How to Correctly Design a DIY Smoke Sensor System

By Kaushik Hazarika. He is an embedded project consultant and inventor of a low-cost LED inverter.

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Fire sensors are essential elements for detecting fires in homes accurately, quickly and for reliably accomplishing both first-order firefighting and second-order disaster prevention. To effectively design a smoke sensor system, the following functions are required:

1. Capable of prompt discovery of fires. This is essential for minimizing the damage, recognizing that with longer, bigger tunnels, the number of cars passing through will be high and much time is required for restoring service after the fire; and

2. Accuracy in identifying the point of fire.

The main methods of detecting fires include: detecting the infrared rays generated by the flame, IR scattered light, detecting the temperature rise caused by the fire etc. Let’s explore some sensing methods that can give a good idea of normal fire detection methods. Then we will learn how to correctly design a DIY smoke sensor system.

As sensors of infrared rays, CO2 resonance radiation detectors can be used. This type of detector detects the unique spectrum of carbon dioxide gas which is generated when material is burned and senses a fire. Because detecting the infrared rays generated by a flame is a straight forward method, fires are discovered immediately. Customarily, such sensors are installed every 25 meters and thus the point of origin of the fire can be identified easily.

We have an array of sensors that detect rise in temperature. The advanced temperature sensors are using optical fibers. They can add a great value in making the fire systems sensitive. Using optical fiber temperature sensors, the temperature at any desired point on an optical fiber can be measured. However, the heat must reach the sensor panel and hence detachable sensors are required, the temperature rise may be small and combined logic of fire-smoke sensing can be used. This is a must have device that can save lives, on top early detection poisonous gases generated by the fire is also recommended. For this reason, such sensors may be effective means for accurately grasping the total situation of a fire in a tunnel, when used in combination with an infrared type fire detector.

Other types of fire detecting sensors include smoke detectors which detect reduction in light transmitted due to smoke and methods which use modern cameras. In the case of smoke detection, it has the disadvantage that it is hard to identify the fire point because smoke disperses through the lower areas. Also, now-a-days with Artificial intelligence even more lower cost systems were developed, by means of image processing via CCTV cameras, the software monitors the effect of light disturbance. The turbulent air becomes dominant and resultant change of image is detected in case of fire. The camera sensing faces issues if there is dark smoke, where IR camera systems work well. But if infrared cameras are used, the issue of high cost is a consideration.

Development of Fire Detectors

In the field of fire detectors, the IR reflectance models has a long history and significantly works well. In the specific area of CO2 resonance fire detectors, the factors DIY designers must consider are

1) Battery life
2) Sensing accuracy
3) Alarm loudness
4) Fire retardant housing

Out of these battery life is critical factor and hence smaller microcontrollers must be used. There are custom ICs for photoelectric detection but that limits the configurability.

Let’s see an example circuit:

smoke sensor system

In the absence of any smoke, the gap of photo interrupter module is clear and the light from LED falls on the phototransistor through the slot. As a result, the collector of phototransistor is pulled towards ground. This causes reset pin 4 of IC 555 to go low. Accordingly, the timer is reset and hence the alarm does not sound.

When smoke is present in the gap of the photo interrupter module, the light beam from LED to the phototransistor is obstructed and sets alarm.

The circuit has its own set of problems

It cannot be housed in compact case. This circuit uses like the bigger sized parts viz. the bulky speaker which is not suitable for compact product size.

The biggest defect is the power drainage that DIY guys don’t consider. The LM555/NE555 standby current is typical 6 mA maximum. This makes the battery to drain in few days. The DIY sensor becomes useless. An adoption to better design is to use smaller microcontroller like Attiny85. This can have sleep mode when no operation is needed. Polling sensor at intervals is the way to go.

In intermittent operation is recommended with a factor of 800 the operating current to be reduced to 20mA/800 = 0.025mA plus the current that is drawn in sleep mode 0.05mA.

Now with 2000mA battery/ (0,025+0,05) we come to 27000 hours lifetime which is about 3 years. There are many small controllers to do this.

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