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Temperature Sensor
by admin | Jan 1, 2018 | Electronics Projects, Heat Sensor, Sensor Circuit | 0 comments
This project uses IC LM35 as a sensor for detecting accurate centigrade temperature. Linearity
defines how well over a range of temperature a sensor’s output consistently changes. Unlike thermistor, Linearity of a precision IC Sensors are very good of 0.5°C accuracy and has wide temperature range. its output voltage is linearly proportional to the Celsius (Centigrade) temperature. The LM35 is rated to operate over a -55° to +150°C temperature range.It draws only 60 μA from its supply, it has very low self-heating, less than 0.1°C in still air. LM35 Operates from 4 to 30 volts.
Output of IC is 10mv/degree centigrade for eg if the output of sensor is 280 mV then temperature is 28 degree C. so by using a Digital multimeter we can easily calculate the degree temperature. For trigger point you should set the voltage of pin 2 of IC 741 by using preset or potentiometer. Our aim of this project is not to construct a thermometer but to activate or deactivate a device at a particular margin temperature. For simplicity we have used 2 LED for indication of both low (Green) and high (Red) temperature.
Working: The output of IC2 increases in proportion to the temperature by 10 mV per degree.
This varying voltage is feed to a comparator IC 741 (OP Amplifier). OP Amplifier are among the most widely used electronic devices today.The op-amp is one type of differential amplifier. It has two input inverting (-) and non-inverting (+) and one output pin. We have used IC741 as non- inverting amplifier which means pin 3 is the input and the output is not reversed. This circuit amplifies the difference between its input terminals.
As a comparator, Bistable output of an op amplifier is as follows :-
Part list:
IC LM35, IC LM Resistance: 10K Ohms, 470 Ohms X 2Pcs Preset or P.O.T of 2K Ohms LED 2pcs (Red and Green) 9V Battery with Snap Switch, wire *By making this Temperature Sensor Project, student will be capable of making many similar project i.e Automatic room heater controller, determine hotness of Tea or Coffee to avoid burning your tongue, Automatic Fan Controller etc.
You may also be Interested in our experiment: Making a Digital Thermometer.
Temperature Sensors
The most commonly used type of all the sensors are those types of sensors which detect Temperature or heat.
These types of temperature sensor vary from simple ON/OFF thermostatic devices which control a domestic hot water heating system to highly sensitive semiconductor types that can control complex process control furnace plants.
We remember from our school science classes that the movement of molecules and atoms produces heat (kinetic energy) and the greater the movement, the more heat that is generated. Temperature Sensors measure the amount of heat energy or even coldness that is generated by an object or system, allowing us to “sense” or detect any physical change to that temperature producing either an analogue or digital output.
There are many different types of Temperature Sensor available and all have different characteristics depending upon their actual application. A temperature sensor consists of two basic physical types:
- (^) Contact Temperature Sensor Types – These types of temperature sensor are required to be in physical contact with the object being sensed and use conduction to monitor changes in temperature. They can be used to detect solids, liquids or gases over a wide range of temperatures.
- Non-contact Temperature Sensor Types – These types of temperature sensor use convection and radiation to monitor changes in temperature. They can be used to detect liquids and gases that emit radiant energy as heat rises and cold settles to the bottom in convection currents or detect the radiant energy being transmitted from an object in the form of infra-red radiation (the sun).
point, and the slower “creep-action” types that gradually change their position as the temperature
changes.
Snap-action type thermostats are commonly used in our homes for controlling the temperature set point of ovens, irons, immersion hot water tanks and they can also be found on walls to
control the domestic heating system.
Creeper types generally consist of a bi-metallic coil or spiral that slowly unwinds or coils-up as the temperature changes. Generally, creeper type bi-metallic strips are more sensitive to temperature changes than the standard snap ON/OFF types as the strip is longer and thinner making them ideal for use in temperature gauges and dials etc.
Although very cheap and are available over a wide operating range, one main disadvantage of the standard snap-action type thermostats when used as a temperature sensor, is that they have a large hysteresis range from when the electrical contacts open until when they close again. For example, it may be set to 20oC but may not open until 22 oC or close again until 18oC.
So the range of temperature swing can be quite high. Commercially available bi-metallic thermostats for home use do have temperature adjustment screws that allow for a more precise desired temperature set-point and hysteresis level to be pre-set.
The Thermistor
The Thermistor is another type of temperature sensor, whose name is a combination of the words THERM-ally sensitive res-ISTOR. A thermistor is a special type of resistor which changes its physical resistance when exposed to changes in temperature.
Thermistor
Thermistors are generally made from ceramic materials such as oxides of nickel, manganese or cobalt coated in glass which makes them easily damaged. Their main advantage over snap-action types is their speed of response to any changes in temperature, accuracy and repeatability.
Most types of thermistor’s have a Negative Temperature Coefficient of resistance or (NTC) , that is their resistance value goes DOWN with an increase in the temperature, and of course there are some which have a Positive Temperature Coefficient, (PTC) , in that their resistance value goes UP with an increase in temperature.
Thermistors are constructed from a ceramic type semiconductor material using metal oxide
technology such as manganese, cobalt and nickel, etc. The semiconductor material is generally formed into small pressed discs or balls which are hermetically sealed to give a relatively fast response to any changes in temperature.
Thermistors are rated by their resistive value at room temperature (usually at 25oC), their time
constant (the time to react to the temperature change) and their power rating with respect to the current flowing through them. Like resistors, thermistors are available with resistance values at room temperature from 10’s of MΩ down to just a few Ohms, but for sensing purposes those types with values in the kilo-ohms are generally used.
Thermistors are passive resistive devices which means we need to pass a current through it to produce a measurable voltage output. Then thermistors are generally connected in series with a suitable biasing resistor to form a potential divider network and the choice of resistor gives a voltage output at some pre-determined temperature point or value for example:
Temperature Sensors Example No
The following thermistor has a resistance value of 10KΩ at 25oC and a resistance value of 100Ω
at 100oC. Calculate the voltage drop across the thermistor and hence its output voltage (Vout) for both temperatures when connected in series with a 1kΩ resistor across a 12v power supply.
At 25 oC
At 100 oC
By changing the fixed resistor value of R2 (in our example 1kΩ) to a potentiometer or preset, a voltage output can be obtained at a predetermined temperature set point for example, 5v output at 60oC and by varying the potentiometer a particular output voltage level can be obtained over
a wider temperature range.
It needs to be noted however, that thermistor’s are non-linear devices and their standard resistance values at room temperature is different between different thermistor’s, which is due
mainly to the semiconductor materials they are made from. The Thermistor , have an exponential change with temperature and therefore have a Beta temperature constant ( β ) which can be used to calculate its resistance for any given temperature point.
