【導(dǎo)讀】隨著社會的進步發(fā)展，人們的生活水平日益提高，物質(zhì)財富加速積累，人們對優(yōu)質(zhì)防盜系統(tǒng)的需求日漸增多。與此同時，為了進一步規(guī)范住宅小。區(qū)智能化建設(shè)，必須加快發(fā)展家庭智能防盜系統(tǒng)的開發(fā)與應(yīng)用。傳感設(shè)備利用紅外熱釋電傳感器探測闖入信號，機相連，通過上位機顯示報警信息來及時通知業(yè)主有人闖入監(jiān)控區(qū)。該系統(tǒng)可以通過OLED顯示屏及蜂鳴器等及時顯示報警信息并發(fā)出警報。改善了昔日傳統(tǒng)家庭防盜系統(tǒng)可靠性差、易受損、失效快、維護。困難、成本高、功耗大等問題，經(jīng)濟實用，是一套完整可行的系統(tǒng)。

　　

【正文】 of the cost of sensor installations. And then there are applications for sensors where wiring isn39。t practical or even possible. The problem, though, is that most wireless sensors use too much power, which means that their batteries either have to be very large or get changed far too often. Add to that some skepticism about the reliability of sensor data that39。s sent through the air, and wireless sensors simply haven39。t looked very appealing. A lowpower wireless technology called ZigBee is rewriting the wireless sensor equation, however. A secure work technology that rides on top of the recently ratified IEEE radio standard (Figure 1), ZigBee promises to put wireless sensors in everything from factory automation systems to home security systems to consumer electronics. In conjunction with , ZigBee offers battery life of up to several years for mon small batteries. ZigBee 49 devices are also expected to be cheap, eventually selling for less than $3 per node by some estimates. With prices that low, they should be a natural fit even in household products like wireless light switches, wireless thermostats, and smoke detectors. Figure 1: ZigBee adds work, security, and applicationservices layers to the PHY and MAC layers of the IEEE radio Although no formal specification for ZigBee yet exists (approval by the ZigBee Alliance, a trade group, should e late this year), the outlook for ZigBee appears bright. Technology research firm InStat/MDR, in what it calls a cautious aggressive forecast, predicts that sales of nodes and chipsets will increase from essentially zero today to 165 million units by 2020. Not all of these units will be coupled with ZigBee, but most probably will be. Research firm ON World predicts shipments of 465 million wireless sensor RF mod ules by 2020, with 77% of them being ZigBeerelated. In a sense, ZigBee39。s bright future is largely due to its low data rates—20 kbps to 250 kbps, depending on the frequency band used (Figure 2)—pared to a nominal 1 Mbps for Bluetooth and 54 Mbps for WiFi39。s technology. But ZigBee won39。t be sending and large documents, as WiFi does, or 50 documents and audio, as Bluetooth does. For sending sensor readings, which are typically a few tens of bytes, high bandwidth isn39。t necessary, and ZigBee39。s low bandwidth helps it fulfill its goals of low power, low cost, and robustness. Figure 2: ZigBee39。s data rates range from 20 kbps to 250 kbps, depending on the frequency used Because of ZigBee applications39。 low bandwidth requirements, a ZigBee node can sleep most of the time, thus saving battery power, and then wake up, send data quickly, and go back to sleep. And, because ZigBee can transition from sleep mode to active mode in 15 msec or less, even a sleeping node can achieve suitably low latency. Someone flipping a ZigBeeenabled wireless light switch, for example, would not be aware of a wakeup delay before the light turns on. In contrast, wakeup delays for Bluetooth are typically around three seconds. A big part of ZigBee39。s power savings e from the radio technology of , which itself was designed for low power. uses DSSS (directsequence spread spectrum) technology, for example, because the alternative FHSS (frequencyhopping spread spectrum) would have used too much power just in keeping its frequency hops synchronized. ZigBee nodes, using , can municate in any of several different 51 ways, however, and some ways use more power than others. Consequently, ZigBee users can39。t necessarily implement a sensor work any way they choose and still expect the multipleyear battery life that is ZigBee39。s hallmark. In fact, some technologists who are planning very large works of very small wireless sensors say that even ZigBee is too power hungry for their uses. A ZigBee work node can consume extra power, for example, if it tries to keep its transmissions from overlapping with other nodes39。 transmissions or with transmissions from other radio sources. The radio used by ZigBee implements CSMA/CA (carrier sense multiple access collision avoidance) technology, and a ZigBee node that uses CSMA/CA is essentially taking a listenbeforetalk approach to see if any radio traffic is already underway. But, as noted by Venkat Bahl, marketing vice president for sensor pany Ember Corp. and vice chairman of the ZigBee Alliance, that39。s not a preferred approach. Having to listen burns power, says Bahl, and we don39。t like to do that. Another ZigBee and munications option is the beacon mode, in which normally sleeping work slave nodes wake up periodically to receive a synchronizing beacon from the work39。s control node. But listening for a beacon wastes power, too, particularly because timing uncertainties force nodes to turn on early to avoid missing a beacon. InYourFace Communication To save as much power as possible, ZigBee employs a talkwhenready munication strategy, simply sending data when it has data ready to send and then waiting for an automatic acknowledgement. According to Bob Heile, who is chairman of both the ZigBee Alliance and IEEE , talkwhenready is an inyourface scheme, but one that39。s very power efficient. We did an extensive 52 analysis that led to the best powersaving strategy in various kinds of environments from quiet to noisy, Heile says. We discovered that, hands down, we were better off just sending the packet and acknowledging it. If you don39。t get an ack, it just means you got clobbered, so send it again. You wind up having much better power management than if you listen and determine if it39。s quiet before you talk. Fortunately, this inyourface strategy leads to very little RF interference. That39。s largely because ZigBee nodes have very low duty cycles, transmitting only occasionally and sending only