Build a Simple Motion Detector Circuit using 555 Timer to Control AC Loads

This motion detector circuit combines an IR sensor module with a 555 timer IC to control external loads through a relay switch. At the heart of the timing stage is the 555 timer IC, supported by a 1µF electrolytic capacitor and several resistors including two 220KΩ resistors and a 100KΩ resistor to establish its timing behavior. When the infrared (IR) sensor module detects motion in its field of view, it outputs a trigger signal. This signal interacts with the timing network to change the state of the 555 timer's output. The output feeds into a transistor stage consisting of a BC547 NPN transistor and a BC557 PNP transistor, which amplify the control signal to drive the SPDT relay. The 1N4007 diodes are placed across inductive paths to protect the active components from voltage spikes generated when the relay coil switches. Once energized, the relay acts as an electromechanical switch capable of handling higher-power loads. For visual feedback, an LED paired with a 220Ω current-limiting resistor lights up to indicate when the circuit is triggered. The entire assembly is powered via a DC jack on a perf-board, allowing the low-voltage control circuitry to safely interface with higher-voltage switching applications.