Node Design

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With our architecture in mind, we designed a PCB around the following features and considerations:

  • Low idle power consumption
  • Powered by battery source
  • Charge from auxiliary supplies eg Solar power
  • LoRa Transceiver
  • Storage for buffering data
Schematics: Power

A BQ25504 is used to handle battery management and charging, while a TPS62740 is used to step down battery voltage to 3V, the main supply rail for the PCB.

The TPS62740 was chosen because of its low quiescent current and the built-in load switch, allowing us to turn off a subsection of our PCB when idling.

Schematics: Radio Transceiver

A E22-900M22S is used to handle communication. This SX1262-based module can communicate over LoRa or raw FSK directly.

Schematics: Microcontroller

A STM32L051 is used as the main microcontroller on the board due to its low power modes. A 32.768kHz crystal is added on the board to support RTC operation for providing timestamps for the samples. We intend to use the 1Mbit EEPROM for buffering sample data before transmitting it to the UAV.

Schematics: SD Card

A Micro SD card socket is placed on the PCB. We intend to populate this only for the node on the UAV in order to store received data.

Schematics: Sensor Ports

At this stage, because we have not finalised the number or type of sensors the board needs to collect data from, we have exposed relevant ports on pin headers to allow for modification. A IMU and thermistor is placed on the board as “sample” sensors.

PCB

The smallest component used are 0603 passives and LEDs. The use of lead-less packages for almost all the integrated circuits make it necessary for us to order a solder mask and use reflow to assemble this board.

We estimate a Bill-Of-Materials (BOM) cost of approximately USD30 (DigiKey) for all the components on the board, plus another USD6.40 for the E22-900M22S (TaoBao).

Source files for the PCB above can be found here.