This is how the SAR sensor works in 2021

This is how the SAR sensor works, one of the most used and least known components of our mobile phone

The SAR sensor does not rest. It has a responsibility to perceive the proximity of our body, but it does not take action only when we approach our smartphone in the face to prevent us from touching the touch screen without pretending it; it is also responsible for keeping under control the electromagnetic radiation emitted by our mobile phone. No more, no less.

When we consult the specifications of our terminal and check that in the section dedicated to its sensor provision the manufacturer tells us about the proximity sensor, it is actually mentioning the SAR sensor. But it would be unfair to accept that all it does is identify that our face is close to the screen. Without it our mobiles could not measure the radiation they emit at a given time in order to prevent it from going through a maximum threshold. Nor would they exceed the demands of current electromagnetic radiation emission regulations.

We start at the beginning: what SAR means

The acronym SAR comes from the English expression Specific Absorption Rate, which we can translate as a specific absorption rate or index. This value reflects the absorbed power per unit of body tissue mass, its average can be calculated throughout the body or in some parts of the body and is expressed in watts per kilogram (W/kg). This definition is rigorous, but it is also a bit complicated, so there goes a simpler one: this parameter measures how much radiofrequency energy the human body absorbs when exposed to an electromagnetic field like that emitted by our mobile phones.

The SAR parameter measures how much radiofrequency energy the human body absorbs when exposed to an electromagnetic field like the one emitted by our mobile phones

According to the Official State Gazette (BOE), full-body SAR is a widely accepted measure to link adverse thermal effects to exposure to radio emissions, but local SAR values are also necessary to assess and limit excessive exposure to energy in small parts of the body. It is interesting that it sounds to us and have a rough notion of what identifies this parameter so that those who want to look more can take a look at what the current legislation says.

On the other hand, it is also interesting that we know that Spanish legislation contains the recommendations made by the EuropeanUnion, so the legislation in Spain should not be very different from that of other countries around us. This explains why the smartphones we can purchase in the states that make up the European Union are identical if we stick to their radio emission capacity, but they may be different in this context from the terminals of the same brand and model that are marketed in countries such as China.

This is how our smartphone’s SAR sensor works

Our body has the peculiar ability to generate electricity, a skill that is essential for our life to be possible. A direct consequence of this phenomenon is that our body is wrapped in a very low-intensity electric field, which is nothing more than a region of the space around us in which if an electrical charge enters you will experience an electrical force caused by the presence of one or more additional electrical charges. The definition of the electric field, as we have just seen, is a little convoluted, but it is easy to intuit what we are talking about if we think about how the magnetic field generated by a magnet manages to induce a force capable of acting on a metal object with magnetic properties placed in its vicinity.

As I mentioned, the electric field that surrounds our body is very weak,but the progress that microelectronics has experienced over the last few years has caused us to be able to design and manufacture sensors that have the sensitivity to perceive them. These are the SAR sensors. Typically the integrated circuits that contain them house more than one sensor because the combination of several allows increasing their sensitivity. But this is not all we can find inside these chips. More advanced models also often incorporate the logic needed to accurately identify whether the body near the sensor is a human being or an inanimate object.

Today we don’t need to do much effort to find a device equipped with a SAR sensor. They live inside our smartphones, but also on tablets, smartwatches, and many laptops. In any case, the most curious thing is the strategy they use to read the most subtle electric fields. What they really do is permanently monitor the electric field around them for the purpose of identifying changes in intensity in it. From there the “smart” logic I discussed in the previous paragraph takes the reins to determine whether the disturbance of the electric field we are measuring has been caused by the proximity of our body or another object.

As we have just seen, this strategy is relatively simple, but it poses a challenge: how does the SAR sensor know that the electric field it is measuring is not the one that is generating the mobile phone itself? Because, as we can sense, the presence of electronic components inside causes it to originate its own electric field. The SAR sensor does not know whether or not it is measuring the electrical field of the phone, but the logic that accompanies it has been trained to characterize it and pay attention to the interactions of the field surrounding the sensor with other electric fields, which is what we are interested in.

An interesting note: during the start of the mobile phone its electric field fluctuates, so it is necessary to ignore the SAR sensor reading for a few seconds and wait until the electric field stabilizes. Otherwise, the logic working in tandem with the sensor might interpret that there is an object nearby and turn off the screen. However, as I mentioned in the first paragraphs of the article, the information collected by the SAR sensor is not only used to perceive our proximity and turn off the screen; it is also used to audit in real-time the non-ionizing radiofrequency radiation emitted by the phone, so that if at some point it exceeds a default threshold value the logic of the mobile triggers the actions necessary to reduce it.

Bioelectricity: how our body generates it and what it’s for

The electricity used by our body to carry out biological processes is known as bioelectricity and is crucial for our organs to perform their function properly. Our muscles and heart use it to produce movement; our neurons to issue the orders that govern the functioning of our organism and so that we can see, hear, think and store memories, among many other functions; and our eyes use it to collect the luminous stimuli and send them to the brain, which is responsible for processing them. Here are just a few simple examples that can help us sense with some precision how important bioelectricity is to make life possible as we know it. Not just that of animals; also that of plants.

Our central nervous system, which consists of the brain and spinal cord, is responsible for coordinating the proper functioning of organs and receiving stimuli from outside, processing them, and issuing appropriate orders in the form of electrical signals. On the other hand, the peripheral nervous system consists essentially of nerves that have a dual function: transport the electrical signals emitted by the central nervous system to the organs and structures that must carry out their orders, and also transport to the central nervous system the sensory stimuli collected by the organs that are responsible for collecting them, such as our eyes or ears. Without electricity, none of this would be possible.

But how does our body manage to generate that bioelectricity? The cells are responsible, and they do so thanks to a protein that is present in all cells of our body called sodium-potassium pump. Its function is to keep in balance the ions inside and outside the cell, which are nothing but atoms or molecules that do not have the same number of electrons and protons, which causes their overall electrical charge not to be neutral. If they have more electrons than protons they acquire negative charge and are called anions, and if they have more protons than electrons they acquire positive charge and are called cations. When a neutrally charged atom acquires one or more electrons it ionizes, which means it transforms into an anion. And if he loses them, for whatever reason, he also ionizes, but this time it transforms into a cation. What is really interesting is that this process causes the atom or molecule to acquire electrical charge, and this charge is the true germ of bioelectricity.

This recreation illustrates how ion channels allow the transport of sodium and potassium ions through the cell membrane to generate the electrochemical potential that causes the appearance of the electrical signal.

Let’s pay attention to our cells again. The sodium-potassium pump we discussed in the previous paragraph is responsible for transporting sodium and potassium ions through the cell membrane, the same way altering its electrical balance. Ions are able to pass through the cell membrane using ducts called ion channels, which can be of various types and allow the cell to relate to its environment by emitting signals. Although it is an excessive simplification we can imagine that each ion channel allows the cell to send a signal that encodes a specific message in response to a particular stimulus.

And finally, we reach the climax: when the cell needs to emit an electrical signal to send a message the sodium-potassium pump carries out the exchange of ions in and out of the cell membrane. This mechanism alters its concentration and results in an electrochemical potential, which is not very different from the potential difference that makes it possible to transport loads between two points with different electrical potential in a conventional electrical circuit. This is what we know as electric current. The flow of electrons will cease when the electrical potential of the two points is matched. What is interesting is that precisely the imbalance of the electrical charge inside and outside the cell generates a potential difference that results in the production of a very low voltage “spark”. Here we go. This is the moment when our cells generate bioelectricity without which our life would not be possible.