the salt card is designed to end the tiresome task of manually unlocking a smartphone or tablet , by automatically unlocking it whenever the user is nearby that the credit card-sized gadget also locks devices again as soon as a user moves out of range at a distance of 10 feet -lrb- three metres -rrb- .
it 's three times as thick because it has to be big enough to store the battery , which lasts for 18 months .