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micropower micrologger [mPmL]

Let's build a set-and-forget temperature logger. Size: 25.3x 10x 18mm incl. battery. Runtime: > 1year

JanJan
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This is a spontaneous development from my first logger project. My idea is simple: The logger needs to be small enough to fit inside small spaces, e.g. a bee hive.
With the press off a button it starts/stops logging. The case can be as simple as shrink tubing!

Functional specification

The following table shows the most important features I expect from my logger. There are of course more (e.g. LED for signaling, reset button, ...) but those will make it into the final design anyway!

PARAMETERVALUE/DATAIMPORTANCE
size
≤25.4x25.4mm (1 square inch)
(preferably 25x20mm)
height < 10mm
REQUIRED
DESIRED
DESIRED
operational life
~ 1 year on a CR1225 coin cell (48mAh)
DESIRED
RTCtemp. compensated RTC (DS3231)
REQUIRED
accuracyT ± 0.5°C
REQUIRED
ext. sensor size
small for hive "in cell" measurement
DESIRED
memory(timestamp + 4x temperature) * 1 year
REQUIRED
easy data transfer
3 wire interface (RX, TX, GND)
REQUIRED

Here's a quick jump-to to all my diary entries:

ENTRYLAST CHANGED
01 – choice of components
Friday, 10th, 2018
02 – data storage
draft
03 – test placement of componentsFriday, 11th, 2018
04 – logger and sensor positionSaturday, 18th, 2018
05 – PCB close to final versionSaturday, 18th, 2018

  • 1 × Atmega328P-MU Microprocessors, Microcontrollers, DSPs / ARM, RISC-Based Microcontrollers
  • 1 × DS3231M (SOIC8 package) Clock and Timer ICs / Real-Time Clocks
  • 1 × M24M02 Memory ICs / EEPROMs, alternatives are e.g. 24LC512, 24AA512, ...
  • 1 × SMTM1225 SMT HOLDER FOR CR1225 coin cells
  • 2 × LED, size 0603 whatever color you like

View all 9 components

  • 05 – PCB close to final version

    Jan5 hours ago 0 comments

    Just a quick shot of the near-final board (nothing routed yet):

    The wasted space on the right will be used for one or two additional digital pins, so one could use sensors other than I2C. I finally decided to use CR1632 button cells (switched between them an CR1225 a few times) to get around two years of use out of them. That's because the combs are used around two years and I wanted to shrink tube the circuit so I can't easily replace the battery "in the field".

    Final size is: 25,3x18mm (1x0.71").

  • 04 – logger and sensor position

    Jan6 days ago 0 comments

    The micrologger has that name for a reason :) It is so small, that it can be placed right in the frames which are put in the bee hives! Here you see a frame with a wax foundation:

    wax foundation

    The top "bar" is 20mm wide, that's why I decided to shrink the logger to 25x20mm. I use very thin battery-pack heat shrink tubing to protect the circuit.

    here goes the logger

    It will be glued in the space you see outlined.

    To not disturb the bees while building the cells, I can't use normal multi stranded wire to connect the sensors. So I chose enameled copper wire of 0.15mm diameter.

    0.15mm enamelled copper wire

    4 strands are first color coded and then twisted with a hand drill. The finished wire is 0.4mm in diameter, which can easily be routed on the pre-made wax foundation plates. It is quite durable, so you can bend it a few times without breaking the insulation.

    The next picture shows two different packages:


    TO-92, left and SOT-23 next to it. I just ordered my TMP112 sensors which are even smaller than the SOT-23 package seen here. Will be quite the hassle soldering wires directly to it but let's see how/if it works.
    I want to add it directly into the cell bottom!

  • 03 – test placement of components

    Jan08/11/2018 at 08:18 0 comments

    Edit August 15th, 2018: Parts ordered.

    All parts are ordered and are expected to arrive in around two weeks from china. I hope to finish my board this weekend to send it to the fab as well, so I can start my first assembly.

    The final design will look quite different from the mock-up...

    Read more »

  • 01 – choice of components

    Jan08/09/2018 at 10:32 0 comments

      This log is all about the why, not the how...

      Space is the limiting factor in this project. I've got a full top PCB pane of 25.4x25.4mm and around 1/3 of that on the bottom side to put parts on. The rest of the bottom side is occupied by the coin cell holder.

      As I intend to put the PCB directly into the brood comb, I need to save as much space (especially in height) as possible.

      The PCB

      1,6mm is standard FR4 for most board houses. I chose 0.8mm and 2oz copper to keep the height down. Board house will be @oshpark.

      Battery

      There's not much to choose from here. A 2032 cell is too large and occupies the whole bottom of my board. A CR1225 fits perfectly, but has only around 48mAh capacity, so I need to do super power saving stuff to get my 1 year of up time. I'll do my calculations with 35mAh to be safe.. The battery holder will be the Renata SMTM1225 SMD coin cell holder.

      For my calculations I like to use the Oregon Embedded Battery Life Calculator, which has proven to be quite accurate.

      With my setup I expect around 380 days of run time with a sleep current of 0.003mA, 4 wake-ups per hour, each 1s long and a current consumption of 0.2mA during logging.

      The microcontroller

      Atmega328P-MU, any questions? Jokes aside, I have two of them sitting in my shelf and didn't solder the small HVQN32 package yet. TIME!

      It will be clocked at 1Mhz internal to keep the coin cell from draining too fast. Current consumption is expected to be around 0.2mA when running/collecting data and 0.1µA when in power down mode.

      Of course the other parts will add to that but will be powered off in sleep mode anyway...

      RTC

      DS3231 in a 8 pin package. Easy to use, drifts a few seconds a year, easy to control + has alarms, which are essential for precise wake/sleep cycles. It uses around 0.002mA when in battery backup mode.

      Memory/EEPROM

      Unfortunately using an micro SD is not possible with such a weak coin cell. It would be like a short circuit drawing 100mA peak from it.

      I'll go with the pretty standard 24LC512, 24LC1026 or 24M02. They all work quite the same + pinout is the same!

      Temperature sensors

      It really breaks my heart, but my beloved DS18B20 sensors aren't too happy working below 3V. As I don't want to use a boost converter I had to switch to other sensors. Requirements:

      1. accuracy of +- 0.5°C
      2. more than one sensor at a bus
      3. small package (<TO92)

      A quick search on lcsc.com gave me 3 options:

      1. Microchip Tech MCP9808-E/MS
        1. size: 4.9x3mm, height 1.1mm
        2. accuracy 0.5°C
        3. 2.7V to 5.5V
      2. TI TMP275
        1. size: 3x3mm, height 1.1mm
        2. accuracy 0.5°C
        3. 2.7V to 5.5V
      3. TI TMP112AIDRLR
        1. size: 1.6x1.6mm, height 0.6mm
        2. accuracy 0.5°C
        3. 1.4V to 3.6V

      I chose number 3, as it seems just solder-able by hot air (0.5mm pitch) and is super small. In a lot of 10 its under 0.8€/piece, so that's alright with me. 4 of them can be used on one bus: nice!

      Power consumption is 10μA active (maximum), 1µA Shutdown (maximum). They won't need turning them on/off to save power. For external sensors I will use 0.1mm diameter enameled copper wire. Don't know yet if I solder the wires directly to the sensors or if I create small break out boards... Hope one of these methods works.

      Bottom line

      Those components will all fit onto my board. I'll add an LED and rest/user button as well in the final layout.

      The Atmega328P-MU and TMP112 are packages I have no experience soldering so I guess I'll learn something on my way too!

View all 4 project logs

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