When you think of a binary display, you may picture a clock with each digit represented by a series of LEDs. Of course, there’s no reason the same thing can’t be done with a thermometer, and as David Johnson-Davies outlines in his Technoblogy post, you can make this type of display with only a single (bi-color) LED.
The setup uses a DS18B20 temperature sensor, capable of a measurement accuracy of 0.5°C, and an ATtiny10 as the main processor. The idea here is that these mini components could eventually be made into a version the same size as the coin cell that powers it. The device is programmed to sleep for 16 seconds, then flash temperature readings in binary, lighting up each time for around 60ms. This makes it efficient enough to last for around 500 days per battery.
Since there is only one — or two if you count both colors — LED, numbers are flashed in sequence with red representing 1 and green 0. Users can choose between displaying temperature in “Easy Binary” or “Negabinary,” both of which will be new to some, but should be easier to read as a series of flashes. Both are explained in his write-up.
This isn’t actually Johnson-Davies’ first try at a tiny binary thermometer either. In fact, he constructed another in 2016 that, while less accurate, is extremely simple as it uses the temperature sensor built into the ATtiny85.
UPDATE: As seen in his latest post, Johnson-Davis has actually been able to fit this apparatus onto a .75" diameter circuit board, which could make an interesting pendant Files for the updated project are available on GitHub.