please guide me experts.

Asked by Ayushi Tarey | 13th Dec, 2012, 07:35: PM

Expert Answer:

Light Dependent Resistor


This type of type of transducer is used to convert change in the light intensity into its equivalent electrical signal. When an LDR is exposed to light its resistance decreases, and when it is dark its resistance becomes very high more than 1MW  (Typically when an LDR is subjected to any colour of light, its resistance drops). The drop in its resistance is directly proportional to the intensity of light incident on it. An LDR is made up of the light sensitive material like CdS – cadmium sulphide or Selenium. It is the property of CdS that when it is dark, all the valance electrons in it are tightly bonded with their parent atoms, and whenever it is exposed to light, the light energy makes these electrons FREE to increase the conductivity of the material. Now in an LDR, the CdS is fitted in a glass envelop to protect it from dust. Its arrangement is made in zigzag style so that more long path can be produced in a small area. The terminals of these zigzag paths (these paths are actually thin wires of CdS) are taken out from the glass enclosure for external connections. When the LDR is exposed to light the free electrons are given out from the CdS and its conductivity increases. However when it is dark, these free electrons are bonded back to their parent atoms and the CdS then behaves like an insulator. So in light it can carry current and in dark it cannot. The schematic & constructional diagrams of an LDR are given below –





Circuit diagram –


Observation table –




Transistor (T1)

Transistor (T2)



Vcc =

Vbe =


Vce =

Vbe =


Vce =



Procedure –

1)     Solder the circuit on the given tag board, with hot soldering iron.

2)     Connect power supply to the circuit and test the circuit under different light conditions.

3)     Check that when light falls on LDR, the relay activates and LED glows.

4)     When LDR is dark, the relay deactivates and the LED goes off.

5)     For proper sensitivity of the circuit, adjust the sensitivity pot in the circuit.

6)     Measure voltages at base and collector of transistors in on and off conditions of relay.

7)     From above voltages of transistor voltages, comment whether the transistors are in cutoff or saturation or in active region.

8)     Switch off the circuit and disconnect the LDR from the circuit.

9)     Measure the light and dark resistance of the LDR using DMM.

10)  Note down the values of LDR, dark resistance _____W, light resistance ____W.

11)  Disconnect the relay coil from the circuit. Measure its resistance using DMM. Note it down as relay resistance = ______W.





Specifications of components –


Type of the component




BC 148/BC 548

SK 100







1N4001, silicon rectifier diode


Small signal amplifier,  silicon NPN transistor, (Gain=100)

Medium power silicon PNP transistor, (Gain = 50)

1000mF/25V electrolytic capacitor

1kW series resistor, carbon type

Dark resistance _____W, light resistance ____ W.

6V, _____ W, single changeover type relay coil

470kW, carbon composition type

230V/6V, 500mA step down type



Brief theory – it is a passive light transducer. It is also called as photoconductive cell because its conductivity changes due to change in light intensity.

When light falls on it its resistance decreases and when it is dark its resistance is maximum. The change in resistance is directly proportional to intensity of light falling on it. 

It is made up of photosensitive material like cadmium sulphide (CdS), Selenium (Se), Cadmium Selenide (CdSe) or Lead Sulphide (PbS). It is deposited on insulating surface like ceramic substrate in the form of zigzag wire as shown in following figure. It is enclosed in round metallic or plastic case and two electrodes are taken out for external connections. The structure is covered with glass sheet to protect it from moisture and dust and allows only light to fall on it.





Answered by  | 14th Dec, 2012, 01:43: PM

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