【Repost】Design of the amplifier circuit of pyroelectric infrared sensor
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With the popularization of information technology, infrared detection technology has achieved rapid development and is widely used in night vision, alarm, medical and automatic control and other fields. In the infrared detection system, the infrared sensor is the core device, and its performance determines the sensitivity of the entire infrared detection system, while the preamplifier circuit is the key part affecting the performance of the infrared sensor. Since the response signal of the infrared sensor is very weak, strict requirements are put forward for the preamplifier, such as low noise, high gain, good low-frequency characteristics and strong anti-interference. The following proposes a new high-gain, low-noise preamplifier circuit design scheme based on the characteristics of the output signal of the pyroelectric sensor. This scheme well meets the requirements of the pyroelectric sensor for the preamplifier with low noise, high gain, good low-frequency characteristics and strong anti-interference ability. 1. Output signal characteristics and noise analysis The amplitude and frequency of the output signal of the pyroelectric infrared sensor are mainly determined by the temperature of the target human body, the background of the detection area, the distance between the human body and the sensor, the speed of the human body, the focal length of the optical lens system and its design. The temperature difference between the human body temperature and the background of the detection area is large. The closer to the sensor, the greater the amplitude of the output signal. When the dual-sensitive element pyroelectric sensor is used with a Fresnel optical lens, the peak-to-peak value of the output signal waveform voltage is about 1mV, and the frequency can be calculated by the following formula: ![]()
[img=114,50] 51)]Where f is the output signal frequency (Hz); Vb is the human body movement speed (m/s); fb is the focal length of the optical system (mm); S is the area of the sensor sensitive element (mm); L is the distance between the human body and the sensor (m). For dual sensitive element sensors, the standard size is 2×1mm2, the human body movement speed range is 0.5~5m/s, and the Fresnel lens focal length used on common detectors is 25mm. From this, we can calculate that the frequency range of the sensor output signal is 0.08~8Hz. Since the output signal of the sensor is very weak, it is easily interfered by noise, and even the effective signal is submerged in the noise. Studies have found that the interference sources of the output signal on the sensor mainly come from the thermal noise, inherent noise, voltage and current noise of the amplifier, etc. Thermal noise is generated by the random thermal motion of the charge carriers in the detector material. To reduce the impact of thermal noise, the distance between the pyroelectric infrared sensor and the preamplifier circuit should be shortened as much as possible, the external thermal interference should be reduced, and a low-pass filter circuit should be connected in series in the preamplifier circuit to limit the noise bandwidth. The peak-to-peak value of the inherent noise voltage of the sensor is about 50μV. The outdoor hot air flow can generate noise close to 250μV, and it is also close to 180μV indoors. Other possible interferences, such as space electromagnetic wave interference and mechanical vibration, have noise amplitudes close to 100μV. The maximum amplitude of the superposition of the three noises is close to 300μV. II. Design of preamplifier circuit According to the output signal characteristics of the pyroelectric infrared sensor, the preamplifier circuit signal processing needs to extract useful weak signals from a variety of noise interferences. Therefore, the preamplifier circuit should have the characteristics of low noise, high gain, good low-frequency characteristics, and strong anti-interference ability. Therefore, it is usually composed of three parts including bandpass filtering, two-stage high-gain amplification, and comparison circuit as shown in the figure. [color=rgb(51, 51, In the figure above, a 10kΩ resistor is connected in series between the D terminal of the pyroelectric sensor and the 5V power supply to reduce radio frequency interference. The G terminal is grounded and the S terminal is connected to a 47kΩ load resistor. The bias voltage is about 1V. The sensor output is directly coupled to the bandpass filter composed of a low-noise operational amplifier (LM324) and the reverse input terminal of the first-stage amplifier circuit, and then coupled to the second-stage reverse amplifier circuit through resistor R6 and capacitor C8 for further filtering and amplification. The upper cutoff frequency is: The upper cutoff frequency is: ![]()
The lower cutoff frequency is: ![]()
The circuit gain is related to the frequency. When the input signal frequency is 1Hz, the first stage amplification gain is approximately: 51)]![]() The second stage amplification gain is: The second stage amplification gain is: ![]()
The calculated bandwidth is 15.83Hz and the total gain of the circuit is 66dB. The dual-limit voltage comparator is composed of the other two amplifiers of the quad op amp (LM324). From the noise analysis in the previous article, it can be seen that the maximum amplitude of the noise source is close to 300μV. After the two-stage amplification circuit, the maximum noise amplitude reaches 600mV. The second-stage amplifier circuit is biased at VCC/2, that is, 2.5V. Therefore, the high and low thresholds of the dual-limit voltage comparator should be set to 3.1V and 1.9V to effectively resist noise interference. That is, when the amplifier output signal level is greater than 3.1V or less than 1.9V, the comparator outputs a high level, indicating that a moving human body is detected. 3. Application of pyroelectric infrared sensors The application of pyroelectric infrared sensors is very wide, which can be roughly divided into qualitative measurement and quantitative measurement. Quantitative measurement is to measure the temperature T of the infrared light source, which is a non-contact method of measuring temperature. Its basic basis is as follows. First, the radiation energy flux density ωλ can be expressed as: 51)]![]() Where, ωλ is the emissivity, which is equivalent to the correction of the absolute black body, and is a number less than or equal to 1; h is Planck's constant; λ is the wavelength, c is the speed of light; k is the Boltzmann constant; and T is the absolute temperature. 51, 51)]From the formula, it can be seen that if the radiation rate of the light source is constant, the relative position of the infrared sensor and the light source and the optical system are fixed, then the temperature of the sensor has a certain relationship with the temperature of the light source. In fact, the chopper plate is used to chop the light source, so the temperature of the pyroelectric device sometimes reflects the temperature of the light source and sometimes reflects the temperature of the chopper plate. Therefore, the amplitude of the pulsating voltage output by the pyroelectric device is the difference between the two functions with the temperature of the light source T and the temperature of the chopper plate as independent variables. Qualitative measurement is the largest application field of pyroelectric infrared sensors. Its basic principle is to detect the existence of the target based on the different properties of the detected object and the background radiation. The surface temperature of the human body is about 34℃, the infrared peak is about 10μm, and the signal generated by the movement of the human body corresponds to a frequency of 0.1～10Hz. Compared with the visible light sensor, the pyroelectric method of detecting the human body has several characteristics. First, it uses the human body's own light instead of other light sources, so the working device is simple and reliable. In addition, since infrared rays are not felt by people, they have the advantage of good concealment. In recent years, in addition to being used in military and industrial occasions such as remote sensing, guidance, night vision, active radar, thermal imaging, gas analysis, radiometers, and temperature measurement, the application of pyroelectric infrared sensors in consumer electronic and electrical products is growing rapidly. At present, the most widely used is the sensor for detecting people, such as for anti-theft alarm systems. 51)]![]() Infrared alarm composition block diagram The infrared rays emitted by the object first pass through the Fresnel lens and then reach the pyroelectric infrared detector. At this time, the pyroelectric infrared detector will output a pulse signal. After the pulse signal is amplified and filtered, it is compared with the reference value by the voltage comparator. When the output signal reaches a certain value, the alarm circuit sounds an alarm. The preamplifier circuit adopts the scheme designed in the first figure (1). The test shows that the signal processing circuit has good low-frequency characteristics, has a relatively high amplification gain for the low-frequency weak signal output by the sensor, and has strong anti-noise interference ability. The circuit design experiment results also prove that the preamplifier signal processing circuit has high sensitivity and low detection error rate when used with the pyroelectric infrared sensor. IV. Conclusion By analyzing the output signal characteristics of the pyroelectric infrared sensor, the noise signal of the designed preamplifier circuit was analyzed and studied. It shows that the weak signal processing circuit has a higher gain at low frequency, the frequency characteristic curve is relatively flat near 1Hz, the distortion is small when amplifying the low-frequency signal, and the 3dB bandwidth is only 15.94Hz. It can effectively filter out high-frequency interference and improve the signal-to-noise ratio, meeting the requirements of the pyroelectric infrared sensor output for the signal processing circuit design. However, in specific applications, the influence of the actual detection environment on the sensor output signal should be fully considered, and the signal processing circuit should be reasonably selected and designed to reduce errors and maximize the detection function of the infrared sensor.
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