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• First 3D Printed Enclosure

  Vishnu Mohanan • 05/19/2018 at 11:28 • 0 comments

  Exciting moment! EAZYFAB, an on-demand 3D printing service provider from Kozhikode, Kerala, India came forward to sponsor a single print for the EXPLOG project and they did. Thanks to Sijah AK and Nabeel BA from EAZYFAB. I sent them the STL files exported from Fusion 360. It was printed with PLA and the print is very sturdy.
  

  This is just a sample print to see if the sizes and placements are satisfying. I'll have to modify this further as I develop the PCB and add more modules. I still don't know how to make the buttons and haven't decide the placements of antennas. I'm thinking of 3D printing the buttons with either flexible rubber or PLA itself. If you have any suggestions on making good buttons, please let me know in the comments :)

• The Grand Plan

  Vishnu Mohanan • 04/23/2018 at 16:29 • 0 comments

  So far we've been discussing about the system design, which sensors to use etc. I think now I'm able to see the big picture. So here's the grand plan for the development of EXPLOG.

  Hardware

  First I need to buy all the sensors and modules from different stores and test them for their performance and possible constraints. Most of the sensors are available as breakout modules from many Chinese online stores. The rest can be bought from DigiKey or Mouser. If you own any sensors I have listed on one of the previous logs, and is willing to test them and share the results, that would really save me some time. Doing so will make you a contributor to this project and will be added to the team.

  Once we finish testing the sensors and the prototyping, we then will have to design a custom SMT PCB as per our form factors.

  Firmware

  The main interfaces will be the 1.8" TFT LCD and 7 button keypad though which the user can interact with the device. Programming can be done on the ESP-IDF framework as it better implements the plethora of hardware features available on ESP32. FreeRTOS tasks will be used for simultaneous operations. The task distribution will look like this,

  1. Task 1 (core 0) - will repeatedly read values from all sensors on the I2C bus and update the global sensor readings variables. The keypad controller will be read when there there's an interrupt occurs at the event of user interaction. A dedicated input will be used for interrupt.
  2. Task 2 (core 0) - will update the display with information from the sensors depending on the current user interaction.
  3. Task 3 (core 0) - will save the data read from sensors to a CSV file on the micro SD card.
  4. Task 4 (core 0) - will read the NMEA data from the GPS module and update the variables.
  5. Task 5 (core 0) - will handle the LoRa protocol.
  6. Task 6 (core 1) - will handle WiFi and Bluetooth.

  Utility Software

  I plan to design a utility software for EXPLOG that can fetch data from the device, visualize them in real-time and save it if required. I'm considering Processing platform for this.

  Smartphone Application

  A smartphone application can be used to connect to the EXPLOG via WiFi or Bluetooth and visualize data. Applications written in Processing can be exported to run on Android phones easily. Will have to see how far we can go with it.

  Cloud Platform

  A cloud platform dedicated for sharing data from EXPLOG devices all around the world sounds a pretty bold idea. Users will be able to save the data from their device to the cloud by connecting to the internet and share the data to the Open Data Pool. Others can use these data for climate studies and a whole lot of other applications. Cool!

  Enclosure

  The enclosure can be 3D printed on PLA material. The 3D models are designed in Fusion 360 and the source files will be made available to the public.

  Marketing

  If this project gains enough attention from the open source community and other interested enthusiasts, I would be able to sell EXPLOG as product at marketplaces such as Tindie for a reasonable price.

• Thoughts On Using A Coprocessor

  Vishnu Mohanan • 04/21/2018 at 16:01 • 0 comments

  ESP32 is a feature packed power house when it come to price-performance ratio which makes it ideal for all IoT applications. But there's one tiny drawback - limited number of GPIO pins. On the ESP-WROOM32 module, some pins are used by the flash chip and can't be used for other purposes. Some other set of pins can only be used as inputs, not outputs. As per the datasheet, ESP32 has 34 GPIOs of which I/O GPIO pads are 0-19, 21-23, 25-27, 32-39, while the output GPIOs are 0-19, 21-23, 25-27, 32-33. GPIO pads 34-39 are input-only. So in total we get 22 I/O pins and 4 input only pins.

  Even though it is not apparent in this initial phase of the project, sooner or later we're going to run out of pins if we're going to add more sensors and stuff in future. That's why I'm considering adding a less powerful but GPIO rich coprocessor to the design. The second processor or microcontroller could offload some tasks from the ESP32 chip giving us more room for some possible number crunching tasks. For that I'm considering the AVR Xmega series ATxmega128A3U which has 50 IO pins (64 pin package) which is plenty. It runs at 32MHz, has 128KB flash, 8KB SRAM, 7 UART, 10 SPI, 2 I2C and one USB modules.

  Let's estimate the pin requirements for each interfaces.

  1. SD card - 4 (hardware SPI)
  2. Display - 4 (3 wire SPI with MOSI only and CD)
  3. I2C for sensors - 2
  4. GPS - 2 (hardware or software UART interface)
  5. USB to Serial - 2 (software of hardware UART)
  6. LoRa module - 4 (hardware SPI)
  7. Keypad controller interrupt - 1

  So that's 19 of available 26 pins and we still have 7 pins left. If we ever use up all the pins left, we'll have to add a coprocessor.

• Choosing the Right Sensors

  Vishnu Mohanan • 03/19/2018 at 11:21 • 5 comments

  EXPLOG needs a lot of sensors obviously and there's a large list to choose from. Some of the criteria for selection of sensors are,

  • Size - must be small with less than 5 mm height, and be surface mountable and reflow solderable  or through-hole solderable.
  • Interface - I2C compatible.
  • Operating voltage - 3.3V
  • Calibration - should be either factory calibrated or user calibratable.
  • Global availability
  • Cost

  We need to make the right choices by considering the global availability and cost of each sensor to make sure that others will be able to get them to replicate/modify the device. These are what I've been able to come up with. Datasheets of all these sensors will be available at the file section of this project.

  Barometric Pressure Sensor

  1. DPS310 from Infineon
  2. BMP280 from Bosch
  3. BMP380 from Bosch
  4. BMP388 from Bosch
  5. BME680 from Bosch
  6. MPL3115A2 from Maxim

  We need a sensor to accurately measure barometric air pressure becasue we need accurate pressure measurements for calculating the altitude accurately. DPS310 was my first choice becasue I had received an evaluation module from Infineon as part of a give away. But not everybody is going to get these as they are not available in the usual maker markets. BMP280 on the other hand is widely available and most are familiar with. But they're not the best ones out there. Let's have a comparison of features of sensors from leading manufacturers.

  	DPS310	BMP280	BMP380	BMP388	BME680	MPL3115A2
  Operation range	300 - 1200 hPa	300 - 1100 hPa	300 - 1250 hPa	300 - 1250 hPa	300 - 1100 hPa	200 - 1100 hPa
  Abs. Max. Pressure	10000 hPa	20000 hPa	20000 hPa	20000 hPa	20000 hPa	5000 hPa
  Max. Resolution (Pressure)	24-bit	20-bit	24-bit	24-bit	20-bit	20-bit
  Precision	0.005 hPa	0.0016 hPa	0.003 hPa	0.0016 hPa	0.0018 hPa	0.0025 hPa
  Absolute Accuracy	± 1 hPa	± 1 hPa	± 0.5 hPa	± 0.5 hPa	± 0.6 hPa	± 4 hPa
  Relative Accuracy	± 0.06 hPa	± 0.12 hPa	± 0.06 hPa	± 0.08 hPa	± 0.12 hPa	± 0.5 hPa
  Temperature Coefficient Offset	0.5 Pa/K	1.5 Pa/K	1.2 Pa/K	± 0.75 Pa/K	± 1.3 Pa/K	NA
  Sampling Rate	128 Hz	157 Hz	200 Hz	200 Hz	182 Hz	100 Hz
  FIFO Size	32 samples	NA	512 Bytes	512 Bytes	NA	32 samples
  Current Consumption @ 1Hz	1.7 uA	2.7 uA	2.7 uA	3.4 uA	4.2 uA	8.5 uA

  -

  From evaluating the specifications, BMP380 and BMP388 seem to be the best choices becasue they provide better absolute and relative accuracies, higher sampling rate, highest bit depth and higher absolute maximum pressure. The BME680 is special becasue it crams a pressure, temperature, humidity and air quality sensors in a single package. It might save some PCB real estate but can't compete with specs of other single purpose sensors, and it's going to cost higher than others. So we won't be using that. The DPS310 has its problems such as lower sampling rate, lower FIFO size etc, and it is priced similar to others. So these reasons also favor BMP380 and BMP388.

  Absolute accuracy is how close the output reading to the actual reading of absolute pressure (absolute pressure is the pressure reading taken with a zero pressure as reference). The more accurate the absolute pressure, more will be the accuracy of our calculated true altitude relative to seal level. So if we need more accurate altitude measurements we need more absolute accuracy. On the other hand, relative accuracy, as you guessed, is the degree of error between two relative pressure readings (absolute error on two relative readings will be same). This reading is crucial because it is what helps us to determine the relative elevation, for example how high a drone from the ground is etc.

  Temperature Sensor

  Most pressure and humidity sensors come...

  Read more »

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