Build a wall mount case for the air quality monitor

This is one suggestion for a low cost case for the air quality monitor for mounting to a wall and with an external power supply. This design uses a single piece of fluted plastic sheet, folded to protect the electronics and sensor from the weather, and providing wings that can be used for mounting holes to attach the monitor to a vertical surface.

This design is to hold a PMS5003 PM sensor, and a BME280 humidity and temperature sensor, and a DS3231 real time clock, and a NodeMCU ESP8266 Wifi board which are mounted on an inner panel, and holes are cut through this panel for the NodeMCU board connector pins. Some designed goals are:

  • The air quality monitor inlet and outlet open to the bottom of the case, to protect them from water and falling dust. They are also recessed slightly to offer some barrier to wind and protection from knocks.
  • The Wifi antenna is located as far away from the sensor as possible in this small case, and at the top front of the case to best avoid obstructing the RF signal.
  • The NodeMCU board power connector exits from the bottom of the case to allow a drip point below the case to keep water away from the connector. The connector also exits directly in line with the connect so that if it is pulled it will pull out of the connector rather than breaking it. The connector side of the NodeMCU board is held lightly so there is some give and take to avoid stress on this connector.
  • The NodeMCU board LEDs are front facing so they can be seen in the dark at least. The board buttons are also front facing, so they can at least be accessed with the front cover unwrapped.
  • The sensor and electronics are mounted on an internal panel to avoid mounting holes on exterior panels. The other panels wrap around this inner panel to keep water out and there are no holes in this outer layer. Even if water does run into an inner wall it will hopefully drip out the bottom of the case, and the inner mounting panel is the best protected.
  • The design attempts low cost and ease of construction and to support experimentation for variations. Components are held in place with cable ties. The fluted plastic is relatively easy to cut and make holes in, and this makes it easier for people to experiment with variations such as compartments for batteries etc. The fluted plastic is light and the white colour and fluted construction might provide some protection from the environment.

Assembled case

Cutting and folding the fluted plastic

The cuts and folds are presented below. The shaded areas are to be cut out. The solid lines are cut lines, and the dashed lines are fold lines. TODO a scaled drawing.

Flutes plastic cut and fold lines

Below is the cut and folded case:

Cut and folded case

Wiring

The design here uses a different cable for the I2C bus devices, one that better fits this case. The image below shows the four wires of the I2C, the two power lines which happen to be blue and orange, and the I2C clock and data lines. The connector with the pins is for the DS3231 real time clock.

The BME280 sensor is mount near the air inlet of the PMS5003, and has simply been taped to the PMS5003 in the example below. The pins visible on the right are simply to hold the connector in place for now. An alternative is to attach the BME280 connector to the side of the case with a cable tie as is done for the DS3231 below.

For the wiring of the PMS5003 please see the instructions for building an air quality monitor. The connector is shown below.

Attaching the NodeMCU board

The NodeMCU board is attached with a single small cable tie. This wraps over the Wifi antenna, and hopefully that does not interfere with the RF signal. It is suggested that it not be over tightened to avoid stressing the boards. The bottom edge of the board is not held to the panel because wrapping a cable tie around it would interfere with the buttons and would also limit movement of the USB connector which would probably it under stress and lead to an early failure of the connector.

Attaching the PMS5003 sensor

The PMS5003 sensor is attached to the inner panel with a single long cable tie. The end of the cable tie is located to avoid obstructing the case, and in the middle of the PMS5003 to avoid obstruction the air inlets and outlets.

Attaching the DS3231 real time clock

The DS3231 real time clock connector is attached to the inner side panel with a small cable tie. Some tape has been used here too, just to hold the connector together. This position is intended to give the clock module some ventilation and to make its temperature reading an good indicator of the temperature inside the case. The BME280 connector can alternatively be attached in a similar manner with a cable tie, rather than taping it to the PMS5003.

Attaching the connectors to the NodeMCU board

The images below show the pins of the NodeMCU board that the cables connect to. The PMS5003 RX cable is currently unused and is not connect and protected from shorting with a pin by some tape. In future it might be connect to one of the possible uart output pins and used to command the PMS5003 sensor to shutdown and restart periodically for power management.

The first image shows the I2C cable connections.

  • Blue: GND
  • Orange: +3.3 Volts
  • Green, I2C Date: NodeMCU pin D2. EPS8266 GPIO4.
  • Yellow, I2C Clock: NodeMCU pin D1. ESP8266 GPIO5.

The first and second images show the PMS5003 cable connections.

  • Purple: VIN, +5V
  • Orange: GND
  • Green, UART RX: NodeMCU pin D7. EPS8266 GPIO13.
  • Blue, UART TX: not connected.

Protective mesh

Here is one example of adding a mesh to try to keeping out modestly sized insects. For example, I have found a small cockroach inside a sensor that was reporting inconsistent large peaks over a number of days, and this mesh is fine enough to keep those out, and to keep out larger spiders and ants and most flying insects. In this suggestion the mesh is placed from the inner top, down the inner back and then folded over the bottom opening and taped to the front of the middle panel. When the case is folded up the pressure of the edges helps hold the mesh in place and seal it. A mesh might change the short term response of the sensor, but the opening has a large area relative to the sensor inlet so hopefully the particles in the air will still quickly diffuse across it.

Closing the case

The case has a single edge join to keep it closed, and a single cable tie can both hold this down and hold the back panel down too. Small holes can be made in the back panel to pass the cable tie through, and these should be close to the edge to about the tension bending the back panel. The USB power cable might also be held to the side of the case with this cable tie to provide strain relief and to create a drip point to keep water away from the connectors.

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