Question 12.3: You want to measure gas levels using a methane sensor....

Low-cost resistive gas sensors are available that can easily be wired to a Raspberry Pi to detect gases such as methane. You can use the step response method that you first used in Recipe 12.1.

To make this recipe, you will need:

• Breadboard and jumper wires (see “Prototyping Equipment” on page 380)

• Methane sensor (see “Modules” on page 381)

• Two 1kΩ resistors (see “Resistors and Capacitors” on page 380)

• 220 nF capacitor (see “Resistors and Capacitors” on page 380)

The sensor contains a heating element that requires 5V at up to 150 mA. The Raspberry Pi is capable of providing this as long as its power supply can supply the extra 150 mA.

The sensor module has rather thick legs—too thick to fit into breadboard holes. One way around this is to solder short lengths of solid core wire to each lead (Figure 12-5). Another is to buy SparkFun’s gas sensor breakout board.

Wire the breadboard as shown in Figure 12-6 if you’re using the SparkFun breakout board, or as shown in Figure 12-7 if you soldered longer leads to the gas sensor.

Note that the direct connection shown in Figure 12-7 uses the same symbol for the breakout board rather than the sensor on its own, but if you look carefully the connections are to the six sensor pins, not the four pins of the breakout.

You can use the exact same program as Recipe 12.1, and you can test out the methane sensor by breathing on it. You should see the readings from the sensor drop when you breathe on it.

Discussion

The obvious use of a methane gas sensor is for novelty fart detecting projects. A more serious use would be for detecting leaks of natural gas. You could, for instance, imagine a Raspberry Pi home watch project that monitored the home with various sensors. It could then send you an email, while you are on vacation, informing you that your house is about to explode. Or maybe not.

These types of sensors (Figure 12-8) use a heating element that warms a resistive surface impregnated with a catalyst sensitive to a particular gas. When the gas is present, the resistance of the catalyst layer changes.

Both the heater and the sensing surface are electrically just resistors. So both can be connected either way around.

This particular gas sensor is most sensitive to methane, but will also detect other gases to a lesser extent. That is why breathing on the sensor alters the reading, as healthy individuals will not normally breathe out methane. The cooling effect of blowing on the element may also have an effect.

See Also

The datasheet for this sensor can be found at http://bit.ly/1gYupsu. This will give you all sorts of useful information about the sensor’s sensitivity to various gases.

There are a range of these low-cost sensors available for sensing different gases. For a list of sensors offered by SparkFun, see the SparkFun website.