Hi,

I would use a different way,

If active they are emmitting a electric field which can be detected.
use this detection an every door and occasionally in the building to
sound
a "warning" device. ( large hammer ... )

especially near the intensive care section ....

everyone using a cellular in a hosptal should get hitten !

My 2 cents...

        Stefan Sczekalla-Waldschmidt
        ssw@oikossw.de