Cheap solar battery + wireless IoT node

[cubeberg 540]

I've got a bunch of sensors in my house uploading data to my phant repository - but I decided to start looking into placing sensors outside as well.  

I purchased some $2 solar sensors from Target (Apparently they're frequently available from the dollar store as well - I'll check into that) in the hopes that I could use them to power an MSP430.  The ones I purchased only provide ~1.2v of power since they're single battery.  They're 200maH as well, but I think that will likely be plenty for my uses.  

 

Since one wouldn't do it - I used two in series.  At full power - that should be around 2.4v - perfect for an MSP430.  I added a diode between the two just in case, and clipped the LED to avoid unneeded battery usage.  Hooking directly into the battery means that I get power all the time - great since I didn't just want power at night.  Plus - the power output for the LED is apparently pulsed - probably not good for the MSP430.  

 

I've been watching the sensor values today - it dropped a bit overnight (very little to be honest - about 0.05v), but it's charging nicely right now.  I need to get a better container - but for now it's in a cheap tupperware container inside of a ziploc bag. Apparently the dollar store also sells locking containers with a gasket that would be perfect for this.  

 

 

Here is my analog.io graph for battery level.  I'm not calculating the actual voltage - but it's out of 255 - 255 being a 3v reference voltage (technically vcc/2 compared to 1.5v reference).  So 198 is ~2.33v.  

 

 

Next up - I've got some radioshack weather sensors (rain, wind speed & direction) that I'll be ripping the guts out of and replacing with MSP430 brains.  They'll get solar power as well. 

[spirilis 1,265]

I'm a little curious peoples' recommendations for cases, in terms of how well to "seal" them and whatnot.  My grill monitor is in a gasket-sealed case and one thing I've come to find is condensation makes its way in there - and has a lot of trouble coming out.  Seems to me it might make more sense to have an enclosure that is "breathable", yet the sensitive electronics shielded well enough from any liquid moisture infiltration.  That's a thought I'm still developing though.

[cubeberg 540]

@@spirilis - just thought of a simple solution - throw in a silica gel packet!  Any moisture that makes its way in should be taken care of pretty easily with that.

[spirilis 1,265]

@@spirilis - just thought of a simple solution - throw in a silica gel packet! Any moisture that makes its way in should be taken care of pretty easily with that.

I've had a few of those in there for a year now...

 

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[cubeberg 540]

I've had a few of those in there for a year now...

 

Sent from my Galaxy Note II with Tapatalk 4

Ouch - and you're still getting condensation?

[spirilis 1,265]

Ouch - and you're still getting condensation?

Yeah I don't think those things are meant to be permanent or even outdoor solutions.  For now I just deal... I may take the enclosure apart some day (it's kind've a pain with the thermocouple leads coming out via grommeted cable bung, that part at least is pretty well sealed IIRC) and see about drilling some tiny holes through it in certain places so it can "breathe" and see how that goes.  The main issue is when too much condensation sits under the top lid, it seems to block the nRF24L01+ signal (2.4GHz is particularly sensitive to water IIRC).

[enl 227]

The options are 1) a complete sealed setup, using the appropriate sealed penetration connectors, capable of holding against the pressure inside and out due to temp changes, 2) run the containment at a slight positive pressure,  3) ensure that moisture can get out before condensing, OR 4) pot that sucker up

 

I have been involved in option 1 a couple of times, and it is expensive to do well. An ammo can is a decent containment, as long as the gasket is good and the temp changes arn't too great. It will breathe under great enough temp change.

 

Option 2 is what communications providers often use on trunk lines. They may pressurize with nitrogen and seal it, or use a compressor and air drier. The second of these is why you see some of the little green lights on the boxes on telephone poles. Those are small compressor/driers, and pressure monitors. They alarm if pressure can't be maintained, and provide a small volume of dry air to make up miniscule leaks. I don't know what the pressure is, but I would guess a few inches of water (24 inches of water is about 1PSI; 1 inch of water is about 2mmHg, or about 2.6millibar. Residential gas is about 10 inches in the US)

 

Option 3 I have used a number of times. Enough ventilation is needed low and high that temperature changes will induce air to move, the electronics are protected from direct impingement, but will follow temp changes fairly quickly so as to not get condensation, and the electronics want to be the warmest thing in the containment. One technique I have used a double roof model: slight overhang with vent at the top, open bottom, like a small garden shed, and the electronics in a box inside with bottom vents, but no top vents. Drip loops on wires entering are a must. Direct solar exposure to the containment is a no-no, as that is asking for after a rain, at the most humid time, the electronics lagging behind the dew point and inducing condensation.

 

Option 4 is common in automotive and ruggedized gear. Pot it solid. Gotta do it right so moisture can't get in where leads enter the potting. Can be done with wax, silicone, some petro-based rubbers and plastics, or many other materials. The antique auto guys still use pitch for some things, both for insulation and moisture protection. Goes in hot, so moisture is removed and can't get back.

 

One other option, used by the tool and gun guys, is not appropriate here, due to practical requirements: a small heater. Raises the dew point to prevent condensation, but takes a bit of power as well as running the electronics hotter than needed.

 

I would use option 3 or 4. Even a low power device, in a small enclosure, running at a few milliwatts, will produce enough heat to keep the containment a degree or two above ambient. As long as no liquid can blow in, and the space around it is generally below the dew point, there isn't an issue, as the electronics will usually be at or above the temp of the surround except during the most extreme rises.

[enl 227]

One other point: Many materials we think of as 'waterproof' are not. Many common polymers, such as styrene plastics and polyethylene films, will allow water molecules to diffuse through, over time leading to issues even without an actual leak. This isn't a short term issue, such as when using Saran wrap to keep leftovers fresh, but over longer periods, it is an issue. Tupperware, plastic boxes sealed with gaskets, etc, are not good long term environmental protection on their own for this reason.

 

Not my specialty, by a long shot, but some of the jobs I have been involved with got me educated enough to know my ignorance. It is AMAZING what is done on oceangoing vessels to protect electronics, and what isn't.

[spirilis 1,265]

Hmm, I wonder if that last point is part of my problem.

 

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[zeke 693]

Another option would be a gore membrane strategically placed on the enclosure.

 

It will allow the inside pressure match the outside pressure while preventing moisture from crossing the membrane.

[enl 227]

@@zeke Short term, yup. I think over long term, some moisture will diffuse through as long as the concentration of water vapour is lower inside the enclosure. It is amazing where water molecules can get, and what they can get through.

[spirilis 1,265]

Yeah, I think gore tex basically allows water vapor while repelling liquid moisture- but that still means you can have condensation inside. However if the rest of the enclosure is breathable then maybe less of an issue, this is basically an extra shield on option #3 above.

 

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[Lgbeno 189]

@@cubeberg great project!  Like we talked about, sensors are always multiplying!  You may consider a low Iq boost converter if you wanted to get down to a single battery but the 2x panels in series is probably a cheaper solution and more power!  Email me your address and I'll send you a couple Si7020 breakouts.  https://www.tindie.com/products/ImpGuru/si7020-temperature-and-humidity-breakout/

 

You can add humidity to your repertoire and possible get more resolution on temperature.  I have the softi2c code for energia and G2553 as well. 

 

I'm a little curious peoples' recommendations for cases, in terms of how well to "seal" them and whatnot.  My grill monitor is in a gasket-sealed case and one thing I've come to find is condensation makes its way in there - and has a lot of trouble coming out.  Seems to me it might make more sense to have an enclosure that is "breathable", yet the sensitive electronics shielded well enough from any liquid moisture infiltration.  That's a thought I'm still developing though.

 

everyone beat me to it but these vents are out there.  No idea where to buy them.

 

 http://www.gore.com/en_xx/products/venting/protective/protective_vents_why_vent.html?isAjax=true?xcmp=idh_google_PTV_general_US&gclid=Cj0KEQjw3M6oBRDnnIywo5i287ABEiQAXRm7S3VXeRKux71jsJLQpgieXZ7I8WBFvU-cS7Vr78lc5d0aAuSk8P8HAQ

 

Otherwise, I'd say that you would need to do an air tight seal in a very dry enviroment.  I like the idea of silca desiccant too, there's always a surplus of those from my digikey orders.

[cubeberg 540]

Interestingly enough - I checked my sensor yesterday - there was water in the zip-loc bag

 

Thanks for all of the feedback on housing, etc. guys!  I'll keep it all in mind - I'm expecting at least 3 outdoor sensors (wind speed + direction, rain, light + temp + humidity) - so taking care of them will be important.  I did see some 3D-printed Stevenson screens mentioned in weather sensor projects for humidity/temp sensors that needed to be exposed, but not directly to elements.  That will likely be a necessary part of the project. 

 

Luke - I'll email you  I'm using the core cpu temperature measurement from the msp430 + calibration - but it's horribly inaccurate (not surprising).  Better sensors would be great!

I know TI has a new chip they were demoing for battery-less sensors - I might look into that as well.  Can't beat $4 though - or $2 if they're available at the $ store

 

Once I've ordered more custom sensor boards (instead of an LP chucked into a tupperware container) - potting seems like a cheap solution.  

 

Today is rainy and overcast - it'll be interesting to see how well the battery holds up.  Considering usage yesterday - I honestly expect it could last a couple of days without charging.  

[cubeberg 540]

So my issue right now - the max charge daily keeps going up!  Slowly - and it's decently below 3v at this point (2.52v today) - but it keeps going up daily (202, 208, 213, 215).  I may have to add some code in to turn on an LED until the value falls below a threshold.