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• SPINES Mote is on Internal Power

  Macro Yau • 07/28/2018 at 17:34 • 0 comments

  Following all the initial testing and debugging throughout the past weeks, it finally comes to the time to let the SPINES Mote run untethered.

  At the time of writing, the SPINES Mote pictured above has already been operating on its internal power (i.e., the attached 14500, or AA-sized, Li-ion cell rated at 750 mAh) for over 30 hours non-stop. The board is running a test sketch to take measurements from its onboard sensors, and transmit them to ThingSpeak every minute (consider this as a stress test to drain the battery). It keeps in the deep sleep mode for the rest of time to conserve energy, which should draw only around 100 µA from my back-of-envelope calculation for the worst case (without using the RTC for power gating), prior to implementing any of my planned optimizations.
  

  Although a solar panel is attached to the board (see the top left cable), it is not actually helping recharge the battery yet: Putting the high duty cycle aside, I found a significant leakage current at ~6.4 mA even when the board is supposedly in the deep sleep mode!

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• Preparing for Solar Cell Tests

  Macro Yau • 07/25/2018 at 17:49 • 0 comments

  Things get much smoother after fixing the ESP8266 I²C issue. While the RV-1805-C3 RTC module is interfering the ESP8266 and hence omitted for now (more details on this later), the rest of the PCB seems to be working fine: The environmental sensors are now fully functional, the LTC2942-1 battery fuel gauge is working (though this might be dropped ultimately due to its high current consumption, more testing needed), and most importantly, the energy harvesting circuit is somewhat charging the battery or supercapacitor.

  Generally speaking, there are two things that you need to understand in order to build a self-sustaining, energy-harvesting system:

  1. Expense: How much energy does the system consume?
  2. Income: How much energy can the system harvest and make available for use?

  Answering the first question is fairly straightforward: You can simply plug a current meter to profile how much power it is consuming at each stage (deep sleep, sensor polling, transmitting, etc.), and derive an upper bound for this.

  On the other hand, determining (or predicting) energy availability is difficult, especially for uncontrollable (though somewhat predictable) energy sources like solar. The software part of this project is meant to address this problem, but that will be a long story to tell in some future posts. For the time being, I will need to characterize the hardware capabilities with different types of solar cells that I have:

  • IXYS KXOB22-01X8F (This is the one that can be placed on the PCB. I do not really expect it to be useful for the ESP8266-based SPINES Mote, but I thought that it would be interesting to evaluate what it can do for some future lower-power SPINES variants.)
  • Seeed Technology 0.5 W Solar Panel
  • Some ultra cheap solar panels "rated" at 1 W (as shown in the picture below)

  I have just set up a halogen lamp test stand to establish the best-case scenario. This measures the charging current (with the multimeter on the right; the left one is probing the battery voltage) to a 240 mAh LiPo battery via the BQ25570 power management IC.
  

  With the halogen lamp turned on (measured at 300 lux with an iPhone app), the KXOB22-01X8F can deliver around 250 µA to the battery, while the 1 W solar panel can achieve up to 8 mA.
  

  I am going to test how well (or bad) the charging goes for these solar cells in more realistic outdoor and indoor (thanks for the mention in the Hackaday newsletter!) scenarios in the next few days. Hope the weather will turn better for outdoor tests. Stay tuned.
  

• Resolving the ESP8266 I2C Issue

  Macro Yau • 07/23/2018 at 13:33 • 0 comments

  TL;DR: Bad soldering. A really dumb mistake that I haven't discovered it until observing an anomaly on the oscilloscope a week later.

  Spent several more days on diagnosing the I²C issue with the ESP-WROOM-02 module. Finally, I managed to get access to an oscilloscope. I hooked up the SDA (CH1) and SCL (CH2) lines, and the root cause became obvious: The SDA line could not be driven low!
  

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• More Testing on ESP8266

  Macro Yau • 07/20/2018 at 14:55 • 0 comments

  I have done some more SMD rework and tests to try to find out the root cause of the weird behavior on the ESP8266 module described in my previous project log.

  It turns out that the issue (probably) has nothing to do with soldering, instead, the I²C bus is causing problems: The two "problematic" boards were resurrected after removing all I²C peripherals, and the same issue appeared on the freshly soldered ESP8266 test bed once I attach any I²C device to it.

  Further investigation using my NodeMCU and old ESP8266 breakout boards (also based on the ESP-12F module) revealed that this is more like an issue related to the ESP8266 Arduino core's Wire library implementation, as the bit-banging I²C (note that there is no hardware I²C on ESP8266) occasionally causes strange failures (watchdog reset, not detecting some devices with an I²C scanner sketch, etc.) on these boards too.
  

  At the moment, I do not really have a conclusive explanation on the root cause. Since the I²C bus is essential to the SPINES design, I hope that I can figure out a workaround for ESP8266 in the next few days (or I will need to move on to another IoT module, maybe one with LoRa or BLE):

  • Revert to an older version of the Arduino core
  • Add some delay(1) to the Wire library function calls
  • Any other suggestion?

• Initial Testing: Unexpected Issues

  Macro Yau • 07/14/2018 at 09:42 • 0 comments

  I ran some preliminary tests on the energy harvesting and ESP8266 functions with the two soldered boards yesterday.

  The energy harvesting part, especially TI BQ25570 power management IC circuitry (though proven in several previous projects), works pretty much as expected. Solar power generation with either the onboard KXOB22 solar cell (this addition to the PCB was initially inspired by the TSEM project, and I found out that this is the cheapest option on Digi-Key for a similar form factor) or an external 0.5 W solar panel appears to be working.

  I will do more testing on the power generation later. It has been raining in Hong Kong for several days, and my halogen lamp burned out...
  

  I believed that the ESP8266 would be the most straightforward part in this project. But the strangest thing so far happened on it.
  

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• Reflow Soldering

  Macro Yau • 07/13/2018 at 16:44 • 0 comments

  Finally I gathered all needed parts today. Assembled two prototypes with a reflow soldering hot plate (yes, there are solder bridges and things need to be fixed). The board looks good so far, despite the oversized inductor footprint...

  I will start testing them tomorrow. Meanwhile, please enjoy this timelapse video of the reflow soldering process!

• PCBs Arrived

  Macro Yau • 07/11/2018 at 09:50 • 0 comments

  Collected the PCBs, stencil, and most of the parts yesterday. Now waiting for the 0603 passives and I can start soldering...

• Modular Design Concept

  Macro Yau • 07/08/2018 at 17:20 • 0 comments

  SPINES Mote embraces modular design. Despite being a highly integrated single-board sensor node, the board is composed of three high-level functional blocks, as illustrated in the figure below:

  1. Energy Harvesting and Power Management Hardware: This block is meant to be reused across multiple designs. I will have a separate post to talk about this soon.
  2. Onboard Sensor Suite: My reference implementation includes three sensors (BME280, CCS811, and VEML6075), as well as a Qwiic expansion port for connecting external I²C sensor modules. This block can be easily replaced when customizing the PCB design.
  3. IoT Module Variant: The "variant" system is the core of SPINES' modularity. Please see below for elaboration.

  IoT projects have heterogeneous requirements, and they often need to utilize different types of wireless networks and application processors. In SPINES, this purple section is designed to be "swappable": Not physically swapping in another hardware module, but you can very easily integrate your own design block into a custom SPINES Mote PCB. Apart from the reference designs of complete sensor nodes, I am going to release "template" design files such that you can easily insert your favorite combination of microcontroller and radio (STM32, RFM95, ESP32, nRF series, etc.), and fabricate your unique flavor of SPINES Mote with its compact form factor.
  

  The preview version of SPINES Mote's schematic (solar-powered ESP8266 variant) is shown below. Note that the "IoT Module Variant" depicted above corresponds to the "Application Processor" building block in the schematic. You can create your own SPINES Mote variant just by pasting your design block into the schematic, and connecting several traces in the PCB layout!

  The EAGLE design files will be released as soon as I receive the boards and finish verifying their basic functionalities. This should happen in the next 3 to 4 days.

• First Design

  Macro Yau • 07/04/2018 at 17:08 • 0 comments

  The first SPINES Mote design features a solar-powered ESP8266 (admittedly, powering a power-hungry Wi-Fi radio with a tiny solar cell might not be the best idea; a LoRa version is coming soon) with three onboard environmental sensors:

  • BME280 for temperature, humidity, and pressure
  • CCS811 for volatile organic compounds (VOCs)
  • VEML6075 for UVA and UVB light

  The first batch of PCBs is now being fabricated on PCBWay's factory floor. While waiting for the PCBs and parts to arrive early next week, I will introduce the modular design of SPINES in the next couple of days.
  

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