Fire smoke detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling
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Smoke sensor controls the degree of air transparency in the room in which it is installed, and in case of smoke (air transparency decreases), a logic zero level is set at its output. The circuit diagram is shown in the figure.
The sensor is based on an optical pair consisting of a VD1 LED and a VD2 photodiode. The photodiode and LED are located at a distance of about 50 mm from each other and directed so. so that there is an optical connection between them.
As long as there is no smoke, the optical coupling is high and the reverse resistance of the photodiode is low, well below the resistance of the resistor R2. Therefore, at the connection point of VD2 and R2, the voltage corresponds to the level of a logical unit. The Schmitt trigger on D1 is in a one state, and the output of the sensor will be a logical one.
When smoke occurs, the transparency of the air deteriorates and the optical connection between VD1 and VD2 weakens. As a result, the resistance of the photodiode VD2 increases, and at a certain moment the voltage at the junction point of VD2 and R2 becomes below the logic zero threshold. The Schmitt trigger on D1 takes a zero position and the sensor output is set to a low logic level, which serves as a fire danger signal.
Details
The circuit uses a photodiode FD-320 (from remote control systems for TVs such as USCT). It can be replaced by another similar one, for example FD-611. An LED can be virtually any visible radiation spectrum.
The sensor has a box-like housing with a rectangular hole at the bottom for the passage of smoke. The box is made in such a way (it has a partition half the height of the box) that direct sunlight (or light from lighting fixtures) cannot enter the sensor through this hole.
Setting
By adjusting the resistor R2, you need to ensure that the sensor is triggered (log. 0 is set at its output) when a sheet of fax paper is placed between VD1 and VD2, both in complete darkness and in normal daylight. If necessary, select the value R1.
The final adjustment must be carried out on smoke, always outdoors and observing all fire safety rules.
During the adjustment process, it is necessary to exclude direct sunlight (or light from lighting lamps) on the sensor.
The sensor must be powered by a stable voltage.
Author: Lyzhin R.; Publication: cxem.net
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