Edgar landivar 
Cell phone charger for Neandertals
A trick that I have performed before in an extreme case (I was camping and didn't have an outlet nearby) is to connect 4 AA batteries in series (they can be taken from any flashlight or electronic device) and connect this directly to the cell phone through a micro-USB cable. I had some spare batteries for a flashlight and I used them. Sure, I also had to strip the wire with a razor (it became useless later), but I solved my problem. However, 4 AA batteries do not give exactly the 5 volts that the cell phone needs to charge, but approximately 6 volts (each provides 1.5 theoretical volts) or maybe even a little more. In short, do it at your own risk :)
A small improvement to the above is to place a small resistance of less than one Ohm between the batteries and the cell phone to slightly lower the voltage and protect the cell phone in the worst case (although practically all cell phones have a built-in voltage regulator, but it never hurts). Obviously if I'm camping in the woods I won't have resistors in my wallet or an electronics store in sight, so I will have to improvise. One thing that we may have on hand and that we can use is a pencil. Between one or two centimeters of pencil lead can give us between 0.25 and 2 ohms depending on the hardness of the lead. I have used a centimeter of 4H pencil lead for the experiment. I will post a video in the next few days.
How to connect the charger to the USB port
We will need at least one old cable to be able to reuse the charger. Remember that a USB cable can have several cables, but to charge the phone we will only need to use only 2 of them, usually red and black. Red for the positive cable and black for the negative cable.
Cell phone charger with voltage regulator
A less primitive circuit than the previous one can be achieved by adding a SINGLE component to our series battery circuit. This has its pros and cons. The construction of this circuit can no longer be improvised in-situ, but we will have to carry a small prefabricated circuit in our backpack. In any case, the manufacture is extremely simple.
The idea here is to use a small voltage regulator component. The most common is the LM7805 (or simply 7805) which can be found at virtually any electronics store for pennies. It is also preferable here to use 5 to 6 batteries instead of 4, as this component needs a certain minimum voltage difference between the input and the output. We could also use a 9 volt battery here. I share with you a small diagram that I made.
Major league cell phone charger
We are getting a little more complicated ... Another more sophisticated but much more precise and efficient solution than the previous one is to use a converter circuit step-down (also called DC-DC step-down or Buck converter), which is a cheap circuit and that we can find in electronics stores. The one I have purchased can be purchased online at various sites such as Amazon, Ebay, OLX or Mercadolibre. It will give us exactly 5 volts. On another article I wrote some time ago In this same blog I explain how to make one, in case someone wants to venture. There are fixed voltage ones like the one I have used, but there are also others in which you have to adjust the output voltage by turning the pin of a precision potentiometer. There are also several capacities. The one in this article will provide us with up to 1 amp, as it is based on the chip AMS1117.
To better illustrate how to connect everything I preferred to make a video. The chosen circuit can even work with a car battery, so it can be used to add a USB port to our vehicle, in case it does not have one. We could also connect a rechargeable battery and use a solar panel to charge it. The general rule for carbon or alkaline batteries is that the larger, the more charge. For example, AA batteries have more charge than AAA, since they are larger.
Now I leave you with the video. In it you will see the circuit used, which we can put in a box to make it look more friendly and put it in our backpack. It will save us in case of emergencies.
[embedyt] https://www.youtube.com/watch?v=xTNuD6QffWE[/embedyt]
