Temperature, Temperature Sensor , Circuit of Temperature Sensor and Algorithm of temperature circuit

• Temperature

Temperature Definition:

• Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot.
  Quantitatively, temperature is measured with thermometers, which may be calibrated to a variety of temperature scales.
• temperature is different from heat, though the two concepts are linked. Temperature is a measure of the internal energy of the system, while heat is a measure of how energy is transferred from one system (or body) to another. The greater the heat absorbed by a material, the more rapidly the atoms within the material begin to move, and thus the greater the rise in temperature.

Effect of temperature on plants

Temperature has a great effect on plants, as very hot and very cold weather can prevent a plant from setting blooms, and it can even cause the plant to start  dropping foliage.

Plants produce maximum growth when exposed to a day temperature that is about 10 to 15°F higher than the night temperature. This allows the plant to photosynthesize (build up) and respire (break down) during an optimum daytime temperature, and to curtail the rate of respiration during a cooler night.

High temperatures cause increased respiration, sometimes above the rate of photosynthesis. This means that the products of photosynthesis are being used more rapidly than they are being produced. For growth to occur, photosynthesis must be greater than respiration.

Low temperatures can result in poor growth. Photosynthesis is slowed down at low temperatures. Since photosynthesis is slowed, growth is slowed, and this results in lower yields.

Problem  definition:

The main problem is how to provide  suitable  temperature for plants growing as there is climatic changes every time which affects on plants in passive manner.

Then the question which arises now how to  control the temperature?

Solving of the problem:

By designing greenhouse control system we can control some parameters which have agreat effect on plants grow and one of  the most effective factors on plants is temperature.

Controlling of temperature can be realized by using temperature sensor which provides accurate measures of temperature degree and according to this measures we can control the temperature and generating the right action that leads to maxmimum product of plant.

Types of tempreture sensors:

When selecting which type of sensor to use there a myriad of considerations that must be made depending specifically on the application.

 Additionally however when selecting a sensor it is important to consider the temperature range, the required accuracy and response time as these will vary with different measuring methods.

When accuracy is of prime importance an RTD sensor is preferable although for those with an eye on finances, thermocouples are clearly the cheaper, more affordable option. Hopefully the following break down of the RTD, thermocouple and infrared thermometer will help operatives make the decision.

The temperature sensor that we used in our system is LM35 General Description

The LM35 series are precision integrated-circuit temperature

sensors, whose output voltage is linearly proportional to the

Celsius (Centigrade) temperature.

The LM35 thus has an advantage

ü over linear temperature sensors calibrated in° Kelvin, as the user is not required to subtract a large constant voltage from its output to obtain convenient Centigrade scaling.

ü The LM35 does not require any external calibration or trimming to provide typical accuracies of ±1⁄4°C at room temperature and ±3⁄4°C over a full −55 to +150°C temperature range.

ü Low cost is assured by trimming and calibration at the wafer level.

ü The LM35’s low output impedance,linear output, and precise inherent calibration make interfacing to readout or control circuitry especially easy.

ü It can be used with single power supplies, or with plus and minus supplies.

ü As it draws only 60 μA from its supply, it hasvery low self-heating, less than 0.1°C in still air.

The LM35 is rated to operate over a −55° to +150°C temperature range,while the LM35C is rated for a −40° to +110°C range (−10°with improved accuracy).

The LM35 comes in many different packages, including the following.

·         TO-92 plastic transistor-like package,

·         T0-46 metal can transistor-like package

·         8-lead surface mount SO-8 small outline package

·         TO-202 package. (Shown in the picture above)

Why  lm35?

Features

ü Calibrated directly in ° Celsius (Centigrade)

ü Linear + 10.0 mV/°C scale factor

ü 0.5°C accuracy guaranteeable (at +25°C)

ü Rated for full −55° to +150°C range

ü Suitable for remote applications

ü Low cost due to wafer-level trimming

ü Operates from 4 to 30 volts

ü Less than 60 μA current drain

ü Low self-heating, 0.08°C in still air

ü Nonlinearity only ±1⁄4°C typical

ü Low for 1 mA loadimpedance output, 0.1

What Does An LM35 Look Like?

Circuit & equation:

·         How Do You Use An LM35?  (Electrical Connections)

o    Here is a commonly used circuit.  For connections refer to the picture above.

o    In this circuit, parameter values commonly used are:

§  V_c = 4 to 30v

§  5v or 12 v are typical values used.

§  R_a = V_c /10^-6

§  Actually, it can range from 80 KW to 600 KW , but most just use 80 KW.

o    Here is a photo of the LM 35 wired on a circuit board.

§  The white wire in the photo goes to the power supply.

§  Both the resistor and the black wire go to ground.

§  The output voltage is measured from the middle pin to ground.

The general equation used to convert output voltage to temperature is:

§  Temperature ( ^oC) = Vout * (100 ^oC/V)

§  So if  Vout  is  1V , then, Temperature = 100 ^oC

• The output voltage varies linearly with temperature

The Algorithm of temperature circuit:

·        The user enter the set point through the computer.

·        The temperature sensor comparing the temperature that it measures with the set point and performing an action which is turning heater or fan according to some condition.

·        If (set point +5) < temperature sensor then turn on fan with high speed.

·        Else If temperature sensor >set point & temperature sensor < (set point +5) then turn on fan with low speed.

·        Else If set point > temperature sensor & temperature sensor > ts1(set point - 5)  then turn on one led

·        Else If ts1(set point - 5)  > temperature sensor & temperature sensor >ts2(set point - 10)  then turn on two leds

·        Else turn on the three leds

·        End if.

The block diagram of the temperature circuit is as the following: