Only of the many sensors available, and one used in many everyday settings is a temperature sensor.

Notably the LM35 sensor.

It is a three terminal integrated chip whose output voltage varies proportionally to the temperature.

The LM35 is commonly used to measure the ambient temperature of a room, or enclosed space. 

But, can a LM35 measure body temperature? Yes, a LM35 can be used to measure body temperature as well. To measure your body temperature using the LM35, you will need to ensure there is contact between the LM35 and some part of your body, like your finger, wrist, head etc.  

Closer look at the LM35

Let’s take a quick look at the LM35 which will give us a better understanding of the question at hand. 

Temperature sensing is crucial for many areas of everyday life.

The heater you may have in your room has a temperature sensor which enables it to maintain the temperature you have set so you can be comfortable during those cold winter nights.

Another crucial application is your car. Temperature sensors are used to make sure the car does not overheat. 

The LM35 is a temperature sensor that enhances the ability of the overall system it is part of by providing it with temperature information.

The LM35

Below is the LM35

The great thing about the LM35 is that it does not require additional circuitry to function. 

The LM35 is an analog device. What this means is that it’s output voltage changes proportional with temperature. 

How much it’s voltage changes according to temperature comes down to the sensor’s sensitivity. 

The LM35’s sensitivity is 10mV/degree celsius. 

So, for example, if a voltage of 300mV appears at its output, the temperature in celsius is 30 degrees (300mV x 10mV).

This temperature is provided in Centigrade (celcius). If you need to convert celsius to fahrenheit, you will need to convert this number in your code. 

Temperature of the LM35

The temperature range of the LM35 is determined by two configurations which can be seen below;

As you can see, the two different configurations yield different temperature ranges for the LM35.

If you do not require to measure temperatures below 0, Configuration A is a better suited option.

However, if you require to sense negative temperatures, the setup in Configuration B will be necessary. 

Contact vs Non-contact temperature sensors

Since we are discussing measuring body temperature, a note needs to be made of the different types of temperature sensors available. 

There are primarily two types of sensors when it comes to measuring the temperature of an object; Contact and Non-Contact. 

Contact temperature sensors as their name suggests need to come in contact with an object in order to measure its temperature. 

Whereas, non-contact (again as the name suggests), does not need to come in direct contact with an object to measure its temperature. 

Non-contact temperature sensors use infra-red sensors which detect the object’s temperature from a distance. 

Can a LM35 measure body temperature?

The LM35 is a temperature sensor which is best suited for applications that measure the ambient temperature.

Ambient temperature is the air temperature surrounding an object or environment where you might store equipment. 

So, like the heater example at the start, the LM35 would be used to measure the air temperature of the room. 

However, it can be used to measure body temperature as well, but direct contact will be needed. 

This is because the LM35 does not have the ability to measure an object’s temperature from a distance.

Placing the LM35 near the body will not give an accurate reading of its temperature so direct contact will be a more effective method. 

Response time of an LM35 measuring body temperature

The time it takes a sensor to react to a change in temperature is important.

A temperature sensor that lags behind changes in temperature is not going to give an accurate representation as the temperature might not match the particular time stamps. 

Also, you might require it to give instantaneous updates. So, the lower the response time the better. 

The LM35 has a relatively fast response time if it is already at ambient temperature and the change in temperature is gradual.

However, an abrupt change in temperature could increase the response time of the LM35 up to 2 minutes. 

When it comes to body temperature, your main concern should be the accuracy of temperature reading. So, the response time of the LM35 shouldn’t be much of an issue. 

Is the LM35 the best option to measure body temperature?

While you can use a LM35 to measure body temperature, a question to ponder is whether it is the best option.

A couple of downsides of using the LM35 to measure body temperature are its response time, and if it is measuring an object’s temperature (like the body) it will need to come in contact with it. 

A non-contact temperature sensor (that uses infra-red) would be more ideal for measuring body temperature. 

That way you do not have to always place the sensor on your body. However, these types of sensors are a bit expensive.

Another great alternative to the LM35 is a Thermistor. 

A thermistor is a negative thermal coefficient (NTC) sensor used to measure surface temperature. 

They are commonly used in medical applications as adult rectal, pediatric rectal, and skin probes. So, this would be another great option as a body temperature sensor. 

Benefits of using a LM35 to measure body temperature

However, the humble LM35 has many benefits.

You might be low on funds and need a cheap way to measure body temperature. Well the LM35 is a great option as it is very inexpensive, costing around $1.50.

It does not require extensive knowledge to get up and running. 

The setup for the LM35 is as easy as soldering wires to its terminals. It does not require any other components (other than a resistor for negative values) to operate.

There is no need for calibration. Once you have connected the LM35, it is good to go to measure your body temperature. 

The last benefit is that it is readily available. Your local electronic store will no doubt have the LM35 in stock.

LM35 circuit to measure body temperature

A great way to use the LM35 to measure body temperature, is to use it in conjunction with a microcontroller (like an arduino).

The microcontroller will need to have an Analog to Digital Converter (ADC), which gives it the ability to convert analog voltages to digital values. 

Below is the circuit setup of the LM35 and microcontroller. 

To calculate the temperature the following steps need to be followed;

1. Micrcontroller converts analog voltage (at output of LM35) to a digital value
2. Use the digital value in the equation below to find out what the voltage is (Vin)
3. Convert this voltage to millivolts
4. Then multiply this value by 10mV to obtain the temperature in degree celsius 

*Note, if you are not placing the LM35 on a printed circuit board, make sure the wires you use are as small as possible. Longer wires will increase the noise interference created by inductance which will affect the temperature reading. 

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They take real world data and then convert it to a voltage which allows our processors like an Arduino to process and analyse it as we choose.

Large applications from power plants to much smaller ones like your toaster will have sensors that make sure that everything runs smoothly.

There exists a multitude of sensors today. You can sense things like force, humidity, pressure, distance, speed, light levels, color and many more. 

One important real world parameter that is the most commonly measured is, Temperature. 

From simple applications of knowing the temperature outside, so you know how to appropriately dress, to more serious ones like being able to measure your body temperature to ensure you do not have a fever. 

The LM35 is a sensor that is capable of measuring temperature. 

In this article I will cover in depth, What is a LM35, from how it works to its many applications.

Sensors

Before we dive into the workings of a LM35, I will quickly cover the basics of sensors.

A sensor is a device that detects changes in the real world and then outputs this information to a microcontroller based system such as an Arduino for further processing. 

Microcontrollers and other computer processors work with voltages.

What a sensor does is react to changes in the physical world and outputs a voltage to the microcontroller. The microcontroller can then process this information based on different criteria. 

The output of the sensor is determined by its input. The sensitivity of a sensor will determine how much the output changes in relation to the input. 

Say a temperature sensor operates from 0V to 5V. The voltage at the sensors output will vary proportionally to the temperature being measured. 

A sensor by itself is quite redundant. Appropriate systems and processing need to be carried out on the output in order to calculate whatever physical quantity is being measured.

A sensor is composed of integrated circuits, transistors and diodes that are all made up of semiconducting material.

Why we use sensors?

But why do you need to use sensors? Can we live without them?

The answer is no! We cannot live without them. 

They provide a valuable service in systems that would not be able to operate without them.

If your air conditioning unit did not have a sensor letting it know what the temperature of the room is, it would not be able to regulate the temperature of that room and you would be either really cold or really hot.

There are 5 reasons why we need sensors; they allow the operation of the system to be smooth and efficient, they keep an eye out for any irregularities, they manage the control of operations, ensure that resources are utilized efficiently and in the case of performance issues make appropriate design changes.

Types of Sensors

There are primarily two types of sensors, Active and Passive.

Active Sensors are a type of sensor that requires an internal energy source to be able to emit radiation.

This type of radiation can be in the form of a light like a laser, infrared radio waves or ultrasonic waves.

This radiation is used to detect objects and changes in the environment.

The radiation is emitted by the sensor and then when it reaches the target object, reflects back to the sensor.

Some common sensors that are active include distance, infrared, and radar. 

A distance sensor sends a wave then calculates the time it takes for the wave to hit the target object and reflect back. Since the speed of the wave is known, the distance of the object can then be calculated.

Passive Sensors on the other hand do not produce their own radiation to detect objects or changes in the environment.

They rely on the target object’s radiation. Radiation such as heat or thermal infrared radiation. 

An electronic thermometer is an example of a passive sensor as it does not produce its own radiation, rather it relies on your body temperature to detect changes.

Applications

There are so many sensors and applications that I could go on and on about them. 

But, for the sake of time I will limit them to the most common applications.

All of these applications that I am going to list would not be able to function without sensors.

Automotive

The Automotive industry uses sensors in many areas from braking and traction control, air bags, avoiding collisions, comfort and engine data. 

Manufacturing

Manufacturing is a wide field and covers things like maintenance of machinery, monitoring performance of machinery, fine tuning quality systems, and reacting to market demands.

Aviation

Flying is a dangerous undertaking and requires systems to help you get from one destination to another.

Applications of sensors in aviation include navigation, measuring engine pressure and oil/fuel, weather conditions, speed of the aircraft, and many more.

Medical 

Having the right equipment can be the difference between life and death.

Here are a few uses of sensors in the medical field; monitoring blood pressure, glucose levels, patients vitals, detection of diseases spread by visitors to patients and robotics in the operation theater.

LM35

So now that we have a better understanding of what sensors are, and the different types of sensors and their applications, let us dive into what is a LM35.

The LM35 is a sensor that measures temperature. The Prefix LM stands for Linear Monolithic.

Temperature is an important parameter that is commonly measured. From weather systems, to air conditioning units, temperature sensors are an essential part of everyday life. 

The LM35 can measure the temperature of its surroundings, or, whatever object it is connected to. 

Temperature sensors come in two forms; Contact and Non-Contact.

Non-Contact

Non-contact temperature sensors do not require physical connection to the object it is trying to measure the temperature off. Rather it measures the temperature given off by the radiation of that object. 

Contact

Contact temperature sensors are further categorised into 3 sub-categories; Electromechanical, Resistance Temperature Detectors and Semiconductor based. 

Electromechanical temperature sensor works almost like a switch. It contains two metals that could be nickel, copper, tungsten or aluminium. These two metals are bonded together to form what is known as a Bi-metallic strip. 

Since the two metals have different expansion rates, when the two metals are subjected to an increase in temperature, they bend.

This way they act as a switch either allowing or preventing the flow of current.

The most common electromechanical sensor is the Thermostat.It was used in applications to control heating elements in boilers, furnaces and vehicle radiator cooling systems.

 However, they are pretty outdated and not used that much anymore.

Resistance Temperature Detectors determine temperature of a wire that is wound around a ceramic or glass. 

The wire is a pure material that has a relationship between its resistance and temperature that is very accurate. Therefore, temperature can be calculated using the right mathematical formulas.

Semiconductor based temperature sensors are aptly named that because they function using semiconductors. They come in the form of an integrated circuit. 

Semiconductors are mainly composed of the material silicon. Silicon is a great material as it is widely available, easy to use, have the right characteristics and are cheap. 

So what category of temperature sensors does the LM35 fall into?

The LM35 is a semiconductor based sensor. It is low cost and readily available among other temperature sensors.

How the LM35 works

The LM35 is a temperature sensor whose electrical output is proportional to the temperature in degrees celsius.

It is a linear device which means that the voltage at the output of the LM35 increases proportionally to the temperature.

This relationship is 10mv/°C. This means that everytime the temperature increases by one degree, the voltage at the output of the LM35 increases by 10mV (0.10V). 

So, if you were at room temperature, which is generally 25°C, the output of the LM35 would be 250mV or 0.25V.

So, you can see how easy it is to calculate the temperature. It is just a matter of multiplying the output voltage of the LM35 by 100. 

Also, the LM35 does not require any form of calibration to attain this accuracy. It is ready to use as soon as you receive it.

And, because of the output impedance, and accurate calibration, interfacing to control circuits is easy. I shall cover interfacing below.

Features

Below is the pinout of the LM35:

As I mentioned above the LM35 is calibrated in degree Celsius, and has a linear scale factor of 10mv/°C. 

Though the output is in degree celsius, you can still obtain the temperature in Fahrenheit by using a Celsius to Fahrenheit formula.

Below are other important features to note

• It can sense temperatures ranging from -55°C to 150°C
• It has an accuracy of 0.5°C
• Low cost
• Its operating voltage is 4V to 30V
• It draws current less than 60mA
• Low impedance output

Interfacing

The LM35 is a standalone device. What this means is that you cannot just hook up its output to a display and expect to see the temperature.

It requires control circuitry that can perform calculations to obtain the temperature and then display it on something like an LCD (Liquid Crystal Display)..

The output needs to be connected to an Analog to Digital Converter (ADC) of a microcontroller based module such as an Arduino (or even just a standalone microcontroller).

Without getting into too much detail, the ADC of the microcontroller converts the output voltage of the LM35 (which is analog) to a digital form (represented in binary).  

Since the LM35 does not require any extra circuitry,interfacing the LM35 to a microcontroller is as simple as connecting its output to the ADC input pin of the microcontroller.

Different types of packaging

Packaging of sensors come in a variety of options. These options include TO-CAN, TO-92, SOIC, TO-220 and many more. 

If you want an in-depth look at all the different types of packaging of components you can find them here.

When it comes to the packaging of the LM35 series of temperature sensors, there are multiple choices available at your disposal listed below

• TO-CAN (3 pin)     —> 4.699 mm x 4.699 mm
• TO-92     (3 pin)   —> 4.30 mm x 4.30 mm
• SOIC      (8 pin)   —> 4.90 mm x 4.90 mm
• TO-220   (3 pin)   —> 14.986 mm x 10.16 mm

Advantages

The LM35 is a great option for Hobbyists, students and DIY projects because it has so many great benefits and advantages that will benefit these applications.

The first main advantage is, ease of use. You do not need any extra circuitry to get the LM35 working. Even if you do not have a control circuit, you can still power up the LM35 and read its output using a multimeter to calculate the temperature.

No calibration is needed. You can use the LM35 as soon as you unpackage it. No need for trimming or calibrating it, as it is ready to go. Just plug and play. 

Since the relationship between the voltage and temperature is proportional, calculating the temperature is as simple as multiplying the output by a factor of 100.

Cost. The LM35 is cheap and readily available. So, for hobbyists or students who want to save on money, it is your best friend.

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