Earth Notes: On IoT Leaf Enclosure Sizing (2015)

Updated 2022-09-28.
What size and shape do Internet-of-Things enclosures/boxes need to be, for IoT Launchpad? #IoT #boxes


(Part of WP1 Research, D13 Enclosures and Connectors. To find out what boxes to use (research and selection of preferred item/design) and the internal connectors (select the physical connector and define pin out).)

For IoT to be successful at least some (I suspect most) of its deployments must be small, cheap and fit-and-forget.

For deployment into bus shelters for the IoT Launchpad project (from April 2015) there are a number of voids in a modern shelter into which an 'IoT' sensor unit could be conveniently deployed and potentially 'see' the people using the shelter/stop for footfall monitoring. (Cost and power are the subject of thinking elsewhere.)

The OpenTRV REV2 valve-control boards of which getting on for 100 have been made and which seem to represent a reasonable form factor for home use, are 5cm by 5cm PCBs, with components on both sides of the board, SMD and through-the-hole (TTH).

Early thinking on how we might stack daughter/shield sensor boards lead to the notion of the 14-pin I2CEXT connector and board, which also supports in-circuit programming of the AVR, and which we have used on the REV7 production-engineered all-in-one valve PCB.

A reasonable starting assumption (given OpenTRV REV2 and REV4 boards) is that the main board housing the MCU (MicroController Unit) is 50mm x 50mm x 15mm with each 'shield' adding another 15mm of height; note that that implies a minimum height of 60mm if power, MCU, radio and sensor boards are all separate (not integrated).

Though less good from an electrical point of view, for vertically-constrained spaces such as within a timetable or similar information panel, it may be possible to place one daughter board beside the main board, ie in the same horizontal plane, for an overall 50mm x 100mm x 15mm. Or have long thin boards say (50mm x 25mm) stacked side-by-side lengthways (less good for SPI and I2C, but maybe better for poking into narrow spaces) or sideways (good for SPI/I2C/power) to keep the vertical height ≤15mm.

Note that to get a good 868MHz quarter-wave PCB antenna the radio board might optimally have a 'long edge' more than 62mm long (to avoid putting in an L-bend, though that need not be a show-stopper) and physically separated from anything else, eg at one end of the board stack or another, else a whip/wire antenna may remain a good choice.

Note that Arduino shields are 10.5mm--15mm vertically, dictated in part by the stacking header pin length, so the vertical 15mm suggested above is possibly erring on the generous.

For reference, two AA cells (eg NiMH rechargeables) in a side-by-size pack take about 32mm x 60mm x 17mm.

Existing Low-power Modular Designs

Some existing pluggable low-power (microwatt or milliwatt capable) designs in this space, either fully modular or with well-understood 'shield' designs to stack extension boards, and well under (say) USD50 per board:

plus of course the Arduino family.

The Microduino looks very interesting and we will order some to evaluate (2015-05-30).

We will look at the TinyDuino and its shield design further.

Bus Stops

Looking at the London bus stop/shelter possibilities for the IoT Launchpad project there are ~19k bus stops in London of which ~13k have shelters (and ~1000 of those are managed by local authorities rather than centrally). The current design for new shelters being deployed is the London Landmark, which is likely in many of the places that we would want to do our ~50 trial deployments mainly in and around Shoreditch.

Note also that the are stops that have no shelters, just 'flags', at which we may try to deploy units possibly on top of the poles, or in the timetable displays, or even conceivably in an Array of Things style unit!

Assuming initially that we work to something like the REV2 OpenTRV PCB size (5cm by 5cm) and allow stacking of some sensor boards on a variant of the IC2EXT connector/board, and that all of our sensors stay within whatever enclosure we use, and that we are battery powered for these deployments, then some plausible (waterproof) enclosures might be:

  • IP67 80x73x53mm (possibly 45mm internal vertically, ie base board + 2 or 3 shields)
  • IP67 106x80x53mm (possibly 45mm internal vertically, ie base board + 2 or 3 shields)
  • IP67 139x80x53mm (possibly 45mm internal vertically, ie base board + 2 or 3 shields)

(Waterproof because apparently the shelter voids can often fill with rain!)

Attaching the boxes to the shelter structure might be as easy as with self-adhesive hook-and-loop tape, implying (say) an extra 5mm or so on one axis to still fit into a void at most, depending on void shape. Easy non-destructive installation and removal is valuable.

Note that for observing footfall there are a number of issues to bear in mind:

  • RF backhaul from the unit must not be blocked eg by unit being totally enclosed in metal structure (Faraday cage).
  • Location of unit needs to have putative passengers in sensors' range, eg within cm for proximity, ~2m line of sight for PIR for areas of interest, maybe ~2m for sensors such as passive Bluetooth/3G/voice detection, maybe ~2m for ultrasound/microwave Doppler/reflection, maybe ~2m directly above seating for body heat sensing.
  • Some sensors will not work through particular materials well or at all, such as directional RF through metal sheeting.

Related constraints apply to environmental monitoring such as:

  • Needing free airflow for PM (particulate) sensors.
  • Needing shade for environmental temperature sensors.
  • Needing exposure to direct sunlight for external sunshine recording (and any PV to power sensor units).

A number of locations may work for different sensor combinations and target users (eg those around the stop versus people actually in the shelter) for footfall/presence:

  • In vertical panels (eg behind maps, no metal in way): UWB radar, Bluetooth, 3G, voice/sound, Doppler, ultrasound.
  • Above seat (enclosed in roof void, no penetrations, no metal in way): UWB radar, Bluetooth, 3G, voice/sound, Doppler
  • In seat: proximity (eg IR reflective or capacitive, per seat), UWB radar, Bluetooth, 3G, voice/sound, light (obstructed in vertical cone), Doppler, ultrasound.
  • In end caps, eg to detect people looking at end-panel ads (no penetrations): UWB radar, Bluetooth, 3G, voice/sound, Doppler, ultrasound.

Environmental monitoring, eg particulates, NOX, temperature, sound pressure, may work best on the top of a shelter or flag, with air flow, natural or forced. (Forced implies fans and much-increased power consumption.)

Some locations, eg higher from the ground, may experience better (more reliable, lower-power) radio comms for backhaul.

Building Health

Bruno's (EnergyDeck CTO) 2015-06-07 D13 note:

Research on physical enclosures and connectors for metering equipment. This
is based on the same use cases as D11.

## General ##

General requirements:
- High gain antenna connector to make it possible to plug in an external
  antenna in areas where wireless communication is difficult.
- Programming connector (optional): USB or similar connector to allow the
  device to be re-programmed on site. If present, this connector should be
  hidden behind a flap that is closed with a screw to ensured that it is
  only accessed by people who know what they are doing.
- Mains power: in general, it is better if the devices work on battery and
  do not needs mains power. This could be provided as an option but is not

## Office ##

General requirements:
- No special treatment,
- Should be relatively unobtrusive and neutral in design and colour,
- Wall mounting option is useful,
- Higher quality enclosure if device is supposed to be on desk top.

## Hotel and social housing ##

General requirements:
- No external connectors,
- Simple neutral enclosure in sturdy material,
- External aspect of enclosure is important: it does not need to be high-tech
  but should be reasonably appealing to the eye (see Switchee enclosure),
- Size of enclosure is important too: it shouldn't be too bulky,
- Wall mounting essential,
- Protection against accidental object intrusion: e.g. light sensors need to
  be protected while able to receive light,
- Battery compartment needs additional protection against access, e.g. single
  screw in addition to keep lid in place (see standards for young children's

Additional requirement for bathrooms:
- Weather sealed (does not need to be completely waterproof but should be able
  to withstand high humidity and light splashing)

## Industrial ##

General requirements:
- No external connectors,
- Ruggedised enclosure,
- Wall mounting essential.

Shape and size can vary so both the stacked shield design or the flat design may be appropriate depending on use case. Wall mounted are probably better as a larger flat design to avoid protruding off the wall too much.


Initially we will probably work with a derivation of OpenTRV's 50mm x 50mm board profile starting with the OpenTRV V0p2 REV2 or REV4 design, and the I2CEXT outline sensor/programming 'shield' connector, using vertical stacking to keep I2C/SPI lines short. However, switching to an (open) third-party base design, or mixing deployments with third-party designs, is on the cards if the most cost-effective way to achieve target results. In particular we will look further at the Microduino and TinyDuino further.

It might be useful to keep power and radio on separate connectors, eg to avoid it being possible to accidentally add more than one of them and more than one MCU. If so, then the XBee pinout may be of interest for a non-stackable radio module slot.

We have identified a preliminary possible set of indoors (R30-3217, R30-3226) and outdoors (see below) cases/enclosures, by default resistant to water ingress for outdoor/exterior implementations, though that seal may have to be forgone in order to monitor sound, RH%, gases, etc.

Box size 1: Evatron DE01S-A-TG-0	80 x 73 x 53	Transparent	Grey	R30-5000
Box size 2: Evatron DE02S-A-TG-0	106 x 80 x 53	Transparent	Grey	R30-5004
Box size 3: Evatron DE03S-A-TG-0	139 x 80 x 53	Transparent	Grey	R30-5009
Box size 4: Hammond 1555HF17GY	ABS	180	120	37	IP67	UL94-HB	R30-5963
Box size 5: HAMMOND  1551KGY  CASE, ABS, GREY, 80X40X20MM Note: IP54 only F3536464
Box size 6: HAMMOND  1551LGY  CASE, ABS, GREY, 80X40X15MM Note: IP54 only F1877312
Note: R Rapid Online, F Farnell
Standard attachment: industrial hook and loop

2015/09/29: boxes 3 and 4 were physically tested in preferred voids. Results:

  • Box 3 fits in timetable void, but sealed bag suggested as alternative.
  • In LL seat height clearance only 25mm and red seat not translucent; operator prepared to make slots in metal tray base if helpful for light detection/occlusion by passengers.
  • LL roof and eaves panels can accommodate boxes 3 and 4, fastened (hook-and-eye) to GRP roof or to metal panel for possibly improved sound transmission.
  • LL back panel above power entry (CU has 2 spare ways) has 6mm ply back panel to which box 3 or 4 can be fastened with self-tapping screws, and in some stops polycarbonate front panel to which boxes could also be fixed and which may be less included to Friday-evening-drunken-thump damage.