Monday, April 28, 2014

The case build (for the weather station), and installed in the field

Around the time I started to plan for the weather station case I came across a post on reddit that peaked my concern about heat management of the Raspberry Pi. Further reading on HackaDay confirmed it was a factor to consider. My RPi weather station is not stressed from a network or CPU perspective, but the summer heat on the sealed case could be show-stopper. From what I've read, 80ÂșC is the critical temp for the RPi. (Link also includes a shutdown script if critical temp is approached, something I may consider implementing.)

What follows is  overview of my case build dealing with both weatherproofing and heat management for the RPi in my remote garden weather station.

For starters, below is a quick review what is in the weather station (case), or external to it. Pictured below are the core components of the station: a remote and base-station. Both include a Raspberry Pi (RPi) with the RPi900 add on board hosting the DNT900 radio and antenna. (The WiFi pictured not used in the final system, only for testing.) The base-station for the system remains in my home, one of the units (the remote end) goes in the case installed at the garden ~2 miles from my home.


Aside from the RPi, the case also needs to house the electronics that interface with the RPi's GPIO. I discussed all the sensors in a prior post and bulk of the board is show below. What is missing from the image below are the sensors attached to the breadboard.



The RPi and breadboard above share a similar footprint, so I ended up picking the Otterbox 3500 for the final case  (better to Google this), it is not produced any longer but deals can be found on Ebay and even Amazon. The Otterbox by design is naturally waterproof. Although I planned to add multiple holes for sensors and power I felt it would be better to start with a waterproof case, then try to make a traditional case water-resistant.

Below is a rather boring picture, but what can be gleamed from it is (and the picture that follows it) is the relative size of the RPi in the case. You can also see standoffs I placed inside the case. The standoffs hold a copper plate slightly elevated from the back of the case to allow for other mounting items, and cables to be run under the plate.



Below is a RPi  in the case, on the copper plate (actually a bad/broken RPI, one I just use for test fittings), as to not rough up my good hardware).


The mount (to attached the case to a wooden pole) being added below. Back of case seen.


The inner attachments of the mount (The nuts do not protrude higher than the copper standoff plat.) Case is actually up-side down in this pick .


So back to the copper plate (mentioned previously.) At fist mention, that may have raised a few eye-brows. The backing copper plate is actually multi-function. One purpose is simple to ease installation mount everything to a plate, then just insert the plate inside the case. The more significant role is actually part of the somewhat elaborate cooling system. Anyone familiar with RPi has likely seen the heat-sinks (of questionable value) that get taped onto the RPi, often made from aluminum.

With my soon to be sealed/waterproof case, I anticipated that simply sticking on heat-sinks wouldn't allow the heat to escape the case , and a cooling fan would be impractical. (Size and power limitations of my solar installation.) My solution was to somehow get the heat-sinks to facilitate heat exiting the case. In comes the idea for a heat pipe solution used to cool laptop computers. My implementation is not a perfect heatpipe setup, but hopefully enough to get the job done.

I chose to go with a full copper system (for heat-transfer properties, and to simplify the soldering/adhesive needed to hold it all together. Below you can see the start of attaching 1 RPi heatsinks (copper) to copper heatpipe, and to the copper backplane. 2 more were done like this and later bent into place.



Next you can see the RPi mounted on the copper backplane (and the 3 heatpipes/heat-sinks not yet attached.)


Now a few pictures outside the case, this time withe heat-sinks in place (not yet attached) and the soon to be external heatpipe/sink that will help transfer heat outside the case.

The heatpipe/sink that will be external to the case is from a Toshiba laptop, used to cool a Tecra M3 (I believe.) Found with some heavy Ebay searching for something just right the right size and shape.



The following few pictures now show how a Dremel was used to create the access points to the case (and external heatsink).  The Dremel was perfect for the job. It spun fast enough to not only cut the case, but also slightly melt the plastic and make smooth access points.


External heat sink


View prior to attaching to backplane


Test fitting of backplane and external heatsink


Near final fitting, RPI in-place with heatsink.  Since soldering the external heatsink was not possible in the case, Thermal Adhesive (available from Amazon) was used. Thermal Adhesive was so convenient if I did it again, I might have chosen to use that to attach the heatsinks to the tubes connected to the backplane.  


Slightly out of order, but hear is a view of all the cut-outs (all part of case that faces bottom) for sensors and antenna feeds existing the case as well power inputs. (Largest hole is power, followed by air sensor, battery and soil sensors, then the smallest for the bulkhead SMA antenna connection. Flat hole is heatsink discussed above)

Ok, now is the unfortunate time in the post (and project) where I got very excited about how things were coming together and spent very little time taking pictures, and more time assembling the case itself. So here is a near final view (in outside on my deck being tested.)


An now with the case opened up. You can clearly see the the backplane and (white) thermal adhesive that attaches the external heatsink. What is not obvious is I added some additional spacers between the RPi and the RPi900 radio to increase the air-space around the RPi. The heattubs also don't block the camera, ethernet port or SD card incase access to those items are ever needed. The RPi900 can remotely control power to the RPi but I chose to also add a switch inline. The GPIO headers extend through the RPi900 and the GPIO extension cable sneaks under the backplane before connecting to the breadboard. 1 air temp sensor remains in the case, 3 extend out. (1 for actual weather monitoring, 1 for battery case temp, and one for soil temp) PG


Here is the whole system in a deck test. (Solar, battery and weather station) Even had some rain and freezing weather to test it out.


Fast forwarding abit, here it is installed at the garden. You can see I have added a camera to the case, painted the case white (to reflect heat), as well as added another standalone heatsink to the DNT900 radio.


A nice clear shot of the bottom of the case


A view from below


Looking right at the case and into the camera that peeks out


Just a nice shot of the solar panel and case


The whole setup in the field.


The project is nearly complete, and sensor data can be seen online at http://mygardenspy.com/ (Unless it happens to be offline for maintenance). A few posts will follow on the subject to include more lessons learned, problems encountered (and worked through), and hopefully a single post summarizing the project.

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