Wireless, battery-less system designed to alert users to windows left open
By Ben Coxworth
September 7, 2012
If a storm rolls in while you’re at work, and you’re wondering if you left your bedroom window open, you can tell via an internet connection – if that window is equipped with a contact sensor. Ordinarily, such sensors require electrical wiring, and a battery or mains power. A new window system developed by Germany’s Fraunhofer Institute for Integrated Circuits, however, works without wires, and draws its power from the environment around it.
The system incorporates a fingernail-sized 3D magnetic field sensor that is based on an existing product known as the HallinOne, which is already used in washing machines to determine the exact position of the drum.
Embedded in the inner window frame, that sensor is able to detect changes in the angle and position of a sliding magnet located in the bottom of the sash – the magnet moves to the right when the window is locked, for instance, which the sensor would register. It can also differentiate between a window that is securely latched and merely pulled shut, and it will notice if someone (such as a prospective burglar) tries to disable the system by removing the magnet.
The window frame also features an embedded RF node, consisting of a radio unit and a microcontroller. This transmits status reports from the sensor, to a range of 20 to 30 meters (66 to 98 feet). That signal is ultimately picked up by an internet-connected base station, such as a computer, smartphone, or purpose-built control box.
Along the way, however, the signal can be relayed by the RF nodes in one or more other window frames. In this way, as long as the windows are no farther apart than about 30 meters, a single base station could monitor multiple windows. Fraunhofer has suggested, for example, that security staff could use the technology to check all the windows of an office building, without having to leave the front desk.
Besides the sensor, magnet, and RF node, each window frame additionally includes a thermoelectric generator and a solar cell. These provide power to the electronics, with the generator running off of ambient heat, and the solar cell (which is mounted on the outside of the frame) harvesting sunlight – even the limited amount of light available on the shaded side of a building is said to be sufficient.
Although the “window sentinel system” is currently still in the prototype stage, Fraunhofer and its manufacturing partner, Seuffler, expect to start production by the end of the year.
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