Structure of passive infrared alarm
1 Structure
Passive infrared alarm is mainly composed of optical system, pyroelectric infrared sensor, signal filtering and amplification, signal processing and alarm circuit. Its block diagram is shown in Figure 2. In the figure, the Fresnel lens can focus the infrared radiation of human body radiation onto the pyroelectric infrared detector element, and at the same time, it also produces alternately changing high-sensitivity regions and blind regions of infrared radiation to adapt to the changing characteristics of the pyroelectric detector element. Pyroelectric infrared sensor is the core device in the alarm design, it can convert the body's infrared signal into an electrical signal for use by the signal processing part; signal processing is mainly to amplify, filter, delay the weak electrical signal output by the sensor. , Comparison, lay the foundation for the realization of the alarm function.
2 How it works
In this detection technique, the so-called "passive" means that the detector itself does not emit any form of energy, but merely receives natural energy or energy changes to complete the detection. Passive infrared alarm device is characterized by being able to respond to infra-red radiation changes caused by intruders moving in the protected area, and can make the monitoring alarm device generate an alarm signal to complete the alarm function.
When the infrared radiation emitted by the human body is focused on the detector element of the pyroelectric infrared sensor through the Fresnel lens, the sensor in the circuit will output the voltage signal, and then the signal is first passed through a combination of C1, C2, R1, and R2. A bandpass filter with an upper cutoff frequency of 16 Hz and a lower cutoff frequency of 0.16 Hz. Because the pyroelectric infrared sensor output detection signal voltage is very weak (usually only about 1mV), but also a changing signal, while the role of the Fresnel lens makes the output signal voltage in the form of pulses (pulse voltage frequency by Since the moving speed of the measurement object is usually about 0.1 to 10 Hz, the voltage signal output from the pyroelectric infrared sensor should be amplified. This design uses an integrated operational amplifier, the LM324, to perform two stages of amplification so that it gains enough gain.
When the sensor detects the infrared signal emitted by the human body and sends it to the window comparator after being amplified, if the signal amplitude exceeds the upper and lower limits of the window comparator, the system will output a high-level signal; if there is no abnormal situation, a low-level signal will be output. In this comparator, R9, R10, and R11 are used as reference voltages. Two operational amplifiers are used for comparison. The main role of the two diodes is to make the output more stable. The upper and lower limit voltages of the window comparators are reference voltages of 3.8V and 1.2V, respectively. This high and low level signal rising edge signal is used as the trigger signal of the one-shot HEF4538B, and it is allowed to output a high level signal with a pulse width of about 10 s. Then use this pulse width signal as the input control signal of the alarm circuit KD9561 to make the circuit generate a 10s alarm signal, and finally use the triode VT1 and VT2 to amplify the electric signal again so that there is enough current to drive the speaker. Send a continuous 10s alarm.