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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Pyroelectric IR sensors. Reference data
Encyclopedia of radio electronics and electrical engineering / Reference materials Today, few people are surprised by the door of an institution or a store that automatically swings open in front of the visitor. In most such cases, the approach of a person is "felt" by a device hanging above the door, equipped with a pyroelectric sensor (receiver) of IR radiation. Such sensors are highly sensitive, durable, easy to use. They are widely used, including in security and fire alarm systems, remote temperature meters. The pyroelectric effect (pyros in Greek - fire) - the generation of electric charges in crystals under the influence of heat - has been known for a very long time, the well-known German physicist Wilhelm Roentgen was studying it back in the XNUMXth century. The effect is akin to piezoelectric, moreover, pyroelectrics, as a rule, also have piezoelectric properties. In crystals of natural origin (quartz, tourmaline), the pyroelectric effect is rather weakly expressed, but the possibility of the existence of substances with an arbitrarily large pyroelectric coefficient - the ratio of the electric charge increment to the temperature increment that caused it, has been theoretically shown. Relatively recently, such substances belonging to the class of ferroelectrics have been synthesized and sensitive sensors have been created on their basis. A typical sensor circuit is shown in fig. 1. The sensitive element B1 is a kind of capacitor - a pyroelectric plate with metal plates. One of the plates is coated with a layer of a substance capable of absorbing electromagnetic (thermal) radiation. As a result of energy absorption, the temperature of the capacitor plate increases and a voltage of a strictly defined polarity appears between the plates. Being applied to the gate-source section of the built-in field effect transistor VT1, it causes a change in the resistance of its channel. The output signal is taken from an external load resistor connected to the drain circuit of the transistor.
After some time, regardless of whether the thermal radiation continues to act on the sensor or not, the capacitor will discharge through the leakage resistance R1 - the output signal drops to zero. Often, sensors are equipped with several sensing elements connected in series with alternating polarity. This ensures the insensitivity of the device to uniform background irradiation and obtaining a sign-alternating output voltage when moving the focused image of the object along the sensitive surface of the sensor. The sensitivity of a pyroelectric sensor is usually measured using the setup shown schematically in Fig. 2. A black body simulator is used as a source of thermal radiation. The flow periodically, with a frequency of 1 Hz, is blocked by a damper-breaker, driven by an electric motor. IR pulses arrive at the sensitive element of the sensor and cause voltage pulses to appear on the external load resistor R1. It is easy to see that the field effect transistor of the sensor is turned on by the source follower here.
Measurements show that the sensitivity of the sensor decreases almost in proportion to the increase in the frequency of the radiation pulses received by it. The reason for this is the significant thermal inertia of the sensing element. Sensors designed to operate at large differences in ambient temperature are equipped with two sensitive elements connected in opposite series - working and compensation. The compensation element can be closed from the external radiation flux, but is in the same temperature conditions as the operating one. The characteristic of the spectral sensitivity of the sensor is determined by the absorbing ability of the coating material of the pyroelectric plate in a particular frequency range of electromagnetic radiation. Finally, it is formed using optical filters installed in front of the sensitive element. Typical characteristics of spectral sensitivity of different versions of pyroelectric sensors are shown in fig. 3.
Sensors with characteristic 1 are designed to detect flames, 2 and 3 are best suited for detecting human movement. Characteristic 4 is optimal for use in remote temperature meters. Pyroelectric sensors for various purposes are produced by several companies. Below will be described in detail about the products of one of them - Murata Manufacturing Co (Japan). The sensors are housed in a cylindrical metal case with three (or four) rigid tinned wire leads (Fig. 4). On the flat end of the case, opposite the terminals, there is a square, rectangular or round window, closed by a filter transparent to IR rays. The same figure shows the pinout of the devices.
The main technical characteristics of pyroelectric sensors of the IRA series by Murata are presented in the table.
The IRA-E710ST0, IRA-E910ST1, IRA-E420S1 and IRA-E420QW1 sensors have built-in blocking capacitors between the gate and source terminals, as well as the gate and drain terminals of the FETs. The body of the IRA-E940ST1 instrument contains two sensors with two sensitive elements each. The device has one common output and a combined drain output, the outputs of the source of the transistors are separate. A typical diagram of the use of a pyroelectric sensor in a security alarm device is shown in fig. 5. Capacitors C1 and C2 are used to suppress high-frequency pickups on the outputs of sensor B1 and should be installed in close proximity to it. These capacitors are not needed if the applied sensor already has built-in ones.
The internal field effect transistor of sensor B1 is connected according to the source follower circuit. Its load is resistor R1. Voltage fluctuations that occur on it when a heated object moves in a sensitive area amplify two op-amps - DA1.1 and DA1.2. Their overall gain peaks at 7500 at 2 Hz, falling off by 3 dB at the 0,5 and 5,5 Hz frequency points. However, the inertia of the sensor itself shifts the total bandwidth of the sensor-amplifier system much lower - up to 0,06...1,2 Hz. As soon as the amplitude of the signal at the output of the op-amp DA1.2 exceeds 0,8 V, the comparator DA2.1 is triggered if the voltage spike is positive, or DA2.2 if it is negative, relative to a certain value close to half the supply voltage (it is determined by the resistor values R10 and R12). The outputs of the comparators (with an open collector) are connected in parallel, so when any of them is triggered, the logic level at the input of the microcontroller changes. As a result of processing the received sequence of pulses (measuring their duration, counting the number for a certain period of time), the microcontroller generates a control signal that activates the actuator or the alarm unit. To increase the spatial sensitivity zone of the sensor, a lens is usually installed in front of its optical window, which focuses the IR rays on the pyroelectric plate. In order to obtain a fan-like shape of the sensitive field of view, similar to that shown in a simplified way in Fig. 6a, a zoned Fresnel lens is used. It consists of many separate focusing areas, each of which forms its own sensitive beam coming from a certain direction. As a result, when moving a moving object from one beam to another, the sensor generates an alternating voltage.
A similar fanning of rays is also formed in the vertical plane (Fig. 6b). Using Fresnel lenses of a special structure, it is possible to vary the shape of the petals in order to obtain the best conditions for detecting an object in a given field of view. In addition to IRA series sensors, Murata produces pyroelectric modules IMD-B101-01 and IMD-B102-01. Along with the sensor itself, such a module contains an amplifier and a pulse shaper suitable for supplying standard logic elements to the inputs (node A3). Block diagram of the module is shown in fig. 7, and the body drawing - in fig. 8.
The pinout of the modules differs little. Both have pin 1 - a common, negative power pin; output 3 - positive power output; pin 4 - digital output. But for the IMD-B101-01 module, pin 2 is the analog output of the sensor signal amplifier, and for the IMD-B102-01, it is the input of the strobe signal of the switch. The main characteristics of the modules:
In systems that automatically turn on the lights when motion is detected in the room, the strobe input of the IMD-B102-01 module is usually supplied with a signal from a photoresistor that responds to the overall illumination. This prevents the system from operating during the daytime. Author: A. Sergeev, Moscow based on materials from the site murata.com.
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