Simple LDR Circuit
Here is very simple circuit for activating a light in darkness using LDR and few components. Here LDR is used as sensor and Transistor is used as electronic switch.
Project Model based on LDR
Simple LDR Circuit Diagram
When light falls on the surface of LDR, the LDR changes its resistance. The more the light, the less the resistance. Resistance R1 (100K) and LDR is connected in series and makes a potential divider circuit. Divided voltage varies according to the intensity of the light. LDR is connected with Ground so voltage drop increases when light intensity increases. It responds to large part of light spectrum.
The centre point of the Potential Divider is fed to a NPN Transistor BC147/BC548. This general purpose NPN silicon transistor conducts when base voltage is approx 0.5V or above when compared to emitter voltage and circuit gets completed thus LED glows in darkness.
b) 100K Resistance
c) Transistor BC548
This Magic Eye circuit is very cheap and easy to make. Will cost you arround 20 Cents (Rs 15) only.
This LDR circuit can be used to make Automatic street lights, Electronic Letter Box, Electronic eye, Automatic Vehical headlights, Automatic Garden lights etc.