 Projects PROJECT BASICS POWER SUPPLY SENSORS INTRODUCTION LIGHT SENSOR LDR PRESSURE SENSOR RELAY CURRENT BUFFER BIDIRECTIONAL CURRENT BUFFER PARALLEL PORT LEVEL SHIFTER WIRELESS COMMUNICATION PROJECT IDEAS

LIGHT DEPENDENT RESISTANCE (LDR):

The LDR is made up of cadmium sulphide and when light falls on this material, it triggers out extra charge carriers and hence conduction of material increases & resistance falls.  The relation between the light intensity (Illuminance) and the resistance is shown as below: The LUX is the unit which is used to measure brightness of light.

The sunlight corresponds to about 50,000 lux while artificial light corresponds to 500-1000 lux.

HOW TO USE: As we have to detect the change in resistance, we use the sensor in series with another resistance and apply voltage across them. We have applied 9V across the sensor and the resistance.

HOW IT WORKS: Suppose the resistance of LDR in dark is 5K ohm and resistance of LDR in light is 200Kohm. Now we see how we detect the resistance change:

Now we see the output voltage in the above 2 cases:

Equations (for 9V):

In the light: In the dark: Hence we can easily detect that when ever, there is light output voltage would be 8.57V and 0.43 otherwise.

WHERE TO USE: We can use the above sensor in various applications. Some of them are as follow:

• Line follower
• Obstacle avoider
• Wall follower
• Detecting the entry of a person in a room
• Counting the number of persons in the room (by detecting both entry & exit)

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