Project Logs

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• Low-power considerations

  martin2250 • 06/11/2015 at 21:13 • 0 comments

  Since one of the key features of this project is the low power consumption, I'll go into some more detail on how to archive similar results on AVR microcontrollers.

  Power Management

  Use the macros provided by <power.h> to turn off modules that you don't need. My usual approach is disabling all modules when going to sleep and only activating modules as you need them.

  Be careful though, registers of disabled modules can't be accessed! I spent a lot of time tracking down a bug where I set up a timer, and enabled it only before starting it.

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• Software V2

  martin2250 • 06/08/2015 at 18:48 • 0 comments

  While performing pretty well in terms of battery consumption (60µA), the old firmware was very buggy and only reacted poorly to the buttons, so I started working on the new Firmware.

  It is available on GitHub, see the main project page.

  Flaws of the old version:

  • processor is woken up from deep sleep every second by the RTC to look for button presses
  • menu completely blocks the processor, is very sluggish and hard to expand
  • battery voltage is determined with a voltage divider
  • no display timeout -> accidentaly drained the batteries in under a day
  • peripherals are accessed during interrupts (not necessarily a bad thing, but only when timing is critical)

  The main difference is that now, interrupts only set flags in the global flag struct which get processed in the main loop. Also the processor goes to deep sleep after it has finished drawing the display, while it is sending data via UART or while it is waiting for the HYT939 to finish it's measurement.

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• Hardware Improvements

  martin2250 • 06/08/2015 at 18:13 • 0 comments

  I fixed some issues with the hardware:

  Buttons

  the buttons reacted very badly, so I replaced them with tactile switches which have spacers glued to them. The old buttons were screwed in, the new ones were secured in true hacker fashion with excessive amounts of hot glue.

  Reverse voltage protection

  The schottky diode that was used to protect the circuit against reverse voltage, which can easily appear with 9V-clips dropped too much voltage under load, especially when the LCD was on, but also when sampling the battery voltage after storing the buffer to EEPROM which resulted in curiously low readings. While it was not too much of an issue for the controller, the LCDs contrast constantly had to be adjusted so I replaced the simple diode with a small MOSFET in this configuration:

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• Software V1

  martin2250 • 06/08/2015 at 17:52 • 0 comments

  If you want to take a look at the old software you can find it here: http://martin2250.blogspot.de/2014/11/avr-temperaturehumidity-logger.html

  Since I created this project log after finishing the improved software, I won't write about it.

• Case Build

  martin2250 • 06/08/2015 at 17:52 • 0 comments

  The Case is made from 4mm plywood and milled on my Shapeoko 2.

  Design (CAD)

  To design the case, I used Cadsoft EAGLE. I have only seen very few people use it for that purpose, although it is quite nice to use (at least if you're already used to the workflow) and the size restrictions only apply to components, not to drawings.

  The design is pretty straightforward: a box with finger joints and a separator for the battery compartment.

  The most exciting part is the battery cover: it slides in and out without using anything but the plywood and no extra parts.

  The design files can also be found in the GitHub repo.

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