Charging battery with sonar power, what modules?

Projects General Guidance
1

Solar powered projects, you read a lot about them but still it is difficult to get signal running.

First of all I know this is not optimal. The are many better and many more expensive options. This is what I have and I wonder if we can make something work with this.

I have my solar charger:

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This will charge my battery. A 18650.

And now I want to run my arduino from this battery. And here is where the problems start.
It is easy to convert the battery voltage to 5v with one of these:

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Works well, led turns off at 2.7v but it has no low voltage cutoffs. So, only this is not enough, it might kill the battery.

So I tried this one:

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Works like a charm. Has low voltage cutoff. But only when charging my phone. The arduino does not use enough power to turn the module on.

So, now my idea is to put one of these modules in between.

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This has the low voltage cutoff, but missing the boost conversion to 5v. So my idea is to hook the battery up this last shown module, and then hook the first shown 5v boost converter up this module. So then I have cutoff protection, and 5v boost that will activate when having my arduino hooked up.

Any thoughts about this?

2

I would simply use the battery to run the Arduino, and 3.3V accessories where possible. No step up converters required.

When using low power techniques, especially with "bare bones" Arduinos, many of the complications go away.

You don't need a rechargeable battery either, as described in this solar powered Arduino project.

3

This project uses a 5 volt ultrasonic. This will not run on 3.3v. I failed to mention that in my original post

4

Having another look at my ultra sonic module or does it works between 3 and 5 volts.

So maybe putting one of these between my battery and arduino setup, fixed at 3.3v cutoff might work? If the battery voltage drops below 3.3v the step down won't provide anything anymore right? Or will it just pass on the voltage that the battery has. If the battery does down to 2.5v it will provide 2.5v

[Edit]
This one needs at least 4.5v, so so won't work but the idea still stands
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5

Why don’t you just buy a 3.3 compatible ultrasonic sensor?

6

Well, my last post states that I have the 3v compatible one. So next issue is the low voltage protection. Just buy or just order is a pain here in the middle of nowhere.

Guess I'll just use the step down and TP4056 charger as low volt cutoff.

7

Simplest solution is to use a protected 18650 cell.
These have a small circuit built into them to prevent over and under voltage.