In order to simplify their installation and operation, modern home security systems must be able to monitor their surroundings wirelessly. This includes perimeter security monitoring, intrusion detection, as well as securing sensitive areas of the home, such as medicine containers, safes, or other places where valuables are hidden.
Although video surveillance is possible, it requires sophisticated image processing and analysis to detect unauthorized activity, and also raises privacy concerns. Conventional motion detectors eliminate the privacy issue, but require power and support electronics, increasing the cost of the product BOM. Motion sensors also require some sort of wired or wireless interface to relay the sensor output to the cloud. In many cases, cameras and motion detectors require professional installation, which makes them uneconomical for many cost-sensitive applications.
This design idea presents an alternative solution more suited to the needs of many consumers and small businesses: the use of RFID tag technology to enable low-cost, battery-less, and easy-to-install home security systems.
RFID tags can be used as the basis for a series of robust security sensors. A motion detector can be realized by integrating the RFID tag with a reed switch. Similarly, a shock event detector can be made by replacing the reed switch with a shock switch. In both cases, the sensor switch is connected between the two terminals of the antenna and the two terminals of the RFID chip as well. In this configuration, the operation of the RFID tag is enabled or disabled depending on the ON or OFF state of the switch.
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The sensors provide an inexpensive and compact solution for the security monitoring system featured in Figure 1. RFID tags equipped with motion detectors can be attached to the window, door and safe, while a shock detection tag can be attached to the medicine box. A commercial RFID reader performs two functions: a) it detects any sensor that has been brought to its active state by its motion or shock sensing switch b) the reader also floods the area with low power RF radiation which powers the labels when activated. Since each RFID tag has its own unique identifier, the RFID reader knows where the detected activity is occurring. Once captured, activity data can be transmitted to the cloud for analysis and, if required, user notification via smartphone or email alert.
Figure 1 Security monitoring using an RFID tag.
Principles of Sensor Operation
The block diagrams in Figure 2 illustrate the operating mechanisms of the shock sensor and motion sensor.
Figure 2 A shock sensor consists of an RFID chip, an antenna and a shock sensor (a). In its normal (undisturbed) state, the metal ball shorts the two contacts of the shock sensor (b). When the sensor is disturbed, its metal ball no longer short-circuits the two contacts, allowing the antenna to receive power from the RFID chip, and to transmit its signal (c). A motion sensor consists of an RFID chip, an antenna and a reed switch (d). In its normal state, the reed switch is held in its ON state by a magnet attached nearby. This shorts out the antenna, preventing the RFID chip from receiving power (e). When the magnet is removed, the reed switch goes to the OFF state, thus activating the RFID device (f).
As mentioned earlier, the shock sensor consists of an RFID chip and an antenna, as shown in Figure 2a. The shock switch consists of a cavity and a small metal ball that rolls inside. When the shock switch is placed vertically, the ball rolls over the two conductive contacts exiting the cavity, creating a conductive path (Figure 2b). If the sensor tilts to another direction, the contacts are disconnected, providing a simple means of detecting motion or orientation. When used as a safety sensor, the shock switch is connected between the two antenna terminals and placed vertically, so that it shorts them in its passive position. The resulting short circuit breaks the impedance match between the RFID chip and the antenna, preventing the RFID tag from transmitting a signal. If the shock sensor is tilted, the metal ball moves out of the contacts, allowing the RFID tag to receive energy and emit a signal, which is then read by the RFID reader.
The motion detection system shown in Figure 2d consists of an RFID chip, an antenna, a reed switch and a magnet. A reed switch consists of two ferromagnetic flexible metal reed contacts in a sealed glass cavity. The two switch contacts are normally open until the presence of a magnetic field causes them to close (figure 2e). When the sensor is moved out of proximity to the magnet, its contacts return to the original open state as in Figure 2f, thereby activating the RFID chip and alerting the reader that the tag has been moved. This is shown in Figure 1 where, for example, the RFID tag with the reed switch is attached to the window, and the magnet is attached to the wall near the RFID tag. When the window is closed, the magnet holds the reed switch in the closed position, thereby bypassing the antenna and keeping the chip inactive. When the window is moved up or down, the magnet is no longer close enough to hold the reed switch closed. At this point the reed switch returns to its open position and the RFID tag becomes active.
Stupid sensors, smart system
Since each RFID tag has its own unique identifier, it can be correlated to the specific object or location being monitored. This information can be used by the RFID reader or cloud-based security application to learn more about the nature of the event, including identifying the number of possible intruders, their location, and making inferences about their activities. Such analysis can also reduce the number of false alarms caused by pets or other benevolent disturbances.
RFID technology is a promising tool for creating robust, reliable and inexpensive security systems. The beacons are easy to install and do not require batteries, ensuring their long-term reliability. In real-world applications, however, care must be taken that RFID readers are positioned to provide full coverage of the area under surveillance. Commercialization will also require the development of “smart” RFID readers capable of performing at least the first layer of processing the activity produced by RFID tags.
Wei Wang, Cihan Asci, and Sameer Sonkusale work in the Department of Electrical and Computer Engineering at Tufts University in Medford, MA. More information about their research activities can be found here.