Control System Lab Activity #1: Characterization and Applications of LDR
A light dependent resistor (LDR) or photoresistor is a resistor whose resistance is inversely
proportional to the incident light intensity.
Part I. Construct the circuit shown on figure 2. Use a tungsten lamp as your light source. Use a
power supply as your voltage source. Use an ohmmeter to measure the resistance of the LDR.
Enclose the circuit so that no other light source will affect the LDR. Complete the table below and
plot your result (Resistance vs Voltage). The voltage readings are proportional to your light
intensity. Describe/characterize the plot.
Voltage supply applied to light
source in volts
2.0 V
2.5 V
3.0 V
3.5 V
4.0 V
4.5 V
5.0 V
5.5 V
6.0 V
6.5 V
7.0 V
7.5 V
8.0 V
8.5 V
9.0 V
Resistance of LDR in
kohms
Part II. Construct a light sensor.
1. Make a circuit using the LDR characteristics such that when light intensity is minimal, an LED
will turn on else it is turned off.
2. Make a circuit using the LDR characteristics such that when light intensity is minimal, an LED
will turn off else it is turned on.
There are many examples of the above circuit. Two circuits using transistors are given below. The
circuits below are simple examples of a control system (On-Off control) using an LDR as sensor
(light intensity as process variable) and an LED as the device being controlled. Other circuits use a
comparator (LM311). Get full credits if your design uses a comparator.