Spoiler alert: there is no battery in the card.
How does a contactless card work then?
The contactless card has its own antenna. This is what it looks like in X-rays:
So what? Where is the battery? How to receive a signal through this antenna if there is no battery? How do I make the chip work?
The whole secret is that electromagnetic radiation carries not only a signal, but also energy. In this case, the power source is a card reader (POS terminal or contactless reader in an ATM). This energy is enough for the chip to work.
Let me remind you that there used to be so-called detector radios. You won't find any battery in the circuit of this receiver, but there is a headphone, and sometimes it sounds very loud. However, all such radios have a huge stationary antenna - a cable several meters long, suspended quite high above the ground.
The broadcasting energy of the radio station is enough to "catch a little of the antenna" and make the membrane of the earphone vibrate. If the antenna is small, then little energy will be caught, and the detector receiver will not sound.
Something similar happens in contactless cards. The reader constantly emits electromagnetic oscillations of sufficient power. When the card gets into this radiation and is close enough to the reader's antenna, enough energy is "caught" in the card's antenna to turn on the chip and start working.
The reader (terminal) does not just give out vibrations, but makes them stronger or weaker. And thus, as it were, it transmits information (used amplitude modulation). The card's chip sees these changes and decodes the message. The energy has become higher - this is "one". Has become weaker - this is "zero". Thus, information is transferred from the terminal to the card chip.
But how does the postback happen?
Here we must remember one physical law, the meaning of which is this: the receiver also affects the transmitter! If the receiver picks up and consumes a lot of energy, then the transmitter becomes "heavier", he has to spend more energy. In the case of the detector receiver, this effect is hardly noticeable, but in the case of the reader-card pair, it is already noticeable. This phenomenon is called mutual induction.
So when the chip wants to transmit something to the reader, it connects and disconnects an additional load to its antenna. Well, as if it closes the antenna inside itself (through a small resistance).
Then the reader becomes sometimes lighter or heavier, which in terms of electrical phenomena means that sometimes more or less current flows through the transmitting antenna. By measuring these changes, the reader can receive a message from the chip. For example, the load has increased (the current has increased) - this is "one". Decreased is "zero". And so, zeros and ones, they exchange.
As soon as the card was pushed too far away from the reader, there was no longer enough energy, the chip did not turn on, and even if it did, the influence on the reader is already too small. The distance for NFC (and the card with the terminal works exactly on this physical protocol) is a maximum of 10-15 centimeters. In practice, even less. To achieve a greater distance, it is required to significantly increase the power of the reader, plus use a directional antenna.
Someone thought that for a meter distance, you need a reader with a microwave power.
How does a contactless card work then?
The contactless card has its own antenna. This is what it looks like in X-rays:
So what? Where is the battery? How to receive a signal through this antenna if there is no battery? How do I make the chip work?
The whole secret is that electromagnetic radiation carries not only a signal, but also energy. In this case, the power source is a card reader (POS terminal or contactless reader in an ATM). This energy is enough for the chip to work.
Let me remind you that there used to be so-called detector radios. You won't find any battery in the circuit of this receiver, but there is a headphone, and sometimes it sounds very loud. However, all such radios have a huge stationary antenna - a cable several meters long, suspended quite high above the ground.
The broadcasting energy of the radio station is enough to "catch a little of the antenna" and make the membrane of the earphone vibrate. If the antenna is small, then little energy will be caught, and the detector receiver will not sound.
Something similar happens in contactless cards. The reader constantly emits electromagnetic oscillations of sufficient power. When the card gets into this radiation and is close enough to the reader's antenna, enough energy is "caught" in the card's antenna to turn on the chip and start working.
The reader (terminal) does not just give out vibrations, but makes them stronger or weaker. And thus, as it were, it transmits information (used amplitude modulation). The card's chip sees these changes and decodes the message. The energy has become higher - this is "one". Has become weaker - this is "zero". Thus, information is transferred from the terminal to the card chip.
But how does the postback happen?
Here we must remember one physical law, the meaning of which is this: the receiver also affects the transmitter! If the receiver picks up and consumes a lot of energy, then the transmitter becomes "heavier", he has to spend more energy. In the case of the detector receiver, this effect is hardly noticeable, but in the case of the reader-card pair, it is already noticeable. This phenomenon is called mutual induction.
So when the chip wants to transmit something to the reader, it connects and disconnects an additional load to its antenna. Well, as if it closes the antenna inside itself (through a small resistance).
Then the reader becomes sometimes lighter or heavier, which in terms of electrical phenomena means that sometimes more or less current flows through the transmitting antenna. By measuring these changes, the reader can receive a message from the chip. For example, the load has increased (the current has increased) - this is "one". Decreased is "zero". And so, zeros and ones, they exchange.
As soon as the card was pushed too far away from the reader, there was no longer enough energy, the chip did not turn on, and even if it did, the influence on the reader is already too small. The distance for NFC (and the card with the terminal works exactly on this physical protocol) is a maximum of 10-15 centimeters. In practice, even less. To achieve a greater distance, it is required to significantly increase the power of the reader, plus use a directional antenna.
Someone thought that for a meter distance, you need a reader with a microwave power.
