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abishur
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Solar Power for security cam

Mon Oct 13, 2014 6:56 pm

Hey guys, I'm setting up a security cam to sit in a window at my house. Ideally since this is a window above my front door with no electrical outlets nearby, I was hoping to use solar power to run everything. I've hit a snag though on making sure that I can appropriately charge and run everything.

My first hurdle is that I know that I'll need 1A @5v and .5A @ 12v for my system. The half amp at 12v coming off a 12v battery is pretty straight forward on considering charge times available, but I don't know that right formula for figuring out the current draw of the 5v power. I'm using a switch-mode DC-to-DC converter to drop the voltage from 12v DC to 5v DC. What's the formula I should use?
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jeanleflambeur
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Re: Solar Power for security cam

Mon Oct 13, 2014 8:46 pm

abishur wrote:Hey guys, I'm setting up a security cam to sit in a window at my house. Ideally since this is a window above my front door with no electrical outlets nearby, I was hoping to use solar power to run everything. I've hit a snag though on making sure that I can appropriately charge and run everything.

My first hurdle is that I know that I'll need 1A @5v and .5A @ 12v for my system. The half amp at 12v coming off a 12v battery is pretty straight forward on considering charge times available, but I don't know that right formula for figuring out the current draw of the 5v power. I'm using a switch-mode DC-to-DC converter to drop the voltage from 12v DC to 5v DC. What's the formula I should use?
Convert everything to power - that's your common currency: watts = amps * volts
So the PI needs 1A@5v => 5W of power. To find out how many amps you need at 12v do the reverse:
amps = watts / volts, so amps = 5 / 12 => amps = 0.42A

So on top of the 0.5A, you'll need 0.42A for the PI.

Note that the 1A is what the 12 -> 5V step-down converter will use. Based on the efficiency of the converter, the PI will always get less amps. For a switching supply it will get ~90% of 1A, so ~0.9A.

Also, I'm a software guy and I only have a limited, intuitive xp with electronics so I probably made a slight mistake somewhere in what I've said..

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abishur
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Re: Solar Power for security cam

Tue Oct 14, 2014 7:09 pm

Hmm... so if I understand that correctly my roughly calculated total power draw would be 1A @ 12v. So I would ideally want a slightly higher than 12 watt solar panel to power the pi and charge the battery for overnight usage.

Of course, the half amp at 12v is for an Infrared light, which wouldn't be used in the day time. So during solar hours, I would really only need a slightly higher than 5w (for running the Pi) solar panel to run the pi and charging the battery.
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Arne Baeyens
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Re: Solar Power for security cam

Tue Oct 14, 2014 7:57 pm

But don't forget that a 12W solar panel will deliver that 12Watts only with a blue sky and the right orientation - but it depends a bit on where you live in the USA.

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abishur
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Re: Solar Power for security cam

Thu Oct 16, 2014 4:02 pm

Well, lamentably, after looking around it appears that I would need to spend around 50 dollars just to get the solar panel, and then I would need to take into consideration where to place it, getting a battery large enough to last overnight (another 30 USD from a cursory search), getting waterproof containers for all of it. And in the end it's just not cost effective to go solar for just one of these.

Now if someday I decide to have 4 of these (1 at front door/ 1 at back door, and 2 for looking down the street) then it might make sense to have some fun with solar. Of course if anyone knows a good way to get some solar cells to get me the 15 Watts I need... well it would certainly still be interesting.
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BMS Doug
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Re: Solar Power for security cam

Fri Oct 17, 2014 2:13 pm

abishur wrote:Well, lamentably, after looking around it appears that I would need to spend around 50 dollars just to get the solar panel, and then I would need to take into consideration where to place it, getting a battery large enough to last overnight (another 30 USD from a cursory search), getting waterproof containers for all of it. And in the end it's just not cost effective to go solar for just one of these.

Now if someday I decide to have 4 of these (1 at front door/ 1 at back door, and 2 for looking down the street) then it might make sense to have some fun with solar. Of course if anyone knows a good way to get some solar cells to get me the 15 Watts I need... well it would certainly still be interesting.
I'm afraid a long drill-bit and length of cable is going to be the easier solution by far.
Doug.
Building Management Systems Engineer.

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