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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Infrared presence sensor. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Safety and security The proposed device is intended for the protection of premises. An alarm signal will sound if a moving or stationary object is detected in the protected area, which was absent at the time the device was turned on. Very often, non-contact sensors are used in security systems to control the near zone. This is the space near the doors, part of the corridor, flight of stairs, a table, a safe, etc. Usually, such problems are solved by means of high-frequency technology. The sensor can be an LC oscillator that detunes when foreign objects approach, a high-frequency bridge that loses balance, etc. But there are other means. On fig. 1 shows a diagram of a device that generates short infrared (IR) pulses and receives their reflection from an object that has appeared nearby. Here BI1 is an IR diode, periodically excited by current pulses, the amplitude of which Iimp = (Upit-3,5)/R5 can many times exceed the average allowable value. The duration of these pulses timp=0,7R3C2=10 µs, and the repetition period T=1,4R2C1=0,2 s.
The reflected IR pulse hits the BL1 photodiode. After amplification and limitation by the DA1 chip, it enters one of the inputs of the DD2.1 element (pin 13). If the reflected pulse coincides with the emitted one (the pulse that excites the IR diode is fed to pin 12 of DD2.1), then a short occurs at the output of DD2.1 ( So the device "sounds" the reflected IR pulses. A series of such impulses will be transformed by it into an alarming sounding sequence, following with the frequency of IR impulses. In table. 1 shows the detection range of a person (Dperson) and a wall (Dst) depending on the current in the IR diode (IBI1), i.e., on the resistance of the resistor R5. The measurements were carried out at a supply voltage of 6 V. The minimum value of Dperson corresponds to a person in a dark coat.
The device is assembled on a printed circuit board made of double-sided foil fiberglass 1,5 mm thick (Fig. 2). The foil under the parts is used only as a common wire. Connections to it with the conclusions of resistors, capacitors, etc. are shown in black squares. Black squares with a bright dot in the center show those pins of microcircuits and oxide capacitors that must be connected to a common wire and at the same time pass through the board. Protective circles with a diameter of 2 ... 2,5 mm (not shown in Fig. 2) should be etched in the foil at the places where the conductors pass. The foil must also be removed under the transistor VT1, which is attached to the board with an M3 screw.
The front panel of the device, on which the photodiode and IR LED are installed, has dimensions of 92x32x3 mm. It is made of black high-impact polystyrene (Fig. 3). In the places where the IR diode and photodiode are installed, it should have thickenings (rings of the same polystyrene are glued to the top and bottom of the panel), which should isolate them optically.
The fully assembled board is mounted on the front panel as shown in fig. 3: to three posts 14 mm high glued to the panel (only one is shown in the figure), the board is fixed with M2 screws. In order to avoid illumination of the photodiode from the side of the leads, the "bottom" parts of the IR diode and the photodiode are sealed with circles of black electrical tape. The DA1 chip includes a highly sensitive amplifier, so it needs to be shielded. The screen is bent from tin in the form of an open box with dimensions of 32x16x10 mm. It is soldered in the corners, a hole for the photodiode is made in the "roof", the bottom is leveled with a wide file with a fine notch and soldered to the board foil in the position shown in Fig. 2 dashed line. If it is necessary to shield the photodiode as well, it is placed in a thin-walled metal tube of a suitable diameter and length, which is soldered directly to the screen box. A properly assembled device usually immediately starts working in alarm mode - the ceiling, walls, furniture give a completely sufficient reflected signal. But if it continues to sound and put "face" on the table, then it will be necessary to detect and eliminate the ways of penetration of IR radiation to the photodiode inside the device itself. After that, it remains to determine the resulting "range" and set the desired one by selecting the resistor R5. Sometimes such a direct reaction of the device, voicing each reflected impulse, is not at all necessary. On fig. 4 shows a part of the device circuit that needs to be changed so that the alarm is generated only when passing through a compact group of reflected signals.
The alarm will sound only if four reflected pulses are received at the CP input of the DD3.1 counter. But this should happen at a time interval of 16T (3,2 s), since the decay of each sixteenth pulse of the master oscillator returns the counter DD3.1 to the zero state (a reset pulse with a duration of 20 μs is formed at the output of the element DD2.2). That is, if at one of these time intervals the sensor detects four reflected pulses, it will turn on the alarm. Its sounding time is tTp<2,4 s (12T). If the object does not leave the control zone, the alarm signal will be repeated. The connection of the output of the element DD2.2 with the input R of the counter DD3.2 is necessary for a reliable reset when the power is turned on. The device can enter the security system as one of its sensors. For her, only the signal that appears at the output of the element DD2.1 will be of interest. In table. Figure 2 shows the dependences of the current consumed by the IR sensor in standby mode (Id), the current consumed by it in alarm mode (Itr), as well as the alarm signal power (Ptr) on the power supply voltage (Upit) with the resistance of the dynamic head HA1 25 Ohm and R5 =16 Ohm.
Reflections from walls, ceilings, furniture, etc., if the device is poorly placed inside the room, can leave considerable "holes" in the built protection, or even completely block its operation. So, if a sensor with R5 = 16 Ohm is installed in a corridor 3,2 m wide in position 1 (see Fig. 5, a), then an uncontrolled passage with a width of at least 1,6 m will remain at the far wall of the corridor. But if the sensor set in position 2, then it will no longer be possible to go through the door unnoticed. And since here it "shines" along the corridor, then, without fear of reflections, the radiation power can be increased (position 3 in Fig. 5, a).
To control the stairway (Fig. 5, b), the resistor R5 is selected so that the sensor stops responding to reflections from the opposite wall. And since Dperson>0,5Dst (see Table 1), a person walking along the nearest flight of stairs will be noticed. In the gate opening (there may not be a gate itself), the device is installed as shown in fig. 5, c. In order to prevent the reflection of IR pulses from the opposite pole, you need to slightly turn the device into the yard (thus, the sensor will not react to passers-by). Even the minimum Dst indicated in Table. 1, it may turn out to be excessive if a close passage, a manhole, a cable corridor, an air duct, etc. are under control. But a decrease in Dst (respectively, and Dperson) is not a problem: you just need to increase the resistance of the resistor R5. If necessary, the "range" of the sensor can be increased. On fig. 6 shows a diagram of a high power IR pulse generator. With the same IR diode AL 156V, Dperson and Dst will increase by 1,5 ... 2 times, and with the IR diode AL123A - by 2,5 ... 3 times.
The radiation pattern of the sensor depends on the radiation pattern of the IR diode, the sensitivity of the photodiode, and how much both are "drowned" in their sockets. All components of the device - the sensor itself, the power supply and the dynamic head - can be combined into a single design. But if the alarm signal should not be universal, the dynamic head and the power source are taken out to another room and connected to the board by a three-wire line. Author: Yu.Vinogradov
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