# Sensors

If system is to be made interactive with the user, then there is big role of sensors. A sensor is a device whose physical property changes in response to the excitations made by the user. But to detect these physical changes by the circuit, we need to convert those changes into electrical signals. The devices which convert the excitation into the electrical signals are transducers. The sensors are also part of transducers. The following are the sensors which are usually used in the application circuits:

1. Light sensor
2. Temperature sensor
3. Sound sensor
4. Piezo-electric sensor

LIGHT SENSOR

The light sensor is used to detect the presence of light. The light sensors are of two types:

• Photovoltaic sensor
• Photo Conductive sensors

The photovoltaic sensors are those which produce electricity when light fall on them. Their response time is less but the relation between voltage produced and the light intensity is proportional while the photo conductive sensors are whose resistance changes when light falls on them. Their response time is less but the relation between the resistance and the light intensity is linear. Hence it is easier to manipulate photo conductive light sensors e.g. LDR.

Next we discuss some of those sensors:

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:

1. Line follower
2. Obstacle avoider
3. Wall follower
4. Detecting the entry of a person in a room
5. Counting the number of persons in the room (by detecting both entry & exit)

PIEZOELECTRIC SENSOR

This sensor is used to detect the pressure. This sensor is based upon piezoelectric effect. The word piezo is a geek word for pressure. When a pressure is applied to a piezoelectric material, it causes a mechanical deformation and a displacement of charges. Those charges are proportional to the applied pressure. Hence due to movement of charges, we get the electrical output. Now we consider the output voltage and pressure or force applied:

When we apply some pressure to this sensor, we get a voltage output but if we applied force remains same then, the voltage starts dropping as shown above. Hence we must detect the voltage produced at the proper time to effectively use the sensor.

HOW TO USE: The output voltage of the piezoelectric sensor is quiet small in the range 2 to 10 mV. Hence to use this in the circuits, we use an amplifier to amplify the output voltage.

HOW IT WORKS: The output of the sensor is amplified to such a value that it can be used in the circuit. The amplified voltage level is dependent on the application. We need to amplify the voltage to about    3-5 V so that it can be detected by the micro-controller.

WHERE TO USE: We can use the piezoelectric sensor in many application where we have to detect the pressure. Some of the applications can be:

1. Electric buzzer
2. Detecting the entry of a person (by placing the sensor below some base at the door entry).

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