Emergency Keyring Phone Charger

Introduction: Emergency Keyring Phone Charger

About: I'm a 18-year-old high school student, I love engineering, film production, design and everything in between.

Have you ever been in a situation where you really needed your phone to make a call or order a ride but it had just died (no rhyme intended) . I know I've been in the situation and it only gets worse with the older a phone gets, as a phone gets older its battery life significantly decreases. Now there are lots of solutions to this like carrying around a power-bank, but these come with their own problems for a start they are big and clunky, aren't always charged and if you don't have your phones USB cable they are unusable so to counter this problem I'm going to show you how to make an emergency phone charger that can clip onto your key ring and use to charge your phone using a very common disposable battery.

Step 1: Parts and Tools

This is a really cheap and easy build so the parts we are going to need are minimal, we will need:

The tools we will need for this project are:

  • soldering iron
  • glue gun
  • hot water
  • hot air gun (a hair blower will work just as well)
  • plyers

Step 2: Buck Converter and Protoplastic?

Let's start with the Buck converter, this is a really cool little circuit that lets us take a larger voltage and slim it down to a smaller voltage of our choice. So, for example, we are going to slim down the 9 volts from our disposable battery down to 5 volts that we can use for our phone. You may be thinking "but that's what a resistor does?" and you are right but the buck converter does it in a completely different way. A resistor lowers the voltage by converting the unwanted power into heat, this is why resistors get hot. A buck converter uses an inductor which results in the buck converter not wasting energy meaning its way more efficient.

Protoplastic is this really cool plastic that comes in small pellets, when these small pellets are heated up they melt together and become moldable but when they dry they become rock hard making it great for making project housings.

Step 3: The Old 9 Volt

The base of the project is going to be the top connector of an old 9-volt battery, to get this out we are going to need to disassemble the old 9-volt battery. We do this by finding the seam of the metal casing around the battery then we slowly peel back the metal casing and pull the whole battery assembly out. inside you should see six 1.5 volt cells that add up to 9 volts and of course our connector, cut the two connections going to the connector and put it aside for later.

Step 4: The Micro USB Connector

To make this as compact as possible we are going to need to make the micro USB cable as short as possible. We start by cutting the cable about 4 cm from the micro connector side, next we crack open the micro USB casing, inside there should be a red and black cable and possibly a green and white wire, cut the green and white wire completely off and set aside the micro USB for later.

Step 5: Setting Up the Circuit

To set this circuit up we start by connecting our other 9 volt connector (not the one we got from the old 9 volt) to the buck converter, we do this by connecting the red wire from the 9 volt connector to the positive input on the buck converter (labeled in +) and the black wire goes to the negative input on the buck converter (labeled in -). Now we connect our positive probe from our multimeter the positive output of the buck converter (labeled Out +) and the negative probe connects to the negative output of the buck converter (labeled Out -). Now we plug a 9 volt supply (I'm using a power supply but using a 9-volt battery will work just as well) into the battery clip and turn the multimeter to voltage mode, now we turn the tiny potentiometer with a screwdriver until the multimeter shows a reading of 5 volts

Step 6: The Circuit

In the last step we soldered our battery connector to input terminals of our buck converter now to finish the circuit we need to solder the micro USB connector to the output terminals of the buck converter we do this by soldering the red wire from the micro USB to the positive output of the buck converter and then the black wire is connected to the negative output of the buck converter. Now we take the circuit and glue the buck converter to the back of that 9-volt connector we salvaged earlier, then the micro USB is glued on top of the buck converter (make sure the 9-volt connector can connect to that other 9-volt connector we glued the buck converter to, check the photos for a better idea of what's going on). Plug an old device in that you don't care about too much (just in case we did something wrong) and give it a test, once everything is verified working we can add a blob of hot glue on top of everything to keep it in place.

Step 7: The Final Casing

So now that our circuit is made we need to make a strong casing to make sure it doesn't take damage while being on a keychain, to do this we use or protoplastic, start by warming up some water and pouring a small amount in, once its warmed up mould it around the buck converter into kind of a half egg shape and let it harden. Once its hardened blow it with a heat gun set on low or a hair dryer set on high all this does is smoothen out the surface and makes it feel better when touched, and that's basically it, we should have a working emergency phone charger that fits on our keyring. Now I wouldn't recommend using this device that often because 9-volt batteries aren't that cheap but it's great for emergencies. I put a backup 9-volt in my backpack just in case as it's small and light. If you have any questions please feel free to send me a message or leave a comment and ill try my best to get back to you.



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    29 Discussions


    1 year ago

    Has anyone determined whether this works with a lightening connector for Iphone? I'm guessing not as Iphones usually also need a lower voltage signal in addition to the 5v

    6 replies

    As far as i know apple requires 2 volts on the data pins to allow charging, so if you short the data pins with a resistor you should be fine.

    Thanks for the reply. This may be a dumb question, but how does shorting the pins with a resistor fool it into thinking it is getting 2v?

    Not a dumb question at all, i didnt explain myself. connect a resistor to the 5 volt pin lowering it to 2 volts, connect it to one of the data pins and then short them together. This is a common issue with apple products so there are tones of resorses online!

    It's a little more complicated than that (as usual apple fails to adhere to standards). But there is a really good article about this:


    And great instructable by the way!

    Have you tested to see how much of a charge a single battery gives you? Very interesting concept, just wondering how much use it could get from my phone in an emergency; just a text and hope the person you have messaged is near their phone and can send help, or enough for a phone call to emergency services.

    6 replies

    A 9V block battery has a capacity of about 500mAh. An iPhone 5 has a battery capacity of over 1500mAh. You are not going to get a full charge without several 9V batteries

    My s5 has a 3300mAh battery in it. But this would work great.

    Most 9V batteries have around 300-600 mAh capacity, so don't expect to fully charge your phone (most phones have around 2000-2500mAh capacity)

    this is true but the point of this project is for emergencies, so like charging your phone enought to send a message or make a call

    it charged my phone up to 24% but all phones have different size batteries. Overall im sure this device will let you make a phone call before running out of juice

    If it gets your phone back up to call for help or a quick message to let people know you are ok and where you are it is worth its weight in gold. You could do this many ways, but this is elegant.


    As I was saying,
    just connect the buck charger power to that and not worry about the other 9v connector. ?

    I suggest buying an extra 9 volt battery connector. Cut the wires off of it, and insulate them with a little silicone or hot glue. Then use it as a safety insulator/protector for your spare 9 volt battery. Or as long as the wires on your protector are safely insulated and separated, they could be used to actually connect the battery to your key chain also, where it would always be handy. The terminals of a 9 volt battery, being close together and on the same end as they are, could easily short out if they should happen to touch anything metal while bouncing around in your purse, backpack, or pocket.

    Nice instructable!

    So the salvaged 9-volt connector is just to hold the purchased one you'll actually use?

    1 reply

    A great project and what's really great is the easy access to power. 9 volt battery, they are everywhere. Thanks.