Dangerous gases surround us everywhere. They are invisible and lack a smell that we can detect with our nose. However, electronic sensors exist that promptly react to hidden threats and protect us from danger.
Safety comes first. Carbon dioxide sensors inform us when to ventilate a room, or even automatically open a window or activate ventilation in smart home systems.
Such sensors are essential in all areas where people are present. We consume oxygen and produce carbon dioxide.
If there are stoves or burners in the room, it's necessary to monitor the presence of carbon monoxide. Carbon monoxide forms during the incomplete combustion of carbon-containing fuel when there's insufficient oxygen in the reaction zone to form dioxide.
Carbon monoxide is not only flammable, since it seeks an extra oxygen atom to convert into carbon dioxide, but it's also extremely toxic.
The fatal toxicity of carbon monoxide is due to the nature of our breathing. Red blood cells are designed to capture oxygen and carbon dioxide. They deliver the former to body tissues and remove the latter, carrying it to the lungs where it returns to the air.
Hemoglobin in red blood cells easily captures oxygen and carbon dioxide molecules and releases them just as easily. However, with carbon monoxide, hemoglobin forms a much more stable compound that decomposes ten thousand times more slowly.
The half-life of carboxyhemoglobin is 4-6 hours, and a person can die from oxygen deprivation and carbon dioxide buildup in tissues if a significant portion of red blood cells is blocked by carbon monoxide.
Also toxic, flammable, and explosive are alcohol vapors and gaseous hydrocarbons, which we use as fuel.
Typical natural gas is methane with traces of ethane, propane, and butane. The boiling point of methane is -258.7°F, so it can only be stored and transported under high pressure.
However, propane and butane remain liquid at room temperature and under very low pressure, allowing them to fill plastic lighters and thin-walled metal containers.
On gas canisters for camping stoves and lamps, you might see labels like "summer," "winter," or "all-season." This is because, at atmospheric pressure, butane's boiling point is 32 ± 2 °F, isobutane is 10.9 °F, and propane's is −44 ± 1 ℉.
Therefore, in warm climates, cheaper pure butane can be used, whereas winter canisters must contain at least 85% propane.
Liquefied propane and isobutane are not only components of convenient household fuel but also propellants for aerosol spray cans. Thus, we might inadvertently release flammable gases into a room. Even if we don't use a canister, it might have a faulty valve and slowly leak gas.
These gases are not highly toxic, but they ignite easily, and when mixed with air, they can explode. A switch flick or a spark from static clothing can lead to disaster.
Fortunately, electronic hazardous gas detectors are widely available and quite affordable. Today, let's experiment with one of them.
MQ-5 is a semiconductor flammable gas sensor. It has a small truncated cone-shaped body with a bakelite bottom and six leads. The body material is stainless steel mesh, performing an important function.
Doesn't the sensor casing resemble the design of the old Davy lamp for coal mines?
If methane was present in the mine air, it would enter the lamp and ignite from the wick. This flash was visible and served as a warning signal, saving many miners' lives.
In low oxygen conditions, the wick's flame would shorten, warning of asphyxiation danger. Advanced Davy lamps even had scales to assess air quality in the mine.
The metal mesh prevented the flame from spreading outside the lamp and causing a destructive explosion. The MQ-5 sensor serves the same purpose.
Under the mesh is a structure similar to a standard film resistor. The same aluminum oxide ceramic tube, but instead of a carbon or metal layer, a semiconductor sensitive to the gases the sensor is designed for is applied to its outer surface.
Of course, the semiconductor layer is not coated with glaze or varnish, unlike regular resistors. The sensor's sensitive layer must be in contact with air, whereas the resistor's conductive film needs protection.
Gold electrodes and platinum wire connect the sensitive layer to the leads. These costly metals are used because of their superior corrosion resistance and chemical inertness, ensuring accurate sensor performance.
Finally, inside the ceramic tube is a nichrome heating coil, similar to a radio tube's cathode. The sensitive layer needs heating for proper functioning.
If you sniff a working sensor, you might recognize a smell familiar from childhood. This is the specific smell of an old grandfather's kettle, its inner surface coated with food tin.
Almost all low-melting heavy metals are highly toxic, but there are two pleasant exceptions: bismuth and tin. Bismuth is used for making medications, and tin coats medical and food utensils, including cans.
Tin dioxide is a semiconductor that reduces its electrical resistance when heated and exposed to various gases.
The sensor is most sensitive to propane, butane, and isobutane, the main components of household liquefied fuels and safe aerosol can propellants.
The MQ-5 also responds well to methane in piped household gas. It is less sensitive to hydrogen, alcohol vapors, and carbon monoxide, for which other sensor modifications should be used.
To demonstrate the sensor's functionality, I will assemble a simple circuit that can run on 3.3 or 5 volts.
Resistor R1 limits the operating current of the heating coil. The sensor's sensitive element is the upper arm of a voltage divider, with R1 as the lower arm.
The LM393 comparator has single-ended open-collector outputs. These outputs can switch loads with a voltage higher than the chip's power supply.
A pull-up resistor R4 ensures a high logic level at the open-collector output.
When the sensor's resistance falls due to gas exposure, the voltage at the comparator's inverting input drops below the reference voltage set by potentiometer R3.
The LED lights up, the module's output switches from high to low logic, and in my model, a buzzer sounds.
As we see and hear, despite the smoothing capacitor C1, the comparator triggers with noise. This is because the circuit lacks comparator hysteresis.
To provide hysteresis, positive feedback is needed. In the PWM brightness controller circuit, it is provided by resistor R9 between the comparator U1A's output and its non-inverting input.
Here, instead of a specialized chip, a regular operational amplifier with push-pull outputs is used as a comparator, requiring no additional pull-up.
Changes in the gas sensor circuit's supply voltage won't affect the comparator's trigger point since both voltage dividers - U1R2 and potentiometer R3 - are powered from the same source.
Thus, we have a simple and reliable sensor, whose heater consumes only 800 milliwatts.
There are also other sensors in the same series. MQ-2 detects smoke, MQ-3 detects alcohol, and MQ-7 detects carbon monoxide. MQ-4 is designed for methane detection, MQ-8 for hydrogen, and MQ-6 and MQ-9 for different flammable hydrocarbon gases.
Miners once took a small bird in a cage into the mine. If the bird became ill, it was a signal for miners to ascend immediately.
Then, liquid-fuel lamps with explosion-proof mesh appeared, followed by electronic gas sensors. We live in an age of technological progress, making life more interesting and safer.
