Mobile, Battery Operated Weather Station Using BME280 Sensor and SSD1306 OLED Display
Introduction
A recent post described the prototype, its software, and results of testing to determine how/if this weather station can be battery operated. That testing clearly demonstrated that two AAA cells in series will operate this device for a very long time only if power is toggled on when a reading is needed and toggled off the rest of the time. This post describes and shows a fabricated, compact, practical device.
Overview
Some images are used to best to explain the device.
Everything is packaged in a Bud Industries ABS plastic snap-together case (5.1×3.0x1.2 inch). The OLED display is bonded in a 3D printed bezel which in turn is held in place with the visible screws. The pushbutton on the left is MOM-ON, MOM-OFF. In words, this means it toggles on and toggles off, which is different from ON when depressed and OFF when released. Therefore, this pushbutton requires the user to turn off the device while the latter would require the user to keep the button depressed while reading the display but would turn off automatically when released.
A single screw mounts the BME280 sensor PCB with the sensor itself positioned behind a small port in the plastic case.
What Is Inside
To open the case the side panels snap off.
The batteries can be replaced with just one side removed, however, removing both sides makes this easier.
The battery case is strongly bonded to the plastic case using a two part cyanacrylate. From left to right, the pushbutton, the custom PCB with the ESP8266, the OLED display, and the BME280 sensor are all visible.
Two items on the PCB to note are a pushbutton for RESET and a three terminal pin header for UART connection. The board is held in place with two #6 screws which in turn are fixed to a platform fashioned from some old PCB material bonded to the ABS case with epoxy.
Operation Video
To the human eye the display does not blink as it appears in the video, which is caused by display scanning together with camera scanning.
It should be noted that the data conveniently appears within about two seconds after the display turns on.
Low Battery Warning
The ESP8266 has an ADC pin that can measure 0 to 1.00 volts. The custom PCB has the ADC pin routed to voltage divder resistors, which when populated, divide down any intended input voltage to this measurement range. Resistors were added and the battery voltage was connected to the input voltage pin for the ADC.
The resistor values were 11K and 27K, which provided a voltage of 0.815 volts at the ADC pin on the ESP8266 SOM when the battery voltage at the supply pin was 2.92 volts. A multimeter was used to make these measurements. The voltage value reported by the software was 0.835 volts. Repeated measurements showed the small offset to persist and remains unexplained. However, this operation is more than adequate to monitor for a low battery condition.
Previous research revealed reports that the ESP8266 may work below 2.5 volts but the datasheet specifies the lower limit to be 2.5 volts. Therefore, a limit voltage of 2.58 volts was picked. The software displays updated results every seven seconds. Prior to each update the battery voltage is measured, and if found to be below the limit voltage, replaces the displayed “SCIENTRIC” logo with “LOW BATTERY”. This is shown in the image below.
