IF you want to protect your house against burglars, you don't need a guard dogjust a few house plants. Like many other common objects, plants can be turned into sensors for burglar alarms simply by connecting them to an electrical circuit, says Hal Philipp, a former IBM researcher based in Southampton.
House plants can act as a capacitoraccumulating an electrical charge and then discharging it. "The amount of charge depends not only on the size of the object but also what it is close to," says Philipp, who invented this type of electronic capacitance sensor.
The burglar alarm works by monitoring the capacitance of the plant. If a burglar walks past the plant, its capacitance changes and the alarm goes off.
The capacitance sensor is normally charged and discharged many times to establish an average reference level. This allows engineers to tune the alarm to ignore harmless changes in capacitancesuch ...
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