Using TCA9548A for multiple MQ-X gas sensors

Question

I have about 8-9 gas sensors I want to use on my Pi but I'm trying to understand how to use The TCA9548A. The TCA9548A Has 8 pairs of SDL and SCL pins to handle multiple I2C devices, but the output from the MQ gas sensors is one line AO for Analog out or DO for Digital out. How would I go about using 8 to 9 gas sensors on a single Pi with the TCA9548A?

Answer 1

It seems to me your earlier question, is a feasible approach. And this one is almost a feasible approach :) All you need to do here is replace the TCA9548A with a Multiplexed A/D Converter. I'll try to explain how this works in general, and you'll see it's not conceptually difficult at all once you understand what's going on.

Before I do that however, I'll bend the rules here a wee bit and recommend a different A/D Converter: This unit from Adafruit should be enough to meet your objectives: it will handle a max of 8 gas sensors, it is considerably less expensive than the unit you found, and Adafruit supplies software and a tutorial for Raspberry Pi. That's only a recommendation of course; there are many hardware solutions that will get you to a functional solution. You may wish to consider this as your initial prototype, and make subsequent improvements.

So, how does this work? The MCP3008 is an IC that combines an Analog-to-Digital (A/D) converter with a multiplexer in a single package. The A/D converter is needed because the RPi has no analog input. The multiplexer simply connects one of its eight inputs to the single input of the A/D converter. The timing and sequencing for making these connections and performing the A/D conversion are controlled in your software - which runs on the RPi.

Now imagine the analog output of each of your 8 gas sensors being wired to one of the 8 channels of the MCP3008: channel 0 - channel 7. Your software communicates with the MCP3008 over an SPI bus, effectively accomplishing the following steps repetitively:

1. tell the MCP3008 to do an A/D conversion of the analog input at channel 0.

2. send the result of the A/D conversion at channel 0 (Step 1) to the RPi as a serial bitstream over the SPI bus.

3. process the received data in the RPi. This may be as simple as comparing the value of the reading to a threshold, and setting an alarm if the threshold is exceeded.

4. Repeat steps 1-3 for each of the other 7 channels.

5. Repeat steps 1-4 repetitively for continuous monitoring.

As far as the wiring interconnections, I'd suggest you forego Adafruit's more elaborate recommendations, and take a minimalist approach - as shown here.

Finally, you will need to enable the Raspberry Pi's SPI bus; this may be done by editing the file at /boot/config.txt or by using the raspi-config software tool. This search should provide explanatory examples for both approaches.

Final recommendation: Learn by doing. If you have questions, please feel free to ask as many as you'd like. Do your homework (read and study the alternatives), and try to ask specific questions.