voltage digital output pins of an Arduino cannot directly drive high-power electromechanical or high-voltage loads, the signal is routed through a 1k resistor into the base of a BC547 NPN transistor configured as a solid-state switch. When the transistor conducts, it energizes the coil of a 12V 1C/O relay, allowing the relay contacts to safely switch a 60W 230V AC incandescent bulb. A 1N4007 flyback diode is placed in reverse bias across the relay coil to suppress voltage spikes and protect the BC547 transistor from inductive kickback when the coil is de-energized. Concurrently, a piezo buzzer is driven to provide an audible alert. This integration of digital logic, discrete transistor switching, and optical and motion sensing creates a fully automated, responsive intrusion detection system.