How do contactless payments work?

1. Leo pays for a $3 juice by holding his pretend card near the reader. Beep! No slot, no swipe. How can the card and the reader talk without touching?
2. Inside the card is a chip, a tiny computer, joined to a thin loop of wire that runs around the edge. The loop is an antenna. There is no battery, so the chip is asleep.
3. The reader has a wire loop too. Electricity racing back and forth in it makes an invisible magnetic field. When the card's loop enters the field, electricity starts to flow in it, and the chip wakes up.
4. Now the two can talk by tiny radio signals. The reader says how much to pay. The card answers with a secret code that it makes up just for this one payment.
5. The reader sends the code to the bank, just like any card payment. The bank checks it and says yes. Four green lights and a beep: paid!
6. The field fades quickly with distance, so the card only works within about four centimeters. That is on purpose. And because the code is new every time, a copied code is useless.

The card and the reader are drawn see-through. Tap each part to find out what it does.

• Card antenna. A thin loop of wire runs around the inside edge of the card. It catches energy from the reader's field and also sends and receives the messages.
• Chip. The chip is a tiny computer with no battery. When the antenna feeds it power, it wakes up, makes a one-time code and sends it.
• Reader coil. Under the reader's top is another loop of wire. Electricity surging back and forth in it makes the magnetic field.
• Magnetic field. The field is invisible. Here it is drawn as blue lines. It is strong near the reader and fades away within a few centimeters.
• Lights and beep. The lights fill up as the reader and card talk. Four green lights and a beep tell you the payment went through.
• Link to the bank. The reader passes the card's code on to the bank through the payment network. Only when the bank says yes is the payment done.

Drag the pretend card toward the reader, or use the slider. Find out how close it must be before the chip wakes up and the payment goes through.

Experiment: How close does the card have to be before the reader can take the payment?

1. Hold the card far above the reader. Does anything happen?
2. Bring it down slowly and watch the loop of wire.
3. Find the distance where the chip wakes up.

What we found: Far away, nothing happens: the field does not reach. Only within about 4 centimeters does enough energy flow into the loop to wake the chip. That short range is on purpose: you have to mean to pay.

Turn on JavaScript to try it yourself.

Three customers cannot pay. Work out what is wrong each time, fix it, then bring the card close.

A contactless card looks like plain plastic, but two things are hidden inside it: a chip, which is a tiny computer, and a thin loop of wire around the edge. There is no battery. Until the card comes close to a reader, the chip is fast asleep.

The reader has a loop of wire as well. Electricity surges back and forth in it, and that makes an invisible magnetic field around the reader. When the card's loop is inside the field, the field pushes electricity around the card's loop too. Now the chip has power and wakes up. Scientists call this induction.

With the chip awake, the two swap short radio messages. The reader says how much to pay. The card answers with a secret code that it makes up for this one payment only. The reader passes the code to the bank, the bank says yes, and you hear the beep. All of that takes about half a second.

The field is strong near the reader and fades very fast, so it reaches only about four centimeters. That is on purpose: you have to hold your card right up to the reader, so you cannot pay by accident from across the room. The whispering comparison is not perfect, though. A whisper needs your own breath, but the card gets all of its power from the reader.

Key words

Contactless

Paying by holding a card or phone near a reader, without putting it into a slot.

Antenna

A loop of wire that catches and sends invisible signals.

Magnetic field

The invisible region around a magnet, or around a wire carrying electricity, where magnetic pushes can be felt.

Induction

Making electricity flow in a wire loop with a changing magnetic field, without touching it.

NFC

Near-field communication: very short-range wireless links between two coils, at 13.56 megahertz.

One-time code

A secret code made for a single payment. It is useless if anyone copies it.

Check your thinking

1. Where does the card's chip get its power?

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   From the reader's magnetic field. The card has no battery. The reader's field pushes electricity around the card's loop.

2. Why does a contactless card only work within a few centimeters?

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   The field fades very quickly with distance. Beyond a few centimeters too little energy reaches the loop to wake the chip.

3. Why does the card make a new secret code for every payment?

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   So that a copied code cannot be used again. A code that works only once is worthless to anyone who records it.

4. A card is wrapped in metal foil. Why can't it pay?

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   Metal blocks the field, so no power reaches the chip. The field cannot pass through the metal, so the chip never wakes.

5. Which design choice most directly stops someone paying by accident from across a room?

   Show the answer

   The very short range of the field. The field reaches only a few centimeters, so the card must be brought right up to the reader.