Tiny Two-Digit Thermometer Has Long Battery Life

Like most of his work, this tiny two-digit thermometer shows that [David Johnson-Davies] has a knack for projects that make efficient use of hardware. No pin is left unused between the DS18B20 temperature sensor, the surface mount seven-segment LED displays, and the ATtiny84 driving it all. With the temperature flashing every 24 seconds and the unit spending the rest of the time in a deep sleep, a good CR2032 coin cell should power the device for nearly a year. The board itself measures only about an inch square.

You may think that a display that flashes only once every 24 seconds might be difficult to actually read in practice, and you’d be right. [David] found that it was indeed impractical to watch the display, waiting an unknown amount of time to read some briefly-flashed surprise numbers. To solve this problem, the decimal points flash shortly before the temperature appears. This countdown alerts the viewer to an incoming display, at the cost of a virtually negligible increase to the current consumption.

[David]’s project write-up explains how everything functions. He also steps through the different parts of the source code to explain how everything works, including the low power mode. The GitHub repository holds all the source files, and the board can also be ordered direct from OSH Park via their handy shared projects feature.

Low power consumption adds complexity to projects, but the payoffs can easily be worth the time spent implementing them. We covered a detailed look into low power WiFi microcontrollers that is still relevant, and projects like this weather station demonstrate practical low power design work.

20 thoughts on “Tiny Two-Digit Thermometer Has Long Battery Life

  1. An interesting variation would be to have it solar powered, along with a simple motion detector that only turns on the LEDs when something is moving, giving you the impression that it’s always on when you turn and look at it.

    1. Americans are the worst at this type of thing yet the first to complain when a non US coin is used for size reference, is the 2032 coin cell on the back not a suitable size reference?

        1. Ha. You troll. There is clearly 50 pence written on the coin. Not shilling… But that is besides the point. There is a banana and quarter lying under the thermometer and 50 pence coin. Just next to the ford 150 and the original citroen DS from 1956…

          I like the thing though. Maybe even better than a button to push to get the readings.

  2. Pushbutton to read temp would be better. You could even measure temp only when button is pushed, so this could have almost unlimited life (as long as battery).

  3. Nice “Brilliant Uncirculated” 50p piece.

    Love the projects on Technoblogy. Lovely clear pictures, clean presentation style, links code etc. All ready for you to pick up and work from. (Now with shiny coin).

    I point my trainees here to start them thinking about design decisions and take thier first real step into engineering. To coin a phrase from Mr Arthur M. Wellington. “Making for a penny, what any fool can make for a pound”.

    Thanks for posting.

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