Hot Beverage Sensor Coaster

Whether coffee or tea, many of us enjoy a hot beverage while doing desk work. Unfortunately, such beverages typically start too hot, work their way down to a more optimal temperature, then become too cold before settling down to room temperature. To take some of the guesswork out of his drink consumption, Tim (AKA Palingenesis) created a temperature-sensitive coaster to show his tea’s thermal status.

The heart of this coaster is a DS18B20 temperature sensor, which is read by an STM8S103F Blue Pill module. With this thermal information, the Blue Pill lights up a series of red, green, and blue (non-addressable) LEDs in pairs to show the temperature range. Each pair of LEDs is driven by a single GPIO pin with a 2N4401 NPN transistor.

The device can use an LCD display as needed, which is especially helpful when setting up your preferred liquid consumption temperature. While most of the build focuses on an in-coaster sensor, there is also another option (shown in the top image) where the probe is inserted into the hot liquid itself. While more accurate, this would present a number of other issues, e.g. dripping, when trying to enjoy your beverage.

The base of the coaster is laser-cut from four layers of 3mm plywood, which forms a nice mug surface and a structure for the electronics. Cut files, wiring diagrams, and programming instructions are all included in the project write-up. More explanation and a breadboard demo of how it works is seen in the video below.