Vending machines determine the value of a banknote using several different technologies. Here are the most common ones:

1. **Magnetic Ink Detection**: Many currencies, including the US dollar, use magnetic ink for the numbers on their banknotes. A vending machine can have a magnetic sensor that reads this ink and can determine the denomination based on the magnetic properties of the note. 

2. **Optical Scanning**: This is another commonly used technology in vending machines. These machines use optical scanners to identify the images, colors, and other features of the banknote. The software inside the machine will then compare these features to the stored images of genuine notes.

3. **Infrared Detection**: Some banknotes have certain patterns or parts that are only visible under infrared light. Vending machines equipped with infrared detectors can be used to identify these unique features.

4. **Ultraviolet Detection**: Similar to infrared detection, ultraviolet detection works by illuminating the banknote with UV light and detecting certain features that only appear under this light.

5. **Thickness and Size Measurement**: Each denomination of a banknote usually has a unique size and thickness. The vending machine can determine the denomination by measuring these properties.

These methods can be used individually or in combination to ensure accurate and secure detection of banknotes. Once the machine determines the value of the note, it will credit that amount to the user and allow them to make a selection.
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