NO₂ - urban air indicator

A few years ago, a nitrogen dioxide display in the form of a social sculpture was installed in Wiesbaden. LED tubes hung from a huge tree directly in front of the main railway station, alerting citizens to the poisonous gas NO₂ in public. Like a traffic light, the "city air indicator" lit up red when the EU limit value of 40 micrograms per cubic metre of air was exceeded and green when the air was OK. This campaign was of course limited in time - but it was intended to be continued. This gave rise to the idea of designing a city air indicator for home use. An initial design in the form of a test tube had already been created for this purpose. My task was now to get this prototype to work or to reproduce it in small series. The idea behind it: Interested citizens and students should be able to build their own city air display and hang it on the windows of public buildings in the city centre, for example, to draw attention to the quality of the air.

Several prototypes were necessary to find out, for example, how the LEDs should be integrated into the tube. At first it seemed easier to use ready-made LED strips and solder them to the board with the ESP-12S. In the next version, I designed the board so that there is a common board for LEDs and microcontroller. In this version, you had to assemble all the components yourself. In the end, however, it proved to be more practical to have one side of the board delivered already assembled. Unfortunately, the company in China can currently only assemble one side. This means that you have to solder the components on the other side yourself using a reflow process. This requires some dexterity, as some of the components, especially the capacitors, are only 1x2mm in size. The layout of the board is built up in four layers. A USB-C socket was used for the power supply in the current version.

The fully assembled circuit board is then inserted into a 22cm long Plexiglas tube. There is a diffusion foil in between so that the light from the LEDs is distributed more evenly. There is a scale from 0 to 100 on this film, which, like a thermometer, shows in increments of ten how good or bad the air is at the selected measuring station in your city (see studies). In a second animation, the tube appears completely in red or green, depending on whether the currently recorded value is just above or below the EU limit value of 40 micrograms per cubic metre of air (annual average). The two animations alternate every 10 seconds. I chose a self-printed cap for the top so that you can see the difference between the electronics and the LEDs even when the device is switched off.