Palm

🌴 Outdoor BLE-connected UV sensor keychain 🗝

power LiPO
wireless BLE
sensor UV Index
mcu nRF52
battery life 5 days
bom items 13
bom cost USD $50.47
vendors 4
completed July 2019

Mechanical sizes

Item Height Width Depth Weight
Adafruit Bluefruit nRF52 Feather 51 mm 23 mm 8 mm 5.7 g
Adafruit UV Sensor VEML6075 17.0 mm 15.3 mm 2.9 mm 0.8 g
LiPo battery 380mAh at 3.7V 37 mm 19 mm 8 mm

User Stories

User

As a user, I want to power ON the device so that it can measure the UV Index periodically.

As a user, I want to power OFF the device so that it can power down completely to save battery consumption.

As a user, I want to switch ON the wireless BLE so that it can transmit the data to another BLE device.

As a user, I want to switch OFF the wireless BLE so that it can stop transmitting the data to another BLE device.

As a user, I want to view the changing colors of the on-board RGB LED so that I know the current range of UV Index.

As a user, I want to view the numerical values of the UV Index and battery level data on another BLE device so that I know those values accurately.

As a user, I want to hang the entire keychain through the M6 drill hole so that I can be hands-free when outdoors.

State Machine

Edit state_machine.md at mermaid js live editor.

State Machine

Power overview

Power source

380mAh LiPo battery

Battery life

5 days

Charging duration

5 hours
Plug in the USB cable to charge. Yellow LED will be on while it is charging.

Power tree schematic

Edit power_tree_drawio.xml at draw.io.

Power Tree schematic

Power consumption of main components

Chip # Chip Operating Voltage Advertising mode (152.5ms) Transmission mode
nRF52832 BLE + MCU 1.7v to 3.3v 60 µA 216 µA
VEML6075 UV Sensor 1.7V to 3.6V 480uA at 1.8V

Power calculation

Advertising Mode

From online power profiler

  • Chip nRF52832
  • Voltage 3.3V
  • Radio TX Power 0dBm
  • Role: Advertising (Connectable)
  • TX payload (Byte)0 byte
  • Advertising interval (ms): 152.5 ms as recommended in Apple developer guide
  • 120mAh battery capacity
Online profiler

Transmission Mode

From online power profiler

  • Voltage 3.3V
  • Role: Connection (peripheral)
  • 3 byte TX payload
  • Connection interval (ms): 1000 ms
Online profiler

Summary

  • If transmitting 100% of the time: ~781 days
  • If advertising 100% of the time: ~83 days
  • if CPU running 100% of the time: ~12.5 hours

Other potential batteries

Name Capacity Voltage Dimension Buy from
CR2450 120mAh 3.6V 4cm x 2cmx 2cm AliExpress
LiPo 380mAh 3.7V 37mm x 19mm x 8mm AliExpress