usb-6008 CO2 measurement and relay controll

[laurentiu]

      Hello

     I'm kind of new in in this DAQ things, but I'm also a PhD student in biotechnology and I need to measure CO2 concentration in a given environment. For that I'm using USB-6008 DAQ board and MG811 sensor (http://www.kosmodrom.com.ua/data/gas/MG811.pdf) - data sheet. I have found on internet some schematics related to this sensor but I do not know if it is any help (http://img.donanimhaber.com/upfiles/29645/508E073F0722470BAB0E9177D13BE3AD.jpg). 

     So, I'm asking your help in this. How should I connect the MG811 sensor to the USB-6008 DAQ ? How  should I make the .vi file for LabView so I can register the values of the CO2? How do I controll three relays knowing that the USB-6008 DAQ has only 2 analog output? How to controll more than 3 relays if I want to controll also themperature (LM35 sensor) and humidity? For controlling relays may I use LPT port or maybe should I use a ULN 2003 (or ULN 2803, ULN 2004) on the I/O ports ? 

       Thanks! 

[Dan_K]

Hello laurentiu,

Taking a look at the documentation you posted I noticed several issues between the circuit diagram and the data sheet.

• The excitation voltage is specified at 6 V ±0.1 V,while the schematic is driving both H pins at 5 V with unnecessary external circuitry.
• -12 V somehow becomes ground.
• Unnecessary components surrounding the op amp.

This entire circuit could be greatly simplified by doing the following:

1. Use an external 6 V power supply to power the sensor and the op amp + power.
2. Connect the ground of the power supply to the other H pin on the sensor and the op amp - power.
3. Configure the op amp as a simple voltage follower as shown in the data sheet.
   1. B to positive input
   2. Negative input wired to the output.
   3. Output wired to AI0 on the USB-6008.
4. Wire A on the sensor to AI4 on the USB-6008.

One thing you should be aware of is that this is not a guarantee that you will be successful with the USB-6008. The output of your sensor ranges from 30-50 mV, which apparently corresponds to a logarithmic scale. According to the USB-6008/6009 User Guide and Specifications the accuracy of the device at the ±1 V range is typically 1.53 mV at 25 ºC and can be much worse depending on the temperature. This may not give you the desired accuracy for your readings, but that would be for you to determine.

As far as driving relays, depending on the current requirement of the relay it would be a better idea to use one of the PFI lines to drive the relay rather than your Analog Output, not only would this be simpler to program, but you have 12 lines you can drive instead of only 2.

Finally, in terms of programming this in LabVIEW I would recommend beginning with an example. These examples can be found through the LabVIEW example finder under Hardware Input and Output»DAQmx»Analog Measurements»Voltage. I would recommend starting with the Cont Acq&Graph Voltage-Int Clock example.

Let me know if you have any further questions regarding the connections of your sensor or the specifications or programming of the USB-6008.

[laurentiu]

       Hello,

 I'll do some research after winter holiday, but till then I'll try to use TLC271 op amp as it is in the sensor datasheet.