SoLoRa - Solar Powered always-on IoT sensor node

Unique, compact power conditioning enables always-on sensor monitoring and Long-Range radio (LoRaWAN) link for IoT applications.

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This wireless IoT node uses solar power and mini power wall-like system to provide always-on power for its sensor and control circuitry. The power conditioning circuit is the heart of this design. I will demonstrate the principals and trade-offs in the power system design and present calculations that can be used to customize component choices and budgeting of sensor and communication duty cycles.

The hardware design I am presenting is a general purpose board and is designed to be customizable for use with almost any sensor and or actuator. I wanted this node to be a starting point and motivation for fellow members of my local Makerspace and members of our local TTN (The Things Network) community. Therefore the priorities of my design are as follows.
* Always-on and no batteries to ever need replacing
* Low cost
* Easy to build
* Easy to program
* Flexible: Room for customization and expansion.
* Small size
* Dust and water-splash tight
* Open source

Detailed design analysis on the power management and software systems used on the SOLoRa board will be presented in an upcoming post.

The hardware features of the SOLoRa board include:

  1. Low-cost components.
    1. Inexpensive standard components in novel configuration are used instead of specialize energy harvesting ICs
  2. Easy (ish) Assembly.
    1. QFP package processor and 0603 Rs and Cs.
    2. Can be soldered by hand using standard soldering iron. One exception is the accelerometer, which can be soldered using a reflow heat gun and a little training.
  3. Arduino compatible: Uses the same device as the Arduino Zero. Uses Arduino bootloader.
  4. Featherwing compatible: Can use with Adafruit’s Featherwing expansion boards or use the header for a protoboard mezzanine
  5. Multiple power options to targeting additional IoT applications, particularly field applications
    1. Solar. The main subject of this blog.
      1. a CR123 battery clip for a LiFePo4 battery used as a power reservoir. (~400mAh)
      2. On-board regulator charges LiFePo4 to constant voltage.
      3. Solar cell size constrains max charge current.
    2. Rechargeable Li-Po or external battery pack
      1. For standard indoor applications
      2. On-board USB micro connector and charger IC
    3. Lithium (primary cell)
      1. Battery clip will also house a standard CR123 3V Lithium, which has ~1000mAH capacity
    4. USB 5V for desktop development (or use with a USB battery bank?)
      1. On- board adjustable regulator
      2. Change Resistor values to adjust VDD
        1. Adjust to 3.3V to regulate Li-Po or external battery pack and USB 5V
        2. Adjust to 3.5V for LiFePo4 constant voltage charging.
  6. Small size: 2.3 x 2 inches
    1. Has mounting holes to match an inexpensive water tight enclosure
    2. All components mount on one-size only. Bottom is free to increase mounting versatility.
    3. 2-layer board, cheaper to fabricate.
  7. Optional on-board sensors, indicators
    1. 3-Axis Accelerometer
    2. Temperature
    3. Programmable Red LED
  8. Development Options
    1. Arduino IDE (use as an Arduino Zero)
    2. Atmel Studio
      1. Atmel Studio has Arduino templates to program like Arduino, or use standard C form
      2. Full debugging capability (breakpoints, watch variables). a  $20 JTAG debugger hardware required for this feature

  • Received PCBs Today

    Joe Miller3 days ago 0 comments

    This is the GIT repo for the PCB design:

    Alternatively, I put my design up on the Seeed Studio Fusion Gallery, so you can order them directly from them. Search for SoLoRa . Cost is ~$1/board with shipping.

    Note: These just arrived so I have not yet tested them. 

  • Power Train Prototype and measurements

    Joe Miller4 days ago 0 comments

    This is the prototype of the power train for the project. I used this to measure its characteristics.

    Power train prototype

    In full sun I measured 90mA of charge current into my LiFePo4 battery when perpedicular to the Sun and 60mA. 30mA on a cloudy day and 23mA when tilted at 45-degrees to the apparent Sun direction.  500uA in shade, not pointing at the sky.  20mA of charge current was measured through a window in my house pointing at a blue sky (the Sun not in view).

    The back-leakage from the battery storage cell to the circuit with no sun was ~1uA, ~6uA at dusk and dawn conditions.

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