How embedded systems work: the hidden computers in everyday things, and how engineers build them
Interrupts and sleep mode: how a battery device can watch all night for a year
A smoke alarm, a TV remote or a door lock can run for a year or more on a few small batteries, and still answer the moment you need it. A computer that never rests would empty them in days, so a battery device keeps watch another way.
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Rono stays the night to watch the flap. At two in the morning, he clips a meter to its batteries, and the needle barely moves, as if the flap were switched off.
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Then the grey cat walks up to the door. A flap that seems switched off now has to notice it, read its tag and keep the latch shut.
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