Getting Current Temperatures with a (Freddie) Mercury Thermometer

Mercury thermometers

For hundreds of years and up until quite recently, people around the world have been using thermometers filled with mercury in order to tell the temperature. They work by relying on the liquid properties of the toxic metal and measuring how much it expands when exposed to heat. However, the introduction of more accurate and safe digital thermometers has rendered them obsolete. So as an homage to this outdated instrument and a clever play on words, Instructables user TurboSnail has created the Freddie Mercury thermometer, which features the Queen singer/writer confidently pointing to the temperature.

The components

The project revolves around an Arduino Nano Every and an accompanying Adafruit AHT20 temperature and humidity sensor in order to read the ambient conditions and process the resulting data. From here, the microcontroller maps the temperature value to a rotation and moves the arm of a servo motor accordingly. Power is provided to each component by a single 1S LiPo cell that gets toggled with a power switch.

Designing the front panel

The primary goal of TurboSnail involved making a design that would look nice, hold components in place securely, and display both the temperature and humidity levels. To combine the art with the electronic components in an inexpensive way, they opted to create a large PCB that had silkscreen graphics and text to minimize extra work. Along the dial are temperature increments of 5C, with each one showing either a title or lyric from a Queen/Freddie Mercury song. The lower-left contains a small area for placing surface mount 0805 LEDs that illuminate to indicate the humidity in the room.

First prototype

After receiving their newly fabricated PCB, TurboSnail got to work by soldering on the necessary LEDs, resistors, and wires to the PCB. Next, the temperature module, Arduino Nano Every, and servo were attached along with a small transistor that cuts off power to the servo for extra battery conservation. However, the Nano still consumed too much current without special sleep modes, meaning that a modification was in order.

A quick update

As a small change, the original Arduino Nano Every was replaced by another custom PCB that was essentially a stripped down Arduino Uno. Due to the simple nature of the circuit and lack of many external components such as a voltage regulator, the board pulls a mere 1mA.

Sensing the temperature

With the hardware and code now complete, TurboSnail cut a set of four identical oak strips and glued them together to form a small frame in which the front panel PCB could be placed. Freddie's arm was made by 3D printing a basic design done in Fusion 360 and then performing some light finishing work on it. As seen in their video, the Mercury thermometer begins by swinging the arm rapidly back-and-forth before settling onto the current temperature. Afterwards, the temperature and humidity are read and used to adjust both the servo motor's position and the currently illuminated LED once every 30 seconds.