【正文】 with this sensor for effective alarm. The block diagram is shown in Block diagram of passive pyroelectric infrared alarm system When man enters into the detecting range of alarm device, human infrared radiation firstly passes through Fresnel lens, which focuses infrared radiation signal on pyroelectric infrared detecting unit and forms alternating change of highly sensitive areas and blind areas of infrared radiation, because the heat is different ,received by two pyroelectric infrared detecting units of sensor and the signal is different, produced by them, two signals can’t be offset and output is achieved. By conversion, voltage signal is obtained,who is weak and variable,then is magnified to acquire enough gain. A bandpass filter is used for suppressing noise, by means of subsequent processing such as window parator, regulated circuit, delay circuit, sound and light alarm is pleted. In this system, each part woks well for ensuring effective alarm. According to characteristic of Fresnel lens, it has different focal length, in that it has different detecting distance, so alarm system has different monitoring field of view, which is more and monitoring is tighter. Working principle of system made of object detected, Fresnel lens, pyroelectric infrared sensor is shown in [3] [4][5] .Detection circuit When this system is designed, pyroelectric infrared sensor is chosen, whose type is D203s, responsive wavelength is 7~ 14181。m and field of view is 139176。126176。, both detecting units whose size is 21mm are sensitive area。 Fresnel lens is chosen, whose type is 7803 rectangle lens, focal length is 20 mm, direction angle is 890,detecting distance is system circuit includes filtering circuit, magnifying circuit, window parator, delay circuit, alarm circuit. BISS0001,infrared signal processor, is adopted, which is special integrated circuit of CMOS digitalanalog hybrid structure , has high input impedance operational amplifier, twoway amplitude discriminator, delay timer, blockade time timer and reference voltage source and so on. This processor can effectively suppress interference ,works stably, regulate widely, it can match with pyroelectric infrared sensor and a few external ponents for achieving passive pyroelectric infrared alarm system[2] [6]. Schematic diagram of PIR working principleBecause pyroelectric infrared sensor is mainly used for responsing man movement, according to human movement frequency, its responsive frequency range is from to 10Hz and other frequency signal should be filtered well[5].The signal whose frequency is less than is subjected to be affected by temperature change of external environment and variety of ponent parameter, noise voltage of the signal whose frequency is more than 10Hz bees larger with frequency increasing, at the same time when the signal frequency is close to 50Hz ,there is electromagnetic interference. When the circuit is designed ,according to the signal noise voltage change and frequency band requirement, active band pass filter is designed for eliminating interference and enhancing the whole system stability. Pyroelectric infrared sensor is posed of pyroelectric infrared units ,low noise JFET and filter. When the preamplifier is selected, operational amplifier of BISS0001 chip is utilized. High input impedance is needed for avoiding effect on system detection after equivalent output noise voltage of preamplifier is magnified by subsequent magnifying circuit because this amplifier ‘s equivalent input noise current is 2 to 3 orders of magnitude less than one of general integrated operational amplifier. As well as it is considered that circuit responsive range is from to 10Hz,the circuit designed is shown in f3=1/2πR3C3 and it is frequency cutoff point for filtering the signal frequency more than f1= 1/2πR1C1 and f2= 1/2πR2C2 are frequency cutoff point for filtering the signal frequency less than is beneficial to improve sensitivity. Equivalent circuit of pyroelectric infrared sensor and two bandpass amplifier Alarm system circuit is shown in man moves in the range of warning area, infrared radiation from man is focused on detecting units of pyroelectric infrared sensor by Fresnel lens. Temperature of these units changes to result in signal output across resistance R. This signal is sent into Input IN+ (14pin)of signal processor BISS0001(IC1) and processed by magnifying , twoway lse delay signal is acquired from output U (2pin) of the control terminal A of IC1 is defined as nonretrigger, in that 1pin connects with ground, Delay time Tx of output pulse signal is unrelated to retrigger, Tx totally depends on external ponent R1 and C10,that is Tx=49152R1C10≈22s；trigger blockade time relies on external ponent R2 and C1 respectively connecting with 5pin and 6pin of IC1,that is T=24R2C1≈（s）. high pulse delay signal from output U (2pin) of BISS0001 inputs in base of transistor Q2 through resistance R13, transistor Q2 connecting with LED2 and resistance R11 forms emitter follower, when there is signal output, LED2 lights. output terminal of emitter follower directly connects with input terminal of analog sound integrated circuit IC2,so when the signal outputs from IC1, IC2 is triggered to work, alarm sound is produced and delays for 22 seconds by IC2 output signal who drives loudspeaker ,then alarm sound stops and waits for the next trigger.Testing experiment Passive pyroelectric infrared alarm system can be only installed indoor, false alarm rate is closely related to installation location and method. so installation points for attention should be emphasized, the following are(1) alarm system should be ~ meter higher than ground.(2) alarm system should be far from the place whose temperature change is sensitive such as air conditioning, refrigerator, stove and so on.(3)in the range of warning area,