Smoke detector. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Smoke detector. 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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This smoke detector can detect smoke or high concentrations of opaque dust particles suspended in the air. The device can be used in hot shops (foundries, welding), cement plants, woodworking plants, garages and closed parking lots. The device will notify you of a high concentration of smoke, harmful dust suspensions and automatically turn on the forced ventilation system.

The device can be set to high power and spread the smoke sensor tubes over a long distance and, conversely, to high sensitivity. The scheme of the device is shown in the figure. The smoke sensor consists of a VD1 LED and a VD2 photodiode operating in the infrared region of the spectrum. The light from the AL301 type LED passes through the gap between the tubes and illuminates the VD2 photodiode of the FD206 type. The photodiode opens, and a positive voltage appears on its cathode, which is fed to the inverting input of the operational amplifier DA1. A reference voltage is supplied to the second input of the amplifier from the output of the variable resistor R4. This resistor sets the sensitivity. In the absence of smoke or dust in the air, the voltage at the cathode of the photodiode VD2 exceeds the voltage taken from the variable resistor R4 engine, so the voltage at the output of the operational amplifier DA1 is close to zero. In this case, the VD3 LED does not light up, but the element DD1.1. locked.

(click to enlarge)

If for some reason the transparency of the air deteriorates (the appearance of smoke, dust), then the illumination of the VD2 photodiode decreases. The current through the photodiode decreases, and the voltage on its cathode also decreases and becomes less than the voltage on the R4 engine. The voltage at the output of the operational amplifier DA1 becomes close to the supply voltage, and the element DD1.1. unlocked. On elements DD1.1., DD1.2. assembled low-frequency generator. At the output of element 11 DD1.2. there are pulses with a frequency of 4 Hz, which periodically allow the operation of another audio frequency generator, assembled on the elements DD1.3., DD1.4. and generating a signal at a frequency of 1000 Hz.

The BA1 piezoceramic emitter reproduces an intermittent alarm signal, while the VD3 LED also lights up. Structurally, the sensor is made in the form of two opaque tubes with a diameter of 4 mm and a length of 75 mm. A VD1 LED and a VD2 photodiode are inserted along the edges of the tubes. The other edges of the tubes are turned towards each other. With this arrangement, the photodiode is not affected by extraneous infrared radiation.

Literature

Radioamator 10/99

Publication: N. Bolshakov, rf.atnn.ru

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