This is how the SAR sensor works, one of the components that we use the most and know the least about our mobile phone

The SAR sensor does not rest. It has the responsibility of perceiving the proximity of our body, but it does not come into action only when we bring our smartphone close to our face to prevent us from accidentally touching the touch screen; is also responsible for keep electromagnetic radiation under control emitted by our mobile phone. No more no less.

When we consult the specifications of our terminal and verify that in the section dedicated to its provision of sensors, the manufacturer tells us about the proximity sensor, in fact you are 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 would not be able to measure the radiation they emit at a given moment in order to prevent it from crossing a maximum threshold. And possibly they would not exceed the requirements imposed by current regulations regarding the emission of electromagnetic radiation.

Let’s start at the beginning: what does SAR mean

The acronym SAR comes from the English expression Specific Absorption Ratewhich we can translate as index or specific absorption rate. This value reflects the power absorbed per unit mass of body tissue, can be averaged over the entire body or parts of it, and is expressed in watts per kilogram (W/kg). This definition is rigorous, but it’s also a bit complicated, so here’s a simpler one: This parameter measures how much radio frequency energy the human body absorbs when exposed to an electromagnetic field like the one 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), whole-body SAR is a widely accepted measure to relate adverse thermal effects to exposure to radioelectric 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 to have an approximate notion of what this parameter identifies so that those who want to investigate more can take a look at what the current legislation says.

On the other hand, it is also interesting that we know that Spanish regulations include the recommendations made by the European Union, so the legislation in Spain should not be very different from that of other countries around us. This explains that the smartphones that we can acquire in the states that make up the European Union are identical if we stick to their radioelectric 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 the SAR sensor of our smartphone works

Our body has the peculiar ability to generate electricity, an ability that is essential for our life is possible. A direct consequence of this phenomenon is that our body is surrounded by a very low intensity electric field, which is nothing more than a region of space that surrounds us in which, if an electric charge enters, it will experience an electric force caused by the presence of one or more additional electrical charges. The definition of electric field, as we have just seen, is a bit convoluted, but it is easy to guess 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 metallic object with magnetic properties placed in its vicinity.

As I have mentioned, the electric field that surrounds our body It is very weak, but the progress that microelectronics has experienced in recent years has meant that we are capable of designing and manufacturing sensors that have the necessary sensitivity to perceive them. These are the SAR sensors. Normally the integrated circuits that contain them house more than one sensor because the combination of several allows their sensitivity to be increased. But this is not all that we can find inside these chips. And it is that the most advanced models also usually incorporate the necessary logic to accurately identify if the body next to the sensor is a human being or an inanimate object.

Nowadays you don’t have to try very hard to find a device equipped with a SAR sensor. They live inside our smartphones, but also in the tablets, smart watches and on many laptops. In any case, the most curious thing is the strategy they use to read the most subtle electric fields. And what they really do is permanently monitor the electric field that surrounds them with the purpose of identifying intensity changes in it. From there, the “intelligent” logic that I have talked about in the previous paragraph takes over to determine if the disturbance of the electric field that 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 generated by the mobile phone itself? Because, as we can guess, the presence of electronic components inside it causes create its own electric field. The SAR sensor doesn’t know whether or not it’s measuring the phone’s electric field, but the accompanying logic 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’re interested in. .

An interesting note: during the start-up 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 could 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; also used for audit radiation in real time of non-ionizing radiofrequency that the phone emits, so that if at any time it exceeds a predetermined threshold value, the logic of the mobile triggers the necessary actions to reduce it.

Bioelectricity: how our body generates it and what it is for

The electricity used by our body to carry out biological processes is known as bioelectricity, and it is crucial for our organs to carry out their function correctly. Our muscles and our 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 light stimuli and send them to the brain, which is in charge of processing them. These are just some simple examples that can help us understand with some precision how important bioelectricity is for life as we know it to be possible. Not only that of animals; also that of plants.

Our central nervous system, which is made up of the brain and the spinal cord, is responsible for coordinating the proper functioning of the organs and receiving stimuli from the outside, processing them and issuing the appropriate orders in the form of electrical signals. On the other hand, the peripheral nervous system is essentially made up of nerves that have a dual function: to transport the electrical signals emitted by the central nervous system to the organs and structures that must carry out their orders, and also to transport them 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 eleectricity none of this would be possible.

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

Let’s go back to paying attention to our cells. The sodium-potassium pump that we talked about in the previous paragraph is responsible for transporting sodium and potassium ions through the cell membrane, thereby altering its electrical balance. Ions are able to cross the cell membrane using channels called ion channels, which can be of various types and allow the cell to interact with its environment by emitting signals. Although it is an oversimplification, we can imagine that each ion channel allows the cell to send a signal that encodes a specific message in response to a specific stimulus.

And, finally, we come to the culminating moment: 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 gives rise to an electrochemical potential, which is not very different from the potential difference that makes possible the transport of charges 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 becomes equal. What is interesting is that, precisely, the imbalance of the electrical charge inside and outside the cell generates a potential difference that causes the production of a very low voltage “spark”. Here we have it. This is the moment in which our cells generate bioelectricity without which our life would not be possible.

Cover image | cottonbro
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