Smoke detector circuit

Radioamateur 1999, 10

This device detects smoke or dust particles in the air. Its circuit diagram is shown in figure 1. The smoke sensor contains of LED VD1 and photodiode VD2, working in the infrared frequency of the electromagnetic spectrum. The infrared light from LED VD1 goes through the gap between two pipes (see figure 2) to the photodiode VD2. The resistance of the photodiode VD2 starts decreasing and the voltage across it also decreases. So the positive voltage goes to the inverting input of the op-amp DA1. The non-inverting input of the op-amp is connected to the the potentiometer R4. This potentiometer sets the sensitivity of the device. When there is no smoke or dust particles in the air then voltage at the cathode of the photodiode VD2 is higher than wiper voltage of potentiometer R4, and the op-amp's output voltage is almost zero. So the LED VD3 doesn't light up and the gate DD1.1 is closed.

Smoke detector circuit diagram

Fig. 1. Smoke detector circuit diagram.
DA1 - 741; DD1 - CD4011; VD1 - AL301; VD2 - FD206; C1,C3 - 10nF; C2 - 47nF

Design of the smoke sensor

Fig. 2. Design of the smoke sensor.

If the visibility of the air drops (because of smoke or dust particles), this reduces the illumination of the photodiode VD2. The current through the photodiode VD2 reduces, and voltage at the cathode reduces and becomes lower than voltage at the wiper of the potentiometer R4. The output voltage of the op amp gets closer to the voltage of power supply +12V, and the gate DD1.1 opens.

The logic gates DD1.1., DD1.2 creates the low frequency oscillator. At the output pin 11 of the logic gate DD1.2 appears the pulses with frequency of 4 Hz, and this pulses periodically turns on the audio oscillator with frequency of 1 kHz, based on the logic gates DD1.3 and DD1.4. The piezoelectric buzzer BF1 produces alarm sound, and the LED VD3 illuminates.

The smoke sensor (see figure 2) is made of two opaque pipes with the diameter of 4 mm and the length of 75 mm. In the pipes is inserted the infrared LED VD1 and the photodiode VD2. The pipes should be located against each other. This arrangement prevents external infrared light from entering the photodiode VD2.

E. Kirjner

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