Enabling Communications, Anywhere, Anytime: January 2017

Monday, January 30, 2017

Looking for ideas for cable and connector parts

I'm currently looking at what to use to build the power cables for the Mesh Extenders.

As previously posted, one end has a D-SUB 25 socket. The other end needs to have connectors for:

a solar panel and battery (4 pins),
USB socket (to allow charging of phones from Mesh Extender, where power budget allows),
12/24v automotive power

and it also needs to have a little serial EPROM in the cable, so that the cable can tell the Mesh Extender which radio regulatory region it is in.

All of this needs to be fairly weather proof, so that it can ideally last for at least a year in tropical maritime or outback conditions.

It also needs to be as cheap as possible.

I'm looking for ideas for all parts of the cable, to make the cables as fast and cheap to build as possible, while maintaining adequate performance.

My current thinking is leaning towards:

Using half of a http://au.element14.com/videk/1125/lead-rs232-25w-d-skt-skt-5m/dp/1525869 for the D-SUB 25 end, then putting some currently unknown small sealed junction box on the end of that, and having the solar/battery, USB and automotive power plugs (not all may be fitted on all cables) come out the other end of the junction box.

Those D-SUB connectors have the problem that they are not UV stabilised or IP rated, however. Are there good simple coatings (like would spraying them with Armor-all be enough) for UV protection? Would the connector, sitting inside a lip at at the bottom of the Mesh Extender exclude dust and water adequeately (we can at least test that once we have complete hardware units on hand)? Or does anyone know an affordable source of similar cables that are UV stabilised, and ideally, IP66 or similar rated?

For the solar/battery connector, perhaps something like http://www.banggood.com/10Pairs-DC-MaleFemale-4PIN-24AWG-Waterproof-IP65-PVC-LED-Connectors-p-1073265.html?rmmds=buy, which are outrageously cheap, and already come with tails fitted, so soldering them up on a tiny PCB in a junction box would be easy. They claim to be UV resistant and IP65 rated, although I must confess I have some suspicions given their very low cost.

I currently have no decent leads on a small junction box that can take a fat cable in one end, and some skinny ones out the other, sealing everything in the middle. Something like the things that Telstra use to join phone cables in pits would be a potential solution (not Scotch-locs, but the bigger things). But I don't know where to source those from (yet).

Friday, January 27, 2017

Mesh Extender 2.0 Exterior and Interior drawings

Just a quick post with some images that have been generated so far during the design process, to give some idea of how the thing will fit together. The exact dimensions and details are likely to have changed before we produce physical units, but give a clear idea on the concept.

First, from the front:

Here you can see the three antenna mounts: 2 for the UHF packet radio, and 1 for the WiFi. The other two bumps are screw bosses, so that you can make your own shade for hot locations to help keep the innards below 70C. We have included this, because the thermal budget is just not quite there for full-sun tropical locations, while sticking to only passive cooling, and with everything in an environmentally sealed case (which also traps heat). Adding some shade fixes this problem.

We are not at this stage designing a shade, as we figure that they can be easily made by users from whatever they have laying around, for example, an old plastic bucket, waxed cardboard box, or even a drink can split open.

From behind:

The main thing to see here is the curved back to make it easier to strap to a tree or pole, and the ribbing to help provide at least a little air-flow between the unit and whatever it is attached to.

From below:

Here you can see the D-SUB 15 utility/IoT and D-SUB 25 power/radio connectors. Also, below those you can see a couple of round markings which are drill guides in case you want to get ethernet out. In their default state, they have gortex moisture seals to allow the unit to maintain equal pressure with the outside, without filling with condensation.

The inside of the top:

Here the main feature to see is the notch and paddle to help hold the PCB firmly in place, to minimise the effects of vibration, e.g., if a unit is mounted on a vehicle.

The PCB itself will sit inside, with fly-leads to the antennae, as shown in the remaining images. Returning to the thermal properties, the main means for heat to exit from the unit is conductance through the antennae connectors. Not ideal, but when you are making a sealed unit that you don't want to cost thousands of dollars per piece, your options can be rather limited.

Wednesday, January 18, 2017

Testing Mesh Extender 2.0 Build Process with Farouk from INRIA

This week Farouk from INRIA in Lille, France has been visiting, as we discuss the potential for collaboration. It looks like what we are doing with Serval and the Mesh Extenders at the moment will be a good fit for the needs of a project that they have at the moment. As a result, we have spent a couple of days making sure that Farouk can replicate our build process, which also gives us confidence for others building firmware images as well, and transferring as much of the necessary know-how as possible before he heads back to France, and then in a just a couple more days, that I head back to Australia.

In this process, we are also trying to document all of the outstanding software and hardware issues for the Mesh Extenders at http://github.com/servalproject/openwrt, so that we can better coordinate our activities, and also hopefully make it easier for others to contribute or replicate our work.

Tuesday, January 17, 2017

Testing the Super-Capacitor

We have included a 1F 5.5V super-capacitor on the new Mesh Extender board.

The idea of this is so that when power is lost, the unit has a couple of seconds to stop writing to the microSD card, and perhaps tidy things up to ensure a clean boot next time.

However, we haven't managed to get it to work properly yet.

The boards have a 20 Ohm resister in-line with the super-cap, so that there is no unmanaged in-rush current when charging the capacitor from empty.

The capacitor takes a few minutes to fully charge to about 4.8V (5V - the voltage of the zener diode that forms part of the power loss detection circuit).

At this point, it should be possible to then remove the power source, and have the unit operate powered solely by the super-capacitor, until its voltage level drops below about 3.3V.

GPIO24 is the line that reads our input-power-available signal (effectively the input power before passing through a zener diode, after which it charges both the super-capacitor, as well as powering the rest of the board. To test the system, I configured that GPIO line for input, and had a little shell script that sits in a loop reading from it.

root@OpenWrt:/# cd /sys/class/gpio

root@OpenWrt:/sys/class/gpio# echo 24 > export

root@OpenWrt:/sys/class/gpio# cd gpio24

root@OpenWrt:/sys/devices/virtual/gpio/gpio24# while [ true ]; do cat value; don

...

It reports 1 when there is an input power supply (other than USB, which is another little problem we need to solve), and 0 when there is no input power supply, but is running on the super-capacitor. At least that is the theory.

However, with the 20 Ohm resister, the voltage on the low-side is only about 2.2V, much too low to be useful. Thus I tried putting a 10 Ohm resister in parallel, dropping the combined resistance to about 6.7 Ohms. That still wasn't enough.

Then I tried bridging the resister by using a multi-meter in current measuring mode after having already charged the capacitor. This gives effectively 0 Ohms, and provides an upper bound of what is possible with this super-capacitor. With that configuration, things were more promising, as I saw u-boot try to boot, but as can be seen, as soon as it tried using the DDR RAM, it would reset, as presumably the DDR RAM draws too much power for the super-cap to sustain the supply above 3.3V:

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* U-Boot 1.1.4 (Jun 6 2015) *

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