Shulou(Shulou.com)11/24 Report--

Take out the payment card

Stick it on the card reader.

Beep! The card is still there, the money is gone.

Such a familiar feeling is really indescribable.

So here comes the question,

It's just a post.

How does the card machine deduct the money?

1. RFID and contactless IC card in order to explain the principle clearly, we should first be familiar with two words: radio frequency identification (RFID) and contactless IC card. Is it suddenly ting (bu) technology (dong)?

In fact, these two concepts are not difficult to understand. As the name implies, Radio Frequency Identification (RFID) is a technology that realizes object identification by transmitting and receiving radios. Its main purpose is that the swiping machine should accurately identify whose card it is and deduct the money in the corresponding card. After all, it's scary to think about "no shopping, no money". How does it come true? Picture above!

Fig. 1 schematic of wireless radio identification system this picture is an illustration of the simplest wireless radio identification system. It can be seen that a RFID system can be divided into four parts:

(1) Electronic tag: the IC chip in the picture stores the identity information of the logo.

(2) Antenna: responsible for sending or receiving electromagnetic waves and realizing communication

(3) Reader: under the card swiping model, it is the card reader, which is responsible for transmitting the received signal to the computer and other software terminals.

(4) Application software: responsible for quietly telling you that the money is gone!

At this point, there must be a partner to ask, how did the card hear the call from the card reader antenna? How did you reply? This brings us into the inner world of IC cards!

Fig. 2 contactless IC card structure schematic contactless IC card, also known as radio frequency card, its layered structure can be regarded as a card on both sides and a circuit structure in the middle. At a cursory glance, the circuit structure can be seen as a loop antenna and an IC chip. However, the blind student found the Chinese point, that is, since this card wants to reply to the message, but where does the energy come from? You don't want the user to recharge the card, do you? Of course the answer is no. The following figure shows the details of the process of receiving and replying electromagnetic waves:

Fig. 3 the communication principle shows that the electromagnetic wave carries energy when it is transmitted from the antenna at the end of the card reader. When the frequency of this electromagnetic wave matches the natural frequency of the LC oscillation circuit in the IC card, resonance will occur. The LC oscillation circuit will absorb part of the energy of the electromagnetic wave and accumulate charge at both ends of the capacitor 1. The other side of the capacitor is connected with a unidirectional electron pump, that is, electrons will only flow from capacitor 1 to capacitor 2 on the other side of the pump and will not flow back. In this way, as electromagnetic waves continue to bring energy, more and more charges will accumulate at both ends of capacitor 2. According to the following formula:

Because the capacitance is constant, when the charge Q increases, the voltage U increases accordingly, until a voltage threshold is reached to start the transmitting unit, and the IC card returns the signal to the card reader.

2. Other kinds of IC cards have read the principle of contactless IC cards, and you may be curious about the appearance of contacting IC cards. And, how can we distinguish between contact and non-contact IC cards? Don't worry, the interpretation will begin!

Fig. 4 Encapsulation of contact IC card | the figure comes from [1]. The figure above shows the encapsulation of contact IC card. Seeing the appearance of the contact in the upper left corner, many readers may want to say, "so this is it!" Yes, the contacts are on exposed metal sheets such as some of our common bank cards, which look something like this when enlarged.

Figure 5 contacts of contact IC cards | figure is derived from [1]. As the name implies, contact IC cards are connected electrically through physical contact, so these contacts are important interfaces for connecting card readers. Don't underestimate these contacts, they have a clear division of labor! C1~C3 contacts are responsible for connecting power (VCC), reset (RST), and introducing clock signal (CLK), while C5~C7 is responsible for grounding (GND), programming voltage (when not in use as normally closed contact N.C.), input and output (I / O). According to the ISO / IEC7816-2 standard, there are generally no more than 8 contacts, but C4 and C8 are not assigned specific tasks, so some cards have only 6 contacts.

Fig. 6 six-contact IC card if you look at the internal structure, the contacts are arranged on the electrode diaphragm, and the semiconductor chip is connected below.

Fig. 7 Internal structure of contact IC card | figure is derived from [5] because the contact IC card uses physical contact and the contacts are exposed, it is easy to have poor contact. However, because there is no need to transmit signals wirelessly, the circuit of the card and the corresponding card reader is simpler and cheaper. To put it simply, the contact IC card is more suitable for places where the card is not used frequently and the environment is not easy to corrode the contacts. The contactless IC card is suitable for frequent use.

Fig. 8 contactless IC card generally speaking, seeing this contactless, fully packaged card is contactless, and common student cards should be of this category.

In addition, there is a simplified version of the contactless IC card called the ID card. It is characterized by being read-only. Therefore, only in the business card printing, a card number will be stored in the card for identification. Its working principle is also the wireless radio identification mentioned above, but its working frequency (mostly low frequency, 125khz) is not the same as that of contactless IC cards (mostly high frequency, 13.56Mhz), so card readers often do not communicate with each other. ID cards are generally used in the access control system of residential areas or parking lots, and can be made into the shape of hanging plates.

Figure 9 different shapes of ID cards to talk about contact, the general determination method is to see if there are any contacts, such as the following card.

Figure 10 contact IC card however, things are not that simple, because contact and non-contact can be merged to form a dual interface card!

Figure 11 double interface card structure shows that you have seen the contact and non-contact structure, you must see at a glance: this is the fusion monster! Yes, there are not only contacts exposed to the surface, but also internal coils. There is no difference between appearance and contact, so there is almost no way to judge by appearance alone. If you want to infer, you can see how to pay by card.

Fig. 12 dual-interface card readers generally use contact circuits as long as they are plugged in, while those affixed on them are generally non-contact. As for the card that can be inserted and pasted, it is a double interface card.

3. Magnetic stripe card and composite card when talking about the card, we have to mention the magnetic stripe card. Careful readers should have noticed that every IC card mentioned above contains a chip, so it is also called a chip card. The magnetic stripe card uses the magnetic stripe affixed to the card to read and write information.

Fig. 13 an example of a magnetic stripe card when using this card, the magnetic stripe needs to be scratched in the slot of the card reader so that the magnetic head of the card reader will read the information in the magnetic stripe.

Fig. 14 Magnetic stripe card reading card magnetic stripe is composed of magnetizable material. If the data needs to be stored in the magnetic card, the magnetic strip moves at a certain speed when swiping the card, and the inner coil of the magnetic head of the card reader will be electrified, and the magnetic field will be generated according to the magnetic effect of the current, and the corresponding position of the magnetic strip will be magnetized. According to the direction and magnitude of the magnetic head current, a specific magnetic pole direction and intensity magnetization can be generated for the magnetic stripe in the corresponding position. In this way, the information is stored in the magnetic strip. When reading the card, the card is also crossed, and when the corresponding position of the magnetic strip passes through the magnetic head, the magnetic field of its specific size and direction will generate the corresponding induced electromotive force in the coil, thus stimulating the electrical signal and completing the reading of the information.

Fig. 15 Magnetic stripe chip composite card however, at one stage, a bank card was a magnetic stripe chip composite card, which, as its name implies, could both swipe and insert cards. However, because the security, anti-interference and storage capacity of magnetic stripe card are far lower than that of chip card, most bank cards cancel the function of magnetic stripe, or even no longer set magnetic stripe.

When you see this, are you impressed by the small card in your hand? No matter swiping, inserting or sticking, they are all cordial greetings from card readers and payment cards.

Of course, we can't do without either one.

Powerful electromagnetism!

Reference:

[1] Yang Zhenye. IC card technology and its application [M]. Science Press, 2006.

[2] Radio Frequency Identification Technology (RFID)

[3] understand ETC and RFID technology in one article

[4] contactless card _ Baidu encyclopedia

[5] contact IC card structure _ principle _ characteristics-Victoria library electronic communication

[6] ID card turned out to be an IC card! What kind of card is the ID card?

[7] what is the difference between IC card, ID card, M1 card and RF card?

[8] what are the advantages of IC cards over magnetic cards?

[9] basic knowledge of ID card and IC card NFC

Note: memes and some pictures come from the Internet.

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Yun Kai Ye Luo

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