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Secret lock

Based on a Labs project | October 2011 | Find it here
This secret lock, unlike a conventional code lock, gives away no hints to the unwanted visitor as to its existence: there are no buttons, switches or keypads. No code sequence need be learnt: you simply need an inconspicuous key.The idea is based on two magnetically-operated switches which, when operated simultaneously, cause two relays to close. These in turn could actuate an electric door latch or start a garage door motor. This would not be particularly noteworthy (and rather easy to defeat) if simple reed switches were used, since they do not depend on the polarity of the magnetic field: they react equally to the north or the south pole of a magnet. Instead we use Hall effect ICs, which only react to south poles.
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