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Food and drink vending machines are built like tanks to stop vandals stealing the cash. They have a mechanical key that controls an electronic bolt. In a power failure, back-up batteries allow the machine to keep running and send a fault warning by phone. But the batteries often die before the engineer arrives, so the lock won’t open and the goods spoil. Instead, Marconi in the US programs most of the system to disconnect from the battery as soon as the fault warning is sent (US 2003/0137399). Only a key sensor and microprocessor remain powered, in sleep mode. When the engineer inserts his key the sensor wakes the microprocessor, which sends power to the bolt so the engineer can get inside and rescue the money and perishables.