WiFi Temperature Sensor Testing Continued

Posted on Posted in Electronics, Projects

Continuing from my last post here about my Electric Imp based WiFi temperature sensor.

I left my prototype sensor running for 14 days before it stopped transmitting which isn’t nearly the batter life I want out of this but as I said in my last post I was getting anywhere between 800uA and 2mA of current consumption when it was asleep which was way too high.

Here is a plot of the battery voltage over the 14 days the sensor was running:

BatteryVoltage

To show it was working and because I like to make graphs here is a plot of the temperature readings over the same time period:

Temperature

From the battery voltage graph you can see it drops off very quickly, also you can see all those sharp dips. I’m not sure what was causing that I did add a 1000uF capacitor in parallel with the battery to help with the peak power consumption from the WiFi transceiver but maybe the ESR of my capacitor was too high, I’m not sure.

After the battery died I decided to investigate why the sleep current consumption was so high. After looking into it I found that that all the current consumption was from the buck converter on the Electric Imp development board. Since it is a 3.3V buck converter and my battery voltage is 3V max I didn’t need it anyway. Anyway after I modified the board to bypass the buck converter the current consumption dropped significantly to around 31uA.

CurrentNot quite the 6uA sleep current that the Electric Imp datasheet quotes but much better. Even through there is no power going into the buck regulator’s input anymore there might be some leakage currents through the output and/or feedback pin. This current consumption would probably drop if i removed the chip altogether.

Picture of board with buck regulator bypassed:

WifiSensor2

With this change I have only had it running for 24 hours so far but the batter voltage looks a lot better and I’m thinking I should get much better battery life out of this.

BatteryVoltage2

Well I am going to leave it running and see how things go. Next step is to design a permanent board for the WiFi sensor with a boost regulator so I can get all the power out of my batteries.

Andrew