A smoke sensor controls the transparency of the air in the room in which it is installed, and in the case of smoke (opacity of the air decreases) its output level is set to logic zero. Schematic diagram shown in the figure.
At the core of the sensor is an optical pair, consisting of led-photodiode VD1 and VD2. The photodiode and the led located at a distance of about 50 mm from each other and directed. that they optical communications.
Don't have smoke optical communications and high reverse resistance of the photodiode is low, much lower than the resistance of resistor R2. Therefore, in the connection point and VD2 R2 voltage corresponds to the level of logical units. Schmitt trigger on D1 is in a single state, and the sensor output will be a logical one.
If you encounter smoke, the transparency of the air is getting worse and optical coupling between VD1 and VD2 is weakening. As a result, the resistance of the photodiode VD2 increases, and at some point, the voltage in the connection point VD2 and R2 becomes lower than the threshold logic zero. Schmitt trigger on D1 takes zero position and the sensor output is set to low logic level that is a signal of a fire hazard.
Details
The circuit uses a photodiode FD-320 (from remote control TV type USCT). It can be replaced with another similar, for example FD-611. The led can be virtually any visible spectrum radiation.
The sensor has a housing in the form of a box with a rectangular hole at the bottom for the passage of smoke. The box is made thus (it is a partition on half of height of the box) through the hole on the sensor could not get direct sunlight (or light from lighting devices).
Setting
The adjustment of the resistor R2 need to ensure that the sensor has been triggered (it's output is set to log. 0) when placed between VD1 and VD2 of a sheet of paper from the Fax machine, as in complete darkness, and in normal daylight. If necessary, - to find the value of R1.
Final setting should be conducted on a smoke, outside of the room and observing all the rules of fire safety.
In the process of setting you need to be exposed to the sensor to direct sunlight (or light from an illumination lamp).
The sensor shall be supplied with stable voltage.
Author: Lyzhin R.; Publication: www.cxem.net