Build Your Own Light/Dark Detector Circuit – 100K Sensitivity

This compact circuit is capable of distinguishing between illumination and shadow, rendering it exceptionally valuable for controlling the activation and deactivation of indicators, exterior lighting, and various devices based on ambient light conditions.

Circuit diagram

Parts:

Notes:

R1 modifies the circuit’s sensitivity.

Integrated Circuits play a crucial role in this design. The **2N3904** is a general-purpose NPN bipolar junction transistor. It acts as a switch, amplifying small currents to control the relay. This transistor is frequently utilized in switching applications due to its robust performance and relatively low cost. It operates by controlling the flow of current between its collector and emitter terminals based on the current applied to its base terminal. The 2N3904’s ability to rapidly switch is vital for quickly responding to changes in light levels. Furthermore, its current gain characteristics enable it to handle the demands of the relay coil, ensuring reliable operation.