SENSORS

SENSORS

In this new Farm automation system, we are using four different sensors to sense the parameters such as temperature, light, soil moisture and atmospheric humidity. The detailed descriptions of these sensors are given below.

3.1 TEMPERATURE SENSOR

We are using IC LM35 for temperature sensing. The LM35 series are precision integrated circuit temperature sensors, whose output voltage is linearly proportional to Celsius temperature. The LM35 has an advantage over linear temperature sensors calibrated in Kelvin, as the user is not required to subtract a large constant voltage from its output to obtain its equivalent centigrade scaling. The LM35 does not require any external calibration or trimming to provide accuracy of +/- 25 ⁰C at room temperature. Low cost is assured by trimming and calibration at water level. The LM35 is low output impedance, linear output, and precise inherent calibration make interfacing to readout.

Features are:

·       Calibrated directly in Celsius

·       Linear =10.0 mv/c scale factor

·       Suitable for remote application

·       Low cost due to water level trimming

·       Operation from 4 to 30 volts

·      
Low impedance output

Fig 3.1.1: Basic centigrade Temperature Sensor

                                       

     Fig 3.1.2: Pin diagram of LM35

3.2 HUMIDITY SENSOR

A humidity sensor also called a hygrometer, measures and regularly reports the relative humidity in the air. This means that it measures both air temperature and moisture. Humidity sensor gives regular, ongoing readings of the relative humidity, so they are usually used for data collection in the weather stations.

The humidity sensor SY-HS-220 can operate up to the range of 95% RH (Relative Humidity). Humidity sensor module itself contain the signal conditioning unit and the voltage out can take out through the connectors.

Fig 3.2.1: SY-HS-220 Humidity sensor

We know that the level of humidity in the air is also a function of temperature. Excess humidity can cause growth of fungus. Too little humidity can cause static discharge or accumulation of unwanted dust, contributing to allergies.

Here we use a humidity sensor known as SY-HS-2 and the module is SY-HS-220 series which produces more accurate and linear voltage output. This is a polymer humidity sensor. The voltage output is directly amplified and given to the LabVIEW to monitor the value of humidity in the air. It is clear that the output voltage indicated is proportional to the intensity of atmospheric moisture content.

Features:

1. Humidity sensor module with voltage output

2. Wide temperature compensation range

3. High reliability and long term stability

4. Linear dc voltage output for humidity range

5. High sensitivity and low hysteresis

6. Compact size and cost effectiveness

These features of the sensor make it more applicable in the measurement field. One of the main advantages is that it has high sensitivity, so that it will respond to the slight change in humidity.

3.2.1 CONNECTOR PIN CONFIGURATION:

Fig 3.2.1.1: Pin configuration for SY-HS-220

   Fig 3.2.3: Relative humidity (%RH) Vs Output voltage (V) Characteristic Graph

3.3 LIGHT SENSOR

The light sensor is made using an LDR (Light Dependant Resistor). The resistance of the LDR varies according to the intensity of light falling on the surface. This change in resistance is connected with a whetstone’s bridge arrangement and the corresponding change in voltage is amplified and given to the PC with LabVIEW through DAQ to monitor it.

Fig 3.3.1: Light Dependent Resistor (LDR)

3.3.2: light intensity Vs. resistance characteristics graph

LDR consists of two Cadmium Sulphide (CdS) photoconductive cells with spectral responses similar to that of the human eye. The cell resistance falls with increasing light intensity.

Features:

·       Wide spectral response

·       Low cost

·       Wide ambient temperature range.

Applications:

·       Smoke detection

·       Automatic lighting control

·       Batch counting

·       Burglar alarm system

3.4  SOIL MOISTURE SENSOR

The existing mechanism to measure the soil moisture content is costly and difficult to use. So we turned to design a soil moisture sensor which is cheap and gives excellent performance. It is made up of two electrodes and a very little quantity of plaster of Paris.

       We know that gypsum is a material which shows water absorbing property and are users in medicine packing fields to avoid the presence of humidity. Depending on the water content in the soil the absorbing rate varies. If we insert an electrode, the conduction through electrode varies with content of water absorbed by gypsum. Or in the words the resistance of conductor varies. But the problem is that gypsum is not a commonly available material. The plaster of Paris is a material which shows the same property and it’s derived from gypsum. When gypsum is heated to 150 ⁰c, it’s converted into plaster of Paris. The absorbing is not changed while changing into this. And we can mould any shape using this plaster of Paris, which makes it more suitable for our use.

       For the ease in use of the sensor we made it in a cylindrical shape. The construction of such a sensor is as follows. Two electrodes of stainless steel of size 6cm * 0.5cm * 0.05cm is selected. It is placed 15mm apart in a cylindrical tube. To avoid any kind of disturbances fix the electrodes together by using some adhesives. And it should be noted that there should not be any conduction through this contact. The adhesive used should be a non-conducting one. Now mix the plaster of Paris with water and make it to a paste form. For two table spoons of plaster of Paris, pour one table spoon of water. This paste is then poured into the cylindrical tube which contains the two electrodes attached together. Then it is allowed to get dried. Then remove the cylindrical cover and solder wires to the electrodes to take the output. Now the sensor is ready.

       Insert this sensor in the soil in which the soil level covers the whole plaster of Paris in the sensor. Before that, calibrate the sensor for dry and wet conditions of the soil. Then we can measure the change in resistance corresponding to the soil moisture content in the soil. This change in resistance is then connected to a whetstones bridge and the corresponding voltage output is amplified and given to the PC with LabVIEW software to monitor the water content in the soil.

Electrodes

Plaster of Paris

Connection Leads

 

Fig 3.4.1: Soil moisture sensor