Data Logging Thermometer with History

2016-12-09 | By Joshua Bishop

License: General Public License

Overview

This idea originated from the fact that I do not like to be freezing when I’m camping or backpacking. I had recently purchased a sleeping bag and while I had its rating, I had no idea how well it would actually work. I wanted something that would tell me how cold it got at night. I searched for small thermometers that had some history as well and was unable to find anything. Most camping thermometers were simple mercury based thermometers or fishing thermometers that were digital, and none of them had memory. So I set out to create a simple thermometer that recorded the previous twenty-four hours of readings and displayed current temperature as well as highest and lowest temperatures over those last twenty-four hours.

Approach

Using a simple 1-wire thermometer, an OLED display and a single button the thermometer will take samples every minute and store that sample to memory in a ring counter style storage. When the button is depressed, the OLED display lights up for five seconds, displaying the current temperature, high and low over the last twenty-four hours and a small graph tracking the changes.

Challenges and Design Considerations

The biggest challenges we expect are keeping a small form factor and minimizing power consumption.

Small form factor is relatively simple on the electronics side of a project this size. We selected a microcontroller in a very small QFN package. The temperature sensor is also a very small SOIC package, with only very small passives to support these. The two items likely to define the overall size are the battery (and its holder) and the screen. The screen needs to be large enough to display information but small enough to be highly portable. Finding this balance will be more of a user interface challenge with engineering ramifications.

To keep power consumption low, it was originally anticipated to use an STN display that constantly showed current and historical values. STN displays have incredibly low power consumption and great daytime visibility. To further minimize power consumption, the sampling rate needs to be reduced to a reasonable rate that gives acceptable time resolution without using too much power. These two items are anticipated to be the largest benefits to power savings, though other small yet crucial component choices, such as voltage dividers, will affect this as well.

End Result

While the plan was originally to use a small STN display, it was difficult to find an off-the-shelf display for prototyping purposes. Plus, the display wouldn’t be visible in the dark. A backlight would have increased cost and complexity and lacking that feature was a serious drawback. While we discussed this, the idea was put forth to have an OLED display that would only display for a couple seconds when a button was pushed. This is idea was first resisted because we thought there was a greater benefit to having the information continuously displayed.

However, it was pointed out that most people are accustomed to pushing a button to see the time or other information on their phone. At this point we asked a group of five or six colleagues for their opinion. This solution was unanimously preferred by those polled and the final interface was chosen to be the OLED.

The OLED selected was a .96” single-color graphical OLED display. This size was perfect for portability while also allowing us to display the current temperature, the high and low over the last 24 hours, and a simple graph showing the trends over that same time period. While the OLED is on, it consumes approximately 7-8 mA, which is still relatively minimal and even with repeated use, should have passable longevity.

For this version, we included two buttons, a button that will turn on the screen and another button on the back in the battery compartment to toggle between imperial and metric units. While this simple interface worked flawlessly, we discovered we could very easily limit it to one button for the entire interface by using a timing scheme. This is a minimal reduction in cost but opens up a decent percentage of real estate on the circuit board and within the enclosure.

When the OLED is off, average current draw is on the order of tens of microamps as most of the time the controller and sensors are asleep, with the occasional sensor sampling. With the 2032 battery used for this first prototype, rough calculations would give a theoretical life of around 6 months on a single battery with moderate usage.

Moving Forward

This is a simple project that has a lot of potential and is definitely something that will be actively pursued. As it is not an original concept and there are already similar products on the market, the main differentiators we will pursue will be the historical data, as well as a robust and completely waterproof enclosure, with the goal of IP68 standards.

To help make waterproofing simpler, we will want to reduce the amount of openings required. We will integrate a LiPo battery with a waterproof microUSB port to make it so the device never needs to be opened. In doing this, we have the benefit of reducing to a single button interface, because the unit changing button would no longer be accessible in the device itself and we’re hoping to minimize case openings.

Code – main.c

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