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A Simple Two-Digit LED Thermometer

A Simple Two-Digit LED Thermometer


A friend of hacker David Johnson-Davies’ now two year-old son recently had a birthday. While he can’t yet read words, he can (impressively) read numbers. Rather than merely running to the store or clicking an order button, Johnson-Davies came up with something truly unique for the little guy — a two-digit PCB thermometer.

The device’s PCB is nicely laid out, and measures just over 25mm x 25mm. This shape would be conducive to insertion in a waterproof case, and with a battery life of around a year on its CR2032 battery, it could be left outside to show what to be prepared for before opening the door.

The thermometer reveals the temperature using a pair of LED 7-segment displays, and everything is controlled by an ATtiny84 MCU. This give it just enough I/O pins to control the numbers, while still getting input from a DS12B20 temperature sensor.

In order to get ~one year of battery life, it’s programmed to stay in sleep mode most of the time, flashing the temperature only once every 24 seconds. As set up, it can display temperatures between -19°C and 99°C, using the left character to display a -1 with three segments.

It’s a brilliantly simple hack, but not Johnson-Davies’ first. For an even simpler (though a bit harder to read) gadget, check out his ATtiny10 Binary Thermometer here!

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