Sometimes a project is given such a name that you (read: me!) do not immediately know what it represents or what it does, an ‘Earth Listener’ for example. The description of the product, however, makes things much clearer: ‘The Earth Listener is an Arduino-based sensor module that can measure multiple environmental variables in order to indicate the air quality’. This module measures temperature, humidity, air pressure, eCO2 and TVOC (Total Volatile Organic Compound) and indicates these values on a TFT touch screen or stores them on an SD card. Furthermore, the circuit contains an AS3935 lightning detector that gives an estimation of the distance to the centre of the storm within a radius of 20 km and gives an alarm when lightning is detected.
The Arduino software in the Earth Listener is open source and the sensor shield design offers the option of adding up to six more sensors yourself. A great number of options for such a relatively small box, which does have to be assembled first, before it can be used.
The kit with its parts.
As is often the case these days, the documentation is only available online, from the website of the manufacturer. I got off on the wrong foot by searching in the section ‘downloads’, but the instructions for the assembly can be found by clicking the link ‘manual’ and subsequently selecting ‘2. Assembly’. On this page are also links to the user manual and a document about the measuring results from the CCS822 (All about Values), bit I will get back to that later.

For the assembly of the Earth Listener, the kit includes a 2.5-mm hex wrench for the bolts. If you need to, you can use pliers to hold the stand-offs, but just holding them with your fingers is probably enough. Otherwise, nothing else is required (no: there is nothing to solder). The English-language instructions are clear, well-illustrated and with more than sufficient photos, and if you follow c.q. look at these carefully, everything will be assembled very quickly and will be ready for use.
In the assembly instructions there is the comment that the processor board can also be mounted in the enclosure using only two instead of four bolts. That is because two of the mounting holes in the ATmega2560 board are too close to the connectors, and it is not possible to insert the bolts without using force. A cosmetic flaw, but for the operation or sturdiness of the Earth Listener it fortunately does not matter if the board is held by only two mounting points.

The firmware is already programmed into the processor board and although the assembly instructions continue all the way to screwing the enclosure closed, I consider it a better idea to test whether the circuit works properly first, before closing the lid on the enclosure. Not that much is likely to go wrong during the assembly, but nevertheless... After switching the power supply on, the LCD should announce that the three sensors have been detected and if this is the case, the circuit should function as intended. In any case, remember to remove the protective stickers from the LCD and the buzzer on the back of the sensor board, before you screw the enclosure together.
The laser-cut wooden enclosure is very neat, there is actually only one thing to comment on: the opening for the SD card. If you are not paying close attention – and I failed to do that the first time – you will push the card past the holder and you will need to reach for the hex wrench again to liberate it. If you are planning to the use Earth Listener as a datalogger (and therefore will frequently change the SD-card), it is recommended to glue a small strip of wood or cardboard on the inside to make the slot just a little smaller.

After the assembly is complete, it is of course useful to know how to operate the device, and the manual can also be found online. The touch screen and the supplied stylus actually make the operation very easy and intuitive, but nevertheless a few factoids from the manual.
In the settings (the well-known gear icon at bottom-right of the LCD), there is an option whether the Earth Listener is used indoors or outdoors, this has to do with the sensitivity of the AS3935 lightning sensor.
When, on power-on, the Earth Listener detects an SD card, it will automatically create the log file DATALOG.CSV to which a line is added every second containing the measured values. If this file is already present on the card, the new data is appended to the end of the file. Personally I find this measuring interval to be very short, normally the conditions do not change that rapidly.
Because of the absence of a Real Time Clock, the time-stamping of the samples simply starts at zero when beginning the measuring session. It is therefore necessary to note the starting time of the measurements (or the end time and work backwards), in order to match the measurements with the correct time afterwards. The manual suggests adding a Velleman module with a DS1302 RTC for absolute time-stamping, but it should be a little clearer that the software for the Earth Listener does not (yet) support this extension.

Then there is the section ‘All about Values’ in the documentation, with an explanation of the measurement results from the CCS811 air-quality sensor (TVOC and eCO2). For most people these quantities will not be very familiar and the explanation in this document was also for me very welcome.
Temperature and humidity have apparently also an effect on the TVOC and eCO2 measurements, that is why the gas sensor is mounted on the board together with BME280 (temperature, humidity and pressure sensors). The enclosure and the heat generated by the electronics cause a deviation between the measured temperature and humidity and the actual values. Apparently this causes a fixed offset for both these measurements, which are corrected for in the software of the Earth Listener using empirically-determined constants. On a similar topic – the warming up of the circuit – I would like to make the comment that the Earth Listener requires a warm-up time before it measures stable values, which I feel is something that should have been mentioned in the manual.

The source code for the Earth Listener is on GitHub and therefore available to anyone who would like have a go themselves with the firmware. The test version was equipped with firmware version V3.2, in the meantime version V3.4 became available for download, but I did not immediately see the need to update it. Apparently the firmware can now switch to American units (Fahrenheit and miles).

The Earth Listener is a practical and nicely designed device, that will not look out of place in the living room or the office. It is completely ready for use, but also offers the possibility of tinkering with it yourself and think of your own extensions.
In addition to the classical measurement values of a thermometer, barometer and hygrometer, the gas sensor, with its measurement of eCO2 and dust particles, gives a good indication of the air quality in a space and when ventilation is desired or even essential. Here at Elektor there are sometimes discussions about whether a window should be opened or not, and it is then quite handy to have an electronic arbitrator on hand.
The lightning detector I have (fortunately) been unable to test, but I do know that the sensor is functional: pushing the desk chair back and standing up can trigger it. So, keep the sensor away from interference sources...

The Earth Listener is available from the Elektor Store for €134.96 for Elektor members.