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Lower power with CCS811 at 50% duty cycle

A project log for STM32L4 Sensor Tile

Small, connected device for smelling and hearing in any environment.

Kris WinerKris Winer 11/15/2017 at 05:100 Comments

11/14/2017

I ran the latest experiment using the same Sensor Tile as I used in the previous log, so it doesn't have the low-power battery charger yet. I modified the sketch to put the CCS811 in idle mode for twenty minutes, and then run in Mode 3 (continuous measurement at once per sixty seconds interval) for twenty minutes; so a 50% duty cycle. I expected this to save half of the 550 uA that I previously measured for Mode 3 or ~275 uA, but it also should saved half of the 200 uA that the CCS811 would ordinarily use keeping the heater on and membrane warm between measurements. So overall, I expected to cut the average power usage by ~375 uA. In fact, using a fully charged 105 mAH LiPo battery the experiment ran for 177 hours (seven plus days) for an average power usage of 593 uA, about what I expected.

The downside of using this strategy is that the heater has to restart every twenty minutes and the air quality data doesn't become reliable until  near the end of the twenty minute active period. I wasn't sure what kind of response to expect but the data looks like the sensor is still able to measure meaningful data even at this erratic pace:

More than half of the air quality (eCO2 and VOC) data points are zeroes but the data do register excursions in air quality that seem reasonable. For the first three of the seven days I was having some windows replaced and the repairmen were in and out of the house (I kept the Sensor Tile indoors in the main hallway). This shows as larger and more frequent variations in eCO2 and VOC. There is a particularly large excursion after noon on the second day. I am not sure what caused this.

So the verdict is still out whether this is a sensible way to measure environmental data. It certainly saves a lot of power, but I need a better test of  the impact on air quality data fidelity. Perhaps a second test with the Sensor Tile placed in a sheltered spot out of doors.

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