Introduction: Pencil Phone Charger

Imagine that you are stuck somewhere in the post-apocalyptic wasteland and you need to power up your phone for one last game of flappy bird before you starve to death. The only problem is that you have a 12-volt car battery and your phone requires 5 volts to charge. How do you convert the power so that you can safely charge your phone without an adapter and without letting the smoke out, well that is what post is all about.

Step 1: You Will Need Some Basic Tools


Knife (a pointy rock will do in a pinch)

Some wire

power source (like a 12-volt battery)

optional extras:




Step 2: Measuring

First you need to measure the length of the pencil, my pencil is new so it comes in at 175 mm. It does not have to be a new pencil just along as it does not have a break inside or you may see some sparks fly. The type of pencil I am using is a HB (number 2 for the Yanks). The darker the pencil the better because there is more clay inside hence more resistance thru the length of the pencil. If you cannot measure in millimeters you can always use the Yanks approach of using your fingers as precision measuring instruments.

Step 3: Time to Math

What we are trying to create is called voltage divider. The diagram above shows you how it works. The Vin is the voltage that we put into the circuit (in this case it is a 12 volt battery).R1 and R2 is usually 2 resisters of witch we would know the resistance of by using a multi-meter. The theory goes that if you have 2 resisters that are equal in resistance and you take a voltage measurement at point A and B you will get the voltage of Vin (in this case 12 volts) and if you take a voltage measurement Between A and C (or B and C) you will get half the volts (6 volts) effectively dividing it by 2.In this case we do not have resisters but the pencil dose give a resistance if measured (in my case it was around 56 kilo ohms). But instead of using resistance we are going to use the length of the pencil to calculate the voltage drop. So in theory if we half the distance the resistance will half and the voltage will half from 12 volts to 6 volts.

But we do not want 6 volts we want 5 volts so what we do is take the length of the pencil divide it by the voltage of the supply then multiply it by the voltage we want, to get the distance we desire for the 5 volts so basically (R1 + R2 / Vin) * Vout or in my case (175mm/12v)*5v = 72.917mm (remember we are replacing the resistance with distance) so between point B (the negative terminal) and point C (the positive terminal) we will get 5 volts

Step 4: Theory Vs Reality

Now we use our trusty knife (or rock) to cut open both sides of the pencil. Then we use our precision measuring apparatus to mark the correct point that we calculated and very carefully cut to the inside of the pencil without going thru or breaking the this pint you can measure the voltage using a multi-meter or pry if you don’t have one. The biggest problem with the voltage divider is that it does not guarantee a stable voltage output as the battery discharges or suddenly increase in voltage due to an engine finally starting after hours of trying, this is the reason Electricians prefer to use a voltage regulator. The last part would be to connect to the phone, if you can open the phone in the first place you should see 3 terminals, two of them indicates polarity and the third one (the center pin in my case) is the temperature sensor’s terminal, Connect the positive leads to the positive terminal and the negative leads to the negative terminal. In about 20 to 30 minutes you will be able to make a phone call to your loved ones or play flappy bird one last time before Immorten Joe hunts you down for stealing his wives.


bob8898 (author)2017-07-14

BE CAREFUL! I'm surprised your lipo didn't explode...

In you instructable you did EXACTLY THIS!!! (At time 2:09)


DonovanP (author)bob88982017-07-15

It will not explode because the battery has a protection circuit on it, this protects against several things like over current draw, over heating etc .i think if you have a look at Scott's video you will see him removing all of this protection. What i did here was limiting the voltage using a pencil because if i get the voltage correct the protection circuit will regulate the current to a max of about 2 amps else if the voltage is incorect this circuitry will burn out kinda like a fuse, of course this all is only to be used in a post apocalyptic hell. so... enjoy??