The circuit below is an arduino compatible, single supply 16-bit volatile organic compound detector using a metal oxide sensor. It uses ADN8810 to precisely control the current running through the sensor to get to within the 1% voltage accuracy needed for the heating element of the sensor.
CN0395 measures indoor air quality by utilizing a Metal Oxide Sensor to detect gases composed of volatile organic compounds. The sensor is composed of a heating resistor and a sensing resistor. When the sense resistor is heated its value changes as a function of the concentrations of different gasses.
The current output DAC, ADN8810, has an output voltage compliance that needs to be met to maintain a linear current output. This can be done by putting a jumper on position 2 and 3 which attaches the ENCOMP pin of the part to ground for output voltages up to 2V.
For higher output voltages that 2V, tie the ENCOMP pin to +5V to maintain the output stability of the part.
P2 header selects the current path from the current output DAC, ADN8810. It is populated across RH+ by default.
|RCAL1||71.5Ω||1 & 2|
|RCAL1||10.0Ω||3 & 4|
|RH+||Heating Resistor||5 & 6|
CN0395 has the TGS8100 on board as default. Using a different set of sensors require a recalculation and replacement of the sense resistors that sets the full scale current for the device. Provision for alphasense sensors or with similar footprint is available and a screw terminal is also in place for any other types of sensors.
If you are using a different sensor, you will need to wire in the appropriate signals into the board via the screw terminal (P8). This also means that you will have to remove the TGS8100 sensor that comes on the board by default. Below is a table of how to use P8 to input other sensors.
|Silkscreen Name||Pin Number||Description|
|RH+||Pin 1||Postive Pin of Heating Resistor|
|AGND||Pin 2||Negative Pin of Heating Resistor|
|RS+||Pin 3||Postive Pin of Sensing Resistor|
|AGND||Pin 4||Negative Pin of Sensing Resistor|
Spacers that could cater various surface mount and through hole packages are available on the board to make some modifications on the board easier.
Resistor spacers, R17_SPACER and R19_SPACER, are used to hold the sense resistors to select the maximum output current of the ADN8810. Changing these maximum output current of the ADN8810 allows the board to cater multiple types of sensor that use up to 300mA of current for measurements.
Sense resistors (RSN) can be calculated by using the formula below:
RSN= 4.096/(10* Full Scale Current)
Pressing push button S2 allows the user to do a hard reset of the temperature and humidity sensor SHT30.
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