Nuts and Volts - August 2015

56 August 2015

large and unwieldy for lookup within a software application. We can automate the process by using a spreadsheet to plot and thus capture the curve fit for the RS/R0 relationship to VRL and then to PPM. A simple regression can be used to derive a formula. The formula can then be applied for code use within Python. As an example, with MQ- 2, the equation is y = 0.1575e1.724*x where LPG gas PPM is y and the MQ- 2 VRL is x. The spreadsheet example is shown in Figure 6.

Most of this spreadsheet “leg work’ has been done for you on the selected gas sensors, and the necessary equations are derived. A copy of this spreadsheet with the gas equations is available for download at the article link.

As you build your own air quality prototype using the equations (Figure 6), keep in mind several fixed required conditions before starting:

1. Configure RL on breakout boards; these can be set using a multimeter when VCC power is not applied to the boards. In our case, we followed manufacturer’s specifications and configured both MQ- 2 and MQ- 7 for an RL of 10K, and HCHO for an RL of 20K.

2. Keep the supply voltage (VCC) to +5V DC.

Prototype Overview

Let’s start with a good old fashioned block diagram (see Figure 7). The computer hardware, as mentioned earlier, is based on an ISaAC adapter/Raspberry Pi board. The application is written in Python which executes on the Raspberry Pi. To this, we added a gas sensor interface breakout boards interface directly to computer hardware. The sensor board also provides an LED feedback.

The various gas sensor breakout boards are supplied with +5V DC power and ground from our sensor interface. Also, it is here where the sensor voltage outputs are buffered and levels are converted for ingestion into the computer hardware.

As noted earlier, each sensor contains individual heating elements that at +5V require current. These typically run hot, and can consume about 160 mA each. It is not practical to use power from the ISaAC and Pi; + 5 volts is needed to supply heater power, so a separate +5V DC supply is integrated in the sensor interface to handle all gas sensor power.