Sewer Gas Sniffer Project –

Sewer gas is a by-product of the breakdown of natural human waste. Plumbing is an important part of the sewer system in your home. In most homes, sewer gas may have a slightly unpleasant odor, but does not often pose a significant health hazard. The hydrogen sulphide in sewer gas is what gives it its signature rotten egg smell.

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It’s uncommon to be exposed to high levels of sewer gas at home. However, those levels of exposure call for immediate medical attention. Moreover, sewer gas can contain methane which is, like hydrogen sulphide, a highly flammable and potentially explosive substance. Since methane in large amounts is extremely flammable, when paired with the flammability of other gases, the complex mixture then creates a fire hazard!

So, in this article, we’ll look at a simple idea to detect sewer gas leaks in your home. Get ready to begin!

The MQ-4 Semiconductor Gas Sensor

The central building block of this little project is the MQ-4 semiconductor gas sensor. Sensitive material of MQ-4 gas sensor is SnO2, which has a lower conductivity in clean air. When the target combustible gas exists, sensor’s conductivity is higher along with the rise in gas concentration. The MQ-4 gas sensor could be used to detect different combustible gas, especially methane. MQ-4 Datasheet

The gas sensor has a built-in variable resistor that changes its value according to the concentration of gas. If the concentration is high the resistance decreases and vice versa. Besides the built-in resistor, there’s a heater used to provide the temperature that the sensor needs to work properly. Below you can see the inside of the MQ-4 sensor.

As you can see in the test circuit provided below, there’s also an external resistor (load resistor) which serves to adjust the sensor’s sensitivity and accuracy. To make a gas sensor system with better performance, an appropriate load resistor is essential.

Even though the MQ-4 sensor has six pins in total, pins 1-3 and pins 4-6 are connected, leaving us with only four connections. Here, 1-3 and 4-6 are the built-in gas sensor element leads whilst H and H (2-5) are the heater leads. RL is the final load resistor.

The sensor requires two voltage inputs: heater voltage (VH) and circuit voltage (VC). VH is used to supply standard working temperature to the gas sensor element, and we can use DC or AC for that purpose. But VC supplies the circuit voltage, and it should be DC. VRL is the variable DC voltage available across the load resistor RL which is actually placed in series with the gas sensor element.

The following “sensor sensitivity curve” depicts the output voltage (VRL) level with different methane concentration. Note that the test was conducted with a 4.7KΩ load resistor (RL).

As it looks now the MQ-4 sensor has a good sensitivity to methane in a wide range. I’d like to recommend that you calibrate your sensor for 5000ppm of CH4 concentration in air and change the value of load resistor (RL) to about 20KΩ (10KΩ-47KΩ) for experiments.

Note that the MQ-4 sensors resistance will drift reversibly if it’s stored for long in an idle state and the drift is related with storage conditions. The sensor should be stored in airproof bag without volatile silicon compound. For the sensors with long time storage, the sensor needs to be preheated for the recommended time (Not less than 48 hours – See datasheet) before the actual usage to get a good stability.

The MQ-4 Sensor Module

Plugging the MQ-4 sensor into a standard circuit board is not easy due to its strange pin assignments. The easiest way then is to use a special MQ-4 sensor break-out board as shown in the picture below.

However, I chose a Chinese MQ-4 sensor module because it’s an easily available, cheap, and compact MQ-4 sensor board.

Like most Chinese sensor modules, this MQ-4 module also has the LM393 comparator chip in its circuitry to provide a digital (high/low) output signal. See its basic ‘web’ schematic below.

The schematic is simple and straightforward, so it hardly needs an in-depth explanation. The potentiometer (multiturn trimpot) Rp can be used to set the detection threshold of the LM393 comparator U1A. The digital output D-OUT is usually in high-state (H) but goes to a low-state (L) when the presence of the methane is sensed in a density level that’s already nailed down through the trimpot. The LED also wakes up at that time to provide a visual indication.

Now to something about my MQ-4 module I bought from an online store. The heater shows a resistance value about 31Ω (at room temperature) when measured with my trusty digital multimeter. The 5V module has a 5.1Ω resistor in series with the heater supply. The module draws about 130mA (at 5V) in standby state and about 140mA in active state.

That’s okay but the value of the load resistor used is 120KΩ (see the microscope capture below) which is bizarre and much higher than the recommended value in the datasheet! Let it be there for a while anyway.

The Sewer Gas Sniffer (Basic Version)

This time I’m giving a design of a sewer gas sniffer, which is entirely based on the aforesaid Chinese module.

Using the MQ-4 sensor module for simply detecting methane (not for measuring it incisively) is quite easy. Just power the module with 5VDC and you will see that the power LED (usually red) on the module shines instantly, and when methane is detected the output LED (usually green) wakes up to indicate that. The output LED is wired to the output pin of the LM393 comparator (D-OUT of the module), so, the same output is tapped to control an external alarm circuitry. You can see the proposed sewer gas sniffer schematic below (basic version).

Luckily in this project, the schematic is very simple. The little low frequency oscillator is wired around cD40106B Hex Inverting Schmitt trigger chip (IC1). Schmitt trigger is a special type of comparator. A comparator compares its input to some reference and output a high or low voltage depending on if the input is higher or lower than the reference. The notable difference between a Schmitt trigger and a common comparator is that a Schmitt trigger moves its reference voltage. This might be a little tough to understand at first!

To get the Schmitt trigger to start oscillating you only need to add a resistor and capacitor. Any resistor and any capacitor will do but try out a 4.7uF capacitor (C2) and a 56K resistor (R3) first. The charging and discharging of the capacitor along with the Schmitt trigger inverting its output are what make the magic happen. One of the other things that makes using the 40106 as a relaxation oscillator so fun and simple is how easy it is to change the oscillator frequency. All you need to do is add a potentiometer wired in series with the resistor.

If you look at the schematic, you’ll notice that two transistors are also there in the scene. Yes, the first PNP one (T1) is a simple power supply switch for IC1 while the second NPN (T2) is the alarm driver. Although a single LED is used in the alarm section, it’s pretty easy to employ more LEDs and/or an active piezo-buzzer there. If you are using an electromagnetic or solid-state relay instead, then that addition will make the design strong enough so that it can smoothly handle external high-voltage blinkers and/or beepers.

This time, only one gate of IC1 is exploited, thus the input of each unused gate is tied to the ground rail. This might seem a little pointless at first. All we seem to have done is made a simple LF oscillator so far, but for now it’s hatful. We’re going to delve into more ideas later, so next time we’ll add more options and control.

In addition, note that the A-OUT pin (analog output) of the MQ-4 module delivers dc voltage in 0-5V range approx. based on the intensity of the gas detected by the gas sensor. Therefore, you can use this analog signal to detect/measure the methane concentration if desired so.

However, just reading the analog output using something (perhaps a microcontroller) is not enough to make a reliable methane detection device. To do that you need a bit complex approach to calculate the gas concentration by implementing the formulas provided by the datasheet. For accurate measurements, influence of temperature and humidity should also be considered. This is a link I found Googling which may help you to get sensible outcomes from your MQ-4 sensor

LEL, %BV, PPM……?

Keep in mind that most portable methane gas monitors will raise an initial alert at 10%-20% of the LEL (Lower Explosive Level). One key thing to remember when designing your own methane detector is that regardless of whether you adopt the %LEL scale, the percent scale or the ppm scale, the gas concentration is all the same. As an example, 10% LEL (% LEL scale) = 0.5% methane (percent scale) = 5000 ppm methane (ppm scale). So now you have three numbers at the alarm level: 10 and 0.5 and 5000. Got it?

The end-user however doesn’t need to study about scale or range – the layman/women just need to know what to do next when the alarm goes off. But if you’re designing/building one, you may need to know all about units of measure, scale, and range. Without scale, all things are meaningless!

This probably just looks like a scary mess of mathematics right now. Fear not, you can work through it step by step. This is a quick guide

End Note

This might seem a strange project, but you’re going to play with a simpler idea. Considering the low components count is and easy is it to implement this way. I consider it very much worthwhile.

I hope you have some inspiration to go further. Well, I will come back with an updated sequel to this post after a few months. Until then, keep your interest alive. Thank you for your time!

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