Experiment: LDR — Light Dependent Resistor Characterization

Objective

To investigate how the resistance of an LDR changes with illumination intensity and to determine the functional relationship between light intensity (lux) and LDR resistance.

Apparatus

• LDR (Light Dependent Resistor)
• Multimeter (resistance or voltage measurement)
• DC power supply or battery
• Fixed resistor (for voltage divider, optional)
• Light source (lamp, LED, or torch)
• Lux meter (or smartphone app)
• Breadboard and jumper wires
• Ruler or measuring tape
• Dark enclosure or blackout cloth (to control ambient light)

Theory

An LDR is a semiconductor device whose resistance decreases as the light intensity increases. The relationship is nonlinear and can often be expressed as:

R = a × L^b

where R is resistance (Ω), L is illumination (lux), and a, b are experimental constants. By plotting log(R) vs log(L), a straight line is obtained, from which a and b can be determined.

Data Table

Procedure

• Set up the LDR on a breadboard and connect it to a multimeter.
• Illuminate the LDR with a light source and measure the lux using a lux meter.
• Record the resistance for each illumination level in the table.
• Repeat measurements for different light intensities (by adjusting lamp distance or brightness).
• Plot Resistance vs Lux and also log(R) vs log(L) to analyze the power-law relationship.

Analysis

• Resistance decreases as illumination increases (inverse relationship).
• On a log-log plot, the data approximately forms a straight line.
• The slope gives the exponent b, while the intercept provides a.
• Report the fitted equation and calculate the coefficient of determination (R²) to check accuracy.