A simple smoke detector. Scheme, description

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Simple smoke detector

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Encyclopedia of radio electronics and electrical engineering / Indicators, detectors, metal detectors

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In civil buildings, smoke detectors of both foreign and domestic production have long been used. Recently, in order to provide timely signaling of a fire that has begun, they began to be installed in residential buildings. But what if there is no such sensor and there is no way to purchase it?

Simple smoke detector
Fig. 1

A homemade smoke detector can be assembled according to the diagram shown in the figure. The sensitive element here is an open channel optocoupler formed by two IR emitting diodes VD1 and VD2. The first of them works for its intended purpose, and the second receives its radiation. The IR signal of the diode VD1 is modulated by pulses with a frequency of 0,5 ... 1 kHz coming from the generator on the elements DD1 1 and DD1.2 through an amplifier on the transistor VT2.

With a good optical connection between the diodes VD1 and VD2, a pulsed voltage is formed at the terminals of the latter, which amplifies the transistor VT3, and then detects the transistor VT4, the collector voltage of which in this case has a high logic level.

If, as a result of the presence of smoke in the air, the optical connection between the diodes VD1 and VD2 is broken, both the amplitude of the voltage at the gate of the transistor VT3 and the constant voltage at the collector of the transistor VT4 will decrease. Schmitt trigger on the elements DD1.3 and DD1.4, to the output of one of the elements which the base circuit of the transistor VT1 is connected, monitors the voltage level at the collector of the transistor VT4.

As a result, in the absence of smoke, the transistor VT1 is closed and the microcircuits DD2 DD3 are disconnected from the power source. With the advent of smoke, this transistor opens to the indicated microcircuits, power is supplied and the audio signal generator assembled on them starts to work. An electromagnetic sound emitter HA5 is connected to the output of the generator through an amplifier on a transistor VT1.

The KPZ0ZA transistor can be replaced with KPZ0ZE, and KT3107A with one of the KT361 series. Any transistor from among KT817A, KT603A, KT503A in this device can be replaced by another from the same list or by a transistor of the KT815, KT817 series.

The design of the sensor can be arbitrary, but the diodes VD1. VD2 should be placed side by side, directing their optical windows to each other and providing them with free access of air from the space surrounding the sensor.

Setting up the device comes down to setting the trimmer resistor R7 to a position in which the presence of smoke (for example, cigarette smoke) between the diodes VD1 and VD2 leads to an audible signal, and after its removal, the signal stops. Achieving a good result, you can select the relative position of the diodes VD1 and VD2. It is recommended to repeat the check and adjustment of the sensor several times.

Author: A. Pavlov. Mirny, Arkhangelsk region; Publication: radioradar.net

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