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Arduino on Battery: Solar Power for $1
#1
Here is an idea I got when browsing garden section. The solar powered garden lights are becoming commodity, got the one below for $0.99 in Walmart. It is a complete rain-proof system with 50-200mW solar panel, rechargeable battery, charging circuit and white power LED. You may even use up empty space inside if available to house your sensor, hence free enclosure. The one below has only 2/3 150mAh AA, larger ones have full size AA NiCD rechargeable. Btw. 150mAh is very weak, basically CR2032 has more capacity, so it's probably an inferior battery with much smaller cell inside. It only needs to power your project for 1 day before it's recharged again so 150mAh should be usable.

I hooked up the LED onto oscilloscope and turns out the output was not DC but 3.5V 460kHz AC signal.

Another problem with using this is that output is off during the day. The internal circuitry uses solar power output to determine day/night.

Following circuit addresses both issues:

   


The AC signal is rectified and filtered by capacitor C.

During the day, when the output would never come on, we need to periodically shut solar panel off to simulate dark - this will enable output and charge capacitor C. When sufficiently charged, the Arduino will need to enable the solar again.

Approximate value of capacitor C: C = I * t / dU where

C: capacitor in [uF]
I: average current by Arduino in [mA], say 10mA
t: periode of recharging the capacitor in [ms], for example 50ms
dU: maximum voltage drop in [V], for example 0.5V

For 10mA circuit and 50ms recharge period and no more than 0.5V ripple you will need 1000uF. But because the battery has only 1.2V 150mAh and we are taking 3.5V 10mA with efficiency around 80%, we can only get less than 4hrs dark runtime from this.

If you do not need to run constantly (e.g. a sensor), you can go to 5uA sleep with WDT enabled and wake up periodically every 1sec to check the sensor and recharge capacitor during the day. The value of capacitor 10uF should be sufficient for 5uA. The software must make sure capacitor is fully charged (by measuring own voltage) before going back to sleep.


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#2
Hi Roman,
I so enjoy when you write posts like this. It is such great food for thought; makes me want to build and test this for myself.
Maybe I will next fall...still have to finish whats on the bench 1st. I have some ideas that I would like to develop further and they will need to have solar and low power microcontroller.
Thanks,
Bob D
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#3
Thanks Bob, appreciate the shoulder pat.

I used to have a notebook where I wrote ideas like this one. Cannot find that notebook, so making my notes here, they say once published online = never forgotten. Next time anyone needs this, it is right here.
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#4
I agree, and I am happy that I have started to put some of my projects on line as well. I also like to read the back story that leads to an idea being developed into a working project. I still have a bunch of ideas from 2007 that I wrote up and sent by registered mail to myself. There are a few that have been made reality by others ( inevitable ) , but there are others that maybe one day... Rolleyes
Bob D
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