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1. Automatic Light
Operated Switch
Using WheatStone Bridge

2. Required Components :
• LDR
• Transistor (BC547)
• LM741op-amp IC
• Potentiometer (10k)
• Resistance (10k, 330ohm)
• Led (red)
• Battery (9v)
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3. LDR (Light Dependent Resistor)
 LDR is type of resistor whose resistance
vary with the strength of light fall over it.
 It’s made up of a semiconductor name
Cadmium sulphide.
 It simply means that the presence of
light decreases the resistance of
LDR and that is how it is used to predict
day and night.

4. Working of LDR
 The LDR works on the principle of photo conductivity,
 when the light falls on the surface of LDR then the resistance of LDR
start decreasing from a high value of it, in the dark resistance of LDR
is in range of Mega ohms and as the light incident on it resistance
decreases to a range of few ohms.
 The electrons in the valence band jumps to conduction band, due
to having high energy of photons in the incident light then the
semiconductor material.
 In dark the resistance is minimum one mega ohm.

5. Op amp IC LM741:
 operationalamplifier is an
integrated circuit that can
amplify weak electric signals.
 An operationalamplifier has two
input pins and one output pin. Its
basic role is to amplify and output
the voltage difference between the
two input pins.

6. Opamp consist of two inputs :
Inverting
Non-Inverting

7. Characteristics of Ideal Op-amp:
 Infinite Input Impedance
 Zero Output Impedance
 Infinite Voltage Gain
 Infinite Bandwidth
 Infinite Slew Rate
 Infinite Common Mode Rejection Ratio (CMRR )
Zero input offset voltage (Output is Zero when
input is Zero)

8. Transistor (BC547)

9. What is Wheatstone Bridge?
 A Wheatstone Bridge is essentially an electrical circuit designed to
compare resistances or measure the unknown value of a resistor’s
resistance by generating a balance between the bridge circuit’s two
legs.
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10. How wheatStone Bridge is used
Automation :
 Wheatstone bridge applications detect electrical and mechanical
parameters.
 In many electronic applications, balanced bridge circuits are used to sense
changes in light intensity, strain, or pressure.
 Potentiometers, LDRs, strain gauges, and thermistors are examples of resistive
sensors that can be utilised in a Wheatstone bridge circuit.
 The Wheatstone bridge is mainly used to measure the low resistance.
 To measure physical characteristics like temperature, light, and strain, a
Wheatstone bridge and an operational amplifier are utilised.

11. Circuit Diagram of Light Detector:

12. Working of Light Detector using
Wheatstone bridge
As we know in Wheatstone bridge, if the difference of voltage drop is zero between
point C and D the ratio of resistance R1 and R2 is equal to the ratio of resistance R3
and R4, where R4 is the unknown resistance, R1 and R2 are known resistors and R3 is
the potentiometer.

13. 13
 Here in our Light detector circuit diagram, Wheatstone Bridge is consists of
one LDR and potentiometer in the first arm and two known resistance of 10k
ohm in the second arm. As the light incident on the LDR, it resistance gets
low and the voltage through point C is increase in comparison with point D.
Here in our Light detector circuit diagram, Wheatstone Bridge is consists of
one LDR and potentiometer in the first arm and two known resistance of 10k
ohm in the second arm. As the light incident on the LDR, it resistance gets
low and the voltage through point C is increase in comparison with point D.

14. Disadvantages :
 If the Wheatstone Bridge is not balanced, it will fail.
 It can produce results rangingfrom a few ohms to a few mega-
ohms.
 The Wheatstone may fail if the quality of the galvanometeris
poor
 The results of testing large resistances are often non-linear.
 The resistance value can decline dependingon the temperature
and EMF applied.
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15. Applications and Uses :
 Street Light Automatic
 Sensor place
 Meters of the strength of light
 Circuits of spinning warning
 Used along with an LED barrier detector
 Automatic lighting for bedroom
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16. Thank you…
By:
Abinesh k
Ashvanth
Harish iyyapan
Akil
Harish kk