NAM measures noise – NAM DNMS kit

NAM DNMS (Digital Noise Measurement Sensor) kit is a kit that allows you to extend the capabilities of NAM to measure noise intensity. The issue of measuring the noise present in our environment has been gaining attention among many people recently. By preparing this kit, we want to meet these people to facilitate the monitoring of noise pollution.

What is DNMS?

The “acoustic” part is built on the DNMS project (developed by people around the Sensor Community project). This project is up and running, but it requires overcoming several obstacles to get it up and running (special PCBs, lack of a ready-made enclosure) , so I decided to create a kit that removes some of these obstacles.

For operation, in addition to this kit you will need NAM – the kit has been tested with kit NAM 0.3.3, it should also work with versions 0.3.0-2 and NAM 0.4 however we have not tested this. You will need to solder NAM DNMS kit, there is one SMD component but it doesn’t create big problems.

The whole project is in beta, so it needs a little more attention and work – read the whole instructions before buying.

Soldering

We start by soldering the SMD component – this is the socket for connecting the microphone. NOTE: there is room on the PCB for two sockets, one on the underside of the board, the other on the side where Teensy is. We solder the latter, on the Teensy side. Double-check, or maybe even triple-check – the footprint is almost identical, so you can make a mistake and solder in the wrong place. 

The socket on the underside is for another microphone, it looks like it won’t be used in the future and we will abandon it in future versions of the board, but at this stage we wanted to maintain flexibility during testing.

If you have a sharp, conical tip and 0.5 mm thick solder tin soldering should be no problem.

Begin by soldering one point on the side of the socket. If it comes out “crooked” – warm up the tin with a soldering iron and, keeping the soldering iron tip pressed against the pad, correct the position of the socket with your finger. Be careful not to touch the housing of the socket with the tip (that you would not touch the tip with your finger I don’t think I need to write?).

You don’t have to try to make the socket as close to the edge of the PCB as possible. The plug from the microphone is flat and should go into the socket without a problem, even if it is not at the very edge. Just make sure that all pins will have good contact. If this is the case – solder all the other pads.

When you use the aforementioned soldering iron and thin tin, you can safely solder the pins without picking up excess tin. When you use thicker tin and a thicker soldering iron – such soldering tape comes in handy to remove the excess that leads to short circuits.

Cut the sockets under the Teensy – right now the NAM DNMS set includes one 1×40 socket – you need to cut it into 2×14 using side cutters. Remember that not like goldpins, which can be divided “losslessly” you have to destroy one socket when cutting. That is, to cut 1×14 you cut the 15th socket. Then you can level the socket with a file if the appearance grosses you out, or you need to put another element right next to it on the contact. Here there is no such necessity, there is room on the PCB even for a “broken” socket, so a file in action only if you care about the final appearance of the board.

In the future, we will try to introduce ready-made 1×14 sockets, then it will be possible to solder directly, without trimming.

Having soldered sockets, you can use them to solder goldpins to Teensy – cut the goldpin to the appropriate length, insert it into the sockets and put Teensy on them. Now you can solder the whole thing.

Mounting

The soldering is basically done. The board has room for pull-up resistors up to 3.3V for I2C, but in a standard configuration there is no need to solder them, as well as additional capacitors on the power rail. In our tests, everything works fine without these components.

In the kit there is an XH connection cable – in the NAM we connect it to the I2C LCD socket. NAM versions 0.3.3 have the XH socket there too, so there is no polarity problem. If you have an older version that has a goldpin as the LCD connection – check the polarity carefully before connecting the cable and power supply.

Note! Using the NAM DNMS, you cannot have devices connected on the I2C 5V bus with I2C bus pull-up to 5V. If the device/sensor on I2C 5V does not have a built-in pull-up for the I2C bus, then you can also connect NAM DNMS. Teensy does not have 5V resistant IO pins, this could end up damaging Teensy. This applies to the NAM 0.3 – the NAM 0.4 beta has 3.3V and 5 I2C rail separation so there you can safely mix NAM DNMS and 5V devices/sensors.

For NAM 0.3 this is why we use the jack for the LCD, so that we do not try to connect the NAM DNMS and the LCD at the same time (AFAIR LCD module has pullups to 5V on I2C). For NAM 0.4 – use some 3.3V I2C connector.

Teensy 4.0 that you get in the kit is already programmed, you don’t need to do anything more with it. Connect the microphone to the PCB socket, screw the NAM DNMS to the spacers and you’re almost done.

How to mount the microphone? The kit includes a PG-16 cable gland. The diameter of the microphone tube is almost 13 mm so it fits perfectly.

The microphone is not truly omnidirectional (a detailed study of what its characteristics look like is planned), so the choice of direction is important. Due to the design of the NAM housing, there is not much choice. That leaves basically only the enclosure cover. 

For installation you need a conical drill bit, with which you drill a hole with a diameter of 22 mm. Be careful not to exceed this value strongly, the collar of the stuffing box is not too wide, I suspect that it will be quite easy to make the hole too large and thus unseal.

At the time of this writing, NAM DNMS is supported in the beta software version (NAMF-47rc7). If a stable version of at least NAMF-47 is not available at the time of installation – change the upgrade source to beta.

What’s next?

The whole project itself is still in beta, so there are still a few things to be resolved.

You need some kind of protection for the microphone. First – from the wind, so that its gusts do not skew the measurement. Second, from water. The microphone itself is waterproof (IP68), however, when it is exposed to heavy rain water can clog the hole in the PCB and the total readings are for a long time (until dry) strongly deviate from reality.

In the test equipment, I use a sponge from a small paint roller. For a short period of time, this is a good solution. The test microphone has been in use for more than half a year, exposed to the sun, and so far the sponge has not yet completely degraded. However, the process is already well advanced after such a long time, and some more “professional” solution will have to be found.

The diameter of the hole in the sponge is larger than 13mm so I made a band out of hot glue to make the sponge hold well.

In the current version there is no way to make adjustments to the readings (after all, such a sponge certainly dampens the noise) but there will be such a possibility in future versions. I also plan to measure how much this type of sponge dampens noise (as far as the readings need to be corrected).