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This circuit converts a key press into an analogue voltage. It's a neat trick because sixteen switches only need one input pin.
The Picaxe ADC converts this into a binary number.
Your code detects this number (or a small range of numbers) and is able to work out which key was pressed.
You'll need the wiring diagram to know which pin is which. Not all keypads have the same pin layout so please check this!
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