“This is a two-digit LED digital thermometer I made as a birthday present for my friend’s son. As he’s only two years old he can’t read yet, but he can read numbers, so I thought it would be fun for him to have something that would tell him the temperature on a seven-segment display:
It uses a DS18B20 1-Wire temperature sensor, and an ATtiny84 to drive the display and read the sensor. It’s just over 25mm square; about the size of a British 50 pence piece.
The two-digit thermometer is designed so it could be put in a waterproof case outside the window, to show the outside temperature in all weathers. To conserve power the thermometer flashes the temperature every 24 seconds, and then goes to sleep, and it achieves a battery life of about a year with a CR2032 button cell.
The thermometer can display temperatures between -19°C and 99°C. To display temperatures between -10°C and -19°C the left-hand display is used to display “-1”. Outside this range it displays ‘Lo’ for temperatures below -19°C or ‘Hi’ for temperatures above 99°C. If you live in a part of the world where temperatures regularly reach below -19°C you could change it to light the decimal points to indicate negative temperatures.”