This is a design for a sensor node powered by two AA cells. It includes the following sensors:
|Texas Instruments TMP102||Temperature||✔||TMP1x2|
|Silicon Labs SI7021||Temperature, Humidity||Si702x|
|Silicon Labs SI1145||Ambient Light Sensor, Proximity Sensor||Si114x|
|STMicroelectronics LPS25H||Air Pressure||✔||LPS25H|
Nora uses a boost supply to provide a minimum voltage of 2.75V when WiFi is enabled. Since the imp can run user code down to 1.8V, and all of the sensors on the board have a minimum supply voltage of 1.9V or less, when WiFi isn’t needed the boost is disabled. This allows for additional power savings. An undervoltage lockout circuit will disable the device if the battery voltage falls below 1.9V, to prevent the device from continuously rebooting when the battery is low.
Battery voltage is also connected to a GPIO pin on the imp002 so it can be monitored.
Note The following was completed using a Nora version 3 design. The current design, version 5, should have equal or better battery life. This section will be updated once testing is complete.
To estimate battery life, we programmed a Nora to wake up, connect to Wi-Fi and report its battery voltage. We graphed the battery voltage over thousands of wakes to estimate the number of wake cycles you could anticipate with different types of batteries. In this test the boost voltage was set to 2.85V to maximize the number of wakes. As you can see, the Energizer L91 Lithium AA batteries provided the greatest number of wakes, then Duracell Alkaline and then Eneloop NiMH rechargable.
Note that wake-time will depend on many variables, including WiFi AP and network performance, and this will affect battery life. There have been further optimizations since these tests were run.