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Solar powered wireless sensor

A solar powered grid of sensors, with supercapacitor storage to run 24/7, for >100 years?

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They say when your infrastructure outlasts your civilization, you overbuilt. I say, it is time to bring back infrastructure that lasts. This project is a solar powered wireless sensor with supercapacitor energy storage (read >1 million charge/discharge cycles). Board includes magnetometer to measure when (not if) the magnetic pole flips. The goal would be to deploy a mesh of these all over the planet and just leave them for future generations to discover and use. Wireless transmission is done over ISM at 2.4 GHz. Accessories can be added to the 4 load switch ports (such as a GPS).

Here is the sensor in the waterproof enclosure:

Here is the sensor outside of the enclosure:

I 3D printed the solar panel holders and secured the panels to a piece of plastic from Home Depot.

Here is the backside of the solar array, with the custom made PCBs.

Why did I make a bunch of PCBs instead of one?  It is easier to make the PCBs and test them individually, and the JST connectors don't add much to the cost.  Plus, then the different parts can get reused later.

The input from the solar array can vary in voltage depending on the sun.  The solar array is fed into a small conditioning circuit (schottky diode, capacitor, and zener diode) to aid and protect the circuits downstream (zener diode is 5.6 V, 3W).

Next, the power is fed to the supercapacitor charger to charge up a couple of 50 F supercaps.

The supercapacitors can supply a tremendous amount of current, so the output is fused with a PTC fuse.

The output of the supercapacitors is fed to an energy harvesting DC/DC converter.  This way, the supercapacitor voltage can vary from ~0.25 V to 5V, and the DC/DC converter will output 5V the whole time.  Basically, you get the whole energy storage potential of the capacitors!  Here is the DC/DC converter:

The stable 5V is sent to the ATMEGA256RFR2 board.  The board has a SMD chip antenna, and a LSM303 (accelerometer and magnetometer).  To power different accessories, there are 4 load switches that can enable/disable the 5V.  There is also a UART multiplexer since the ATMEGA256RFR2 only has two UART ports.

One accessory I made was a GPS using the SAM-M8Q-0-10.

The ATMEGA256RFR2 was coded up using AtmelStudio.  Every few seconds the watchdog timer wakes up the microcontroller.  The magnetometer and accelerometer are read.  The supercapacitor voltage is measured.  If the voltage is really good, then the GPS is turned on and the location/UTC time is captured.  If the supercapacitor voltage is good enough, then the ISM Tx is powered up, and the latest data is sent to the base station.

The base station is just another, simpler ATMEGA256RFR2 board that I made for testing.

  • 1 × ATMEGA256RFR2 Microprocessors, Microcontrollers, DSPs / ARM, RISC-Based Microcontrollers
  • 1 × LSM303AGRTR Accelerometer, magnetometer
  • 1 × 1SMB5919BT3G Discrete Semiconductors / Diodes and Rectifiers, Zener Diode
  • 1 × SAM-M8Q-0-10 RF receiver, GNSS
  • 1 × 313070004 0.5 W solar panel

View all 11 components

  • Data data data!!!

    sciencedude199006/11/2019 at 16:40 0 comments

    Here is some project data.  I started taking data from the base station at 16:18 EST, and stopped at 11:54 EST (basically, to show what happens when the sun sets).

    Here is the supercapacitor stack voltage versus time.  After the sun starts to go down the voltage starts going down too (the GPS is still periodically activated).  Then, once the voltage dips below 3.4 V, then just the low power accel/magnetometer is active along with the ISM radio, so the voltage on the stack decreases more slowly.  It was cloudy this morning, so the charging of the capacitor was slower until the clouds moved off.

    Here is the temperature as reported by the ATMEGA256RFR2.

    Here is the magnetometer data (no pole flip, yet...).  Data looks somewhat correlated to the temperature for the X (black) and Y (blue) reading (temperature compensation is on for register 0x60), but not for Z (red).

    Here is the accelerometer data:

    Lastly, here is the GPS reported UTC time.

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sciencedude1990 wrote 06/27/2019 at 01:26 point

if there is any aspect of the design that you want more details about, please let me know.  This way, I can prioritize the log entries and possible YouTube videos.

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Jan wrote 06/27/2019 at 08:47 point

Yeah, everything related to your circuits used :) Really like your project!

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sciencedude1990 wrote 06/28/2019 at 13:39 point

ok, I’ll start working on some descriptions and YouTube videos.

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Jan wrote 06/26/2019 at 12:40 point

Here's an interesting paper on super-cap lifetime: https://hal.archives-ouvertes.fr/hal-00947824/document you should take into account using numbers like years of lifetime :) Another interesting read is the wikipedia page: https://en.wikipedia.org/wiki/Supercapacitor#Lifetime

Especially hot temperatures (very likely in your design, heh!) kills them super quickly...

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sciencedude1990 wrote 06/27/2019 at 00:14 point

thanks for the link.  My application is low current and not too sensitive to a loss in capacitance.  So even if I lost 50% of the capacitance, I’m okay with that.  As for the temperature, I’ll monitor over a super hot day with direct sunlight and post a log.  We’ve had a cold summer so far up here.

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RasmusB wrote 06/26/2019 at 11:52 point

Nice build! 

With so much dead volume in the enclosure and the fact that it is supposed to be directly exposed to sunlight, I would strongly recommend that you add a waterproof vent. This will keep moisture out over time. 

A watertight enclosure will suck water into itself when the pressure on the inside is negative (cooling down after a long day in the sun, where the air has expanded and escaped through the seals). Moisture can't get out one it has gotten in, and over time more moisture will collect on the inside. 

A vent removes the pressure differential, and lets the humidity in the box vary with temperature. This reduces the risk of having moisture condense on your PCBs.

This is one example of products that will do the job, but there are other (cheaper) manufacturers.

https://www.gore.com/products/categories/venting

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sciencedude1990 wrote 06/27/2019 at 00:08 point

you are absolutely correct - the enclosure is quite old and I did get little drops of water condensing in cold mornings even with packs of desiccant.  I think the best option would be a piece  of polycarbonate, and then pot the whole thing in a layer of resin, leaving an expansion chamber for the supercaps filled with dry air.

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sciencedude1990 wrote 06/11/2019 at 16:48 point

Thanks.  Sure, I'll throw my hat in the ring.

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Tom Nardi wrote 06/10/2019 at 17:53 point

Very nice. Is this something you're thinking of entering in to the Hackaday Prize this year?

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