LDR - Light Depending Resistor

The working principle of a light-dependent resistor is based on the phenomenon of photoconductivity. Photoconductivity is the increase in the electrical conductivity of a material when it absorbs photons (light particles) with sufficient energy.

When light falls on an LDR, the photons excite the electrons in the valence band (the outermost shell of atoms) of the semiconductor material and make them jump to the conduction band (the shell where electrons can move freely). This creates more free electrons and holes (positive charges) that can carry electric current. As a result, the resistance of the LDR decreases.