How does a self-checkout machine work?

1. At a self-checkout you scan your own shopping. Slide a carton past the window and the screen shows its name and price. How does it know?
2. Every package has a barcode: thick and thin stripes with a number underneath. The stripes and the number say the same thing. Each kind of product has its own.
3. The scanner sweeps red light across the stripes. White bounces lots of light back to a sensor and black bounces very little. Bright, dark, bright, dark: that pattern is the number.
4. The stripes do not hold the price. The computer takes the number and looks it up in the store's list. The matching row says: milk, two dollars.
5. The bag sits on a hidden scale. When the milk goes in, the scale should feel the weight of one carton of milk. If the weight matches, the machine is happy.
6. The screen adds up the prices. Mina pays with a pretend play card, the printer rolls out a receipt, and the light on top glows green. All done!

Tap each part of the machine. Each one does a job when you find it!

• Barcode. The stripes on the package. They spell out a number that belongs to this kind of product.
• Scanner. Behind the glass, red light sweeps out in many directions, so the stripes can be read even if you hold the box crooked.
• Computer and price list. The computer takes the number from the scanner and looks it up in the store's list to find the name and price.
• Touchscreen. The screen lists each item and its price, and adds up the total. You can also tap it to pick things with no barcode, like loose fruit.
• Bagging scale. A scale under the bag. After every beep it checks that the bag got heavier by the right amount.
• Card reader. This is where shoppers pay. In our model it reads a pretend play card and says the sale is paid.
• Receipt printer. A small printer rolls out the receipt: a list of what you bought and what it cost.
• Help light. The light on the pole glows green when things are fine. It turns yellow to call a store helper when something needs checking.

Drag each item over the scanner and watch the screen and the scale. Then see what the machine does when a scan is forgotten.

Experiment: What happens if something goes into the bag without being scanned?

1. Drag one item over the scanner and let it drop in the bag. Read the scale.
2. Press "Forget to scan one".
3. Watch the light on the pole and the message on the screen.

What we found: The scanner told the computer how heavy each scanned item should be. When something lands in the bag without a beep, the scale feels weight that is not on the list, so the light turns yellow and a helper is called. The scale is there to catch honest slips, so always scan everything.

Turn on JavaScript to try it yourself.

Match each scanned number to its row in the price list, find the item that was never scanned, then pay with the pretend play card.

Look at almost any package and you will find a barcode: a patch of black and white stripes with a number under it. The stripes and the number mean the same thing. People read the digits; machines read the stripes.

The scanner shines red light across the stripes. White paper bounces most of the light back to a sensor, and black ink soaks it up. As the light sweeps along, the sensor sees bright, dark, bright, dark. The widths of those patches spell out the number.

Here is the surprise: the price is not in the barcode. The barcode only says which product this is. The checkout sends the number to the store's computer, which looks along its list and sends back the name and the price. That is why a shop can change a price without printing new packages.

The bag sits on a scale. The store's list also knows about how heavy each product is, so after every beep the scale checks that the bag got heavier by the right amount. If something lands in the bag with no beep, the weights do not match and the light calls a helper. Our model is simplified, and it uses a pretend play card to pay.

Key words

Barcode

A number written as black and white stripes that a machine can read.

Scanner

The part that reads a barcode by shining light on it and measuring what bounces back.

Database

A giant list kept on a computer, here with every product's name and price.

Check digit

The last digit of a barcode, worked out from the others, which lets the scanner catch a misread.

Load cell

A sensor that turns weight into an electrical signal; it is the heart of an electronic scale.

Tolerance

How big a difference is allowed before something counts as wrong.

Check your thinking

1. What does a barcode tell the checkout?

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   A number that says which product it is. The stripes only spell a number. The computer looks that number up to find the name and price.

2. A shop wants milk to cost less tomorrow. What has to change?

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   The price in the store's computer list. The price lives in the list, not in the stripes, so changing one row changes the price at every checkout.

3. Why do white stripes and black stripes look different to the scanner?

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   White bounces back lots of light and black bounces back very little. The sensor sees bright where the paper is white and dark where the ink is black.

4. You scan a carton of milk but put two cartons in the bag. What does the machine notice?

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   The bag is heavier than the list says it should be. The scale expects the weight of one carton. Two cartons weigh twice as much, so the weights do not match.

5. A store sets the bagging scale's tolerance very tight. What is the downside?

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   Honest shoppers are stopped when an item weighs a little more or less than average. Real items vary in weight. A tight tolerance catches more mistakes but also raises more false alarms.