Analog input with MCP3008

Question

I'm trying to connect a Turner C-Sense pCO2 sensor to an RPi2B using an MCP3008 ADC. The probe is powered via dedicated 9V inverter. The probe's analog output wires include a signal out (+) and AGND (-). I can use a multimeter to read the voltage (0-5VDC) across the two wires, but I'm interested in having the RPi read/log the voltages.

I've completed several tutorials using the RPi/MCP3008 to connect several 10K thermistor resistors using voltage dividers, and thought I could connect this sensor in a similar way. I tried several different connection options that seemed logical to me, but only ended up with either 0 or 1023 outputs readings.

I'm using the rpi.gpio python package to communicate with the ADC and verified with a 10K thermistor that CH0 can send accurate data. So I am interested in the correct wiring configuration for the CO2 sensor's two output wires.

Edit: I think I had it working there for a minute when I connected the ADC VREF pin to 5V and grounded the sensor's AGND and ADC's AGND to a common GND pin on the RPi. I was getting variable voltage readings from the sensor. BUT the ADC got extremely HOT and the values were offset from true.

• joan: A photo of the connections (clearly showing the pins being used and the wires between) and a description of the connections between the Pi, the ADC, and the sensor would be helpful.

Answer 1

If the ADC has become very hot, then it might have been damaged by the circuit. Check the wiring diagram and then try using a fresh ADC.

Not sure why you are using the gpio library rather than spidev. However, if it was working and now is not, it is probably just a dead ADC.

• Laaaars: I rewired the circuit and got the system to work without overheating the ADC. Would you like to make a case for spidev over gpio?
• joan: You don't say how you are talking to the ADC. Given that you are using RPi.GPIO and not spidev I guess you are using a bit bang approach. Generaly people only bit bang SPI if they can't achieve what they want with the standard drivers. Perhaps the tutorial you are following pre-dates the availability of the SPI driver.

Answer 2

The cause of over-heating mostly be passages of excess voltage through the ADC and so it is a sign that the ADC is getting damaged. To connect he CO2 gas sensor using ADC, you can use ADC081C For Python sample code, click here