Smoke detector for fire alarm. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Smoke detector for fire alarm. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling

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The damage from a fire can be even greater than from thieves, and a timely signal of an alarm will allow at least something to be saved.

Rice. 3.21. Smoke sensor wiring diagram (click to enlarge)

At industrial facilities, thermal sensors are mainly used for fire alarms (they are the cheapest). The peculiarity of their device is such that they give an alarm when the protected premises have already burned down.

According to firefighters, smoke sensors are considered the most reliable, but not everyone can afford them.

One of the options for the smoke sensor is shown in Fig. 3.21. The circuit consists of a generator (on the elements of the microcircuit DD1.1, DD1.2, C1, R1, R2), a short pulse shaper (on DD1.3 and C2, R3), an amplifier

Smoke detector for fire alarm
Rice. 3.22. Type of sensor design

(VT1) and emitter (HL1) of IR pulses, as well as a comparator (DD2) and a switch on a transistor (VT2). When receiving IR pulses, the HL2 photodiode triggers the comparator and discharges capacitor C4 with its output. As soon as the passage of the pulses is broken, the capacitor will be charged through the resistor R9 for 1 second to the supply voltage, and the element D1.4 will start working. It passes the pulses of the generator to the current switch VT2. The use of the HL3 LED is not necessary, but if it is present, it is convenient to control the moment the sensor is triggered.

The design of the sensor (Fig. 3.22) has a working area, when smoke enters it, the passage of IR pulses is weakened, and if several pulses in a row could not pass, the sensor is triggered (which ensures noise immunity of the circuit). In this case, current pulses appear in the connecting line, which are distinguished by the control circuit shown in fig. 3.23.

Smoke detector for fire alarm
Rice. 3.23. Control scheme

Many smoke detectors can be connected (in parallel) to one security loop. When setting up the control circuit • resistor R14 set the transistors so that VT3 and VT4 are in the locked state (LED HL4 is off).

One smoke detector in the ARMED mode consumes a current of no more than 3 mA and has been tested for operation in the temperature range from -40 to +50 °С.

The output of the control circuit (collector VT4) can be connected to the security system directly instead of the sensor.

When using several sensors installed simultaneously in different places, the scheme can be supplemented with an indicator of the number of the triggered smoke sensor. To do this, it is necessary that the frequencies of the generators (depending on C1 and R2) differ from each other, and using a digital frequency indicator, for example, proposed by M. Nazarov ("Radio", N 3, 1984, pp. 29-30), it will be easy to determine fire place. This eliminates the need to conduct security loops separately to each sensor, which greatly simplifies the wiring and reduces their consumption.

Transistors VT1 and VT2 can be replaced by KT814. IR diodes will fit many other types, but it may be necessary to select the value of the resistor R6.

Capacitors used C1, C2, C4, C5 type K10-17a, C3 - K53-18-16V, C6 - K50-6-16V. Resistor R14 type SP5-2, the rest of the type C2-23.

It is advisable to install the smoke sensor in rooms where flammable objects are stored, and place it in places where the air flow passes, for example, near a vent - in this case, a fire will be detected earlier.

The circuit can also find other applications, for example, as a proximity sensor for burglar alarms or automation devices.

Publication: cxem.net

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