I would like to power the arduino externally from a 7-25V DC source and at the same time have a backup in the form of a battery with solar charging and charging from a DC source.
At the same time, I want to be able to choose whether I want to go from a DC source or from a solar-charged battery or use both.
I'm looking for a wiring diagram, but I can't find anything at all. Can you help?
“I want . . .”
Creat a block diagram.
I would like to power the arduino externally from a 7-25V DC source
That is a challenge in itself; due to the wide voltage range eg a 7805 can handle an input range of 10 - 35V.
Dropping 25V to 5V with a linear regulator you would waste a LOT of power.
A buck converter would do the job - such as this
Input voltage 4.75~40V, Adjustable output: 0.9~30V
Long time maximum output current: 0.8A(12V to 5V)
Short time maximum output current: 1A(12V to 5V)
Quiescent current: about 1.5MA
Working frequency: 550KHz.
Efficiency is 75-90% .
That seems a lot of quiescent current - perhaps its a misprint? 
I'd suggest you taclke 1 stage of the project at a time.
cevepe:
I would like to power the arduino externally from a 7-25V DC source and at the same time have a backup in the form of a battery with solar charging and charging from a DC source.
What is producing the 7-25v power?
How many watts do you need at maximum consumption and how many watt-hours do you need over a 24 hour period?
I can understand the desirability of a battery backup for occasional power outages but the idea of solar charging suggests that you expect long outages lasting several days.
Without a good description of the overall project and the constraints in which it must work it is difficult to give useful advice.
...R
Feel free to PM me I looooove me some good solar projects. The way I do it is take a 5v solar cell attached to an 18650 charge controller module and a couple of 18650 batteries. Some projects that I have out in the woods have 20 18650 cells as main power. Then you take a step up to the voltage you need. Stepping down burns a lot of wasted energy as mentioned. Stepping up is the way to go. I like to solar charge onto 1 cell batteries and not into 3 cell for a 12 volt battery unless you're using lead acid. Those batteries are heavy but if this thing your building is close to home or you can use lead acid then go with that and a 12v solar panel but that sounds like overkill for an arduino board. anyway let me know if you want a hand.
Stepping up and stepping down has similar losses - assuming you use a buck converter for the step down and a boost converter for the step up.
When working with microcontrollers chances are that you can use the voltage of the LiPo battery directly, without change. That's the most efficient.
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