Introduction: Cell Phone Battery Flashlight

About: Ever since I almost electrocuted myself at age 3 (long story), I've been interested in science. I love to write, and since Instructables combines writing with science, it's one of my favorite websites. I like …

Almost everyone has junk laying around the house that they never use. In my case, some of this "junk" just so happened to be about fifteen used cell phone batteries. They'd just been sitting there for a couple years, so I thought "Hey, why not try to make some use out of them?" This high-brightness light, made of spare bits and pieces, is what followed.

(First Instructable btw; it's so cool to finally be sharing projects on here!)

Step 1: Getting the Stuff

Supply List

  1. Perfboard (or other 'frame' to mount the components on, such as a piece of cardboard or wood)
  2. 16 high-brightness white LEDs (available online for just a couple bucks)
  3. A 10 ohm resistor, preferably rated for at least 1/2 watt.
  4. Two cell phone batteries (3.6 volts)
  5. A switch (not pictured)
  6. Spare copper wire to connect the components together.

Step 2: Assembling the LED Array

In order to create the array of LED bulbs, you'll need 16 high-brightness white LEDs. Generally, white LEDs are designed to consume around 3.2-3.3 volts, so the end result of this light panel will be a system of LEDs that will operate best right around 6.4-6.5 volts- perfect for cell phone batteries.

To begin with, reference the circuit diagram. We're going to create a circuit in which 8 pairs of LEDs will be wired in parallel. Start by identifying the negative and positive leads on the LEDs. The positive lead will be longer than the negative lead by about an eighth of an inch. If you have perfboard, (starting from the left hand side of the board) drop the LEDs into the holes with the leads in this pattern: negative-positive-negative-positive positive-negative-positive-negative. This allows us to combine two positive taps into one, thus saving space. If you don't have any perfboard, take a sheet of cardboard and use a tack or pin to poke holes all the way through it- you'll need four rows of eight holes. After the holes have been poked, insert the LEDs into them to form the circuit. Repeat this step three more times until all 16 LEDs are in place.

Once you have a 4x4 block of LEDs, congratulations! You can now either solder them together, forming a more permanent circuit, or you can simply bend the leads underneath your cardboard or perfboard and twist them together. When you've linked all your LEDs together, find some spare copper wire (it has to be bare), and connect all the leads on the far left and far right sides together. These are your negative terminals. Then, connect the four soldered or twisted joints in the very middle of the circuit together with copper wire. This will become your positive terminal.

I'm sorry about the crude quality of the photo, but it sort-of shows what the end result of the LED panel should look like.

Now, we can move on to the next step: the battery pack.

Step 3: Building the Battery Pack

Remember those old cell phone batteries?

When I was trying to build my battery pack, I ended up accidentally coating one of the battery's terminals with solder, which effectively rendered it useless. I was unable to melt or scrape it off. With the battery dead, I had to go and find a replacement. Luckily, there just so happened to be two identical batteries in my little stockpile. Lesson learned: soldering this battery pack is hard work. Just a little heads-up.

To start off with, you'll need two cell phone batteries. They should be rated for 3.6-3.7 volts, and their milliamp-hour rating should be about the same (this rating is sometimes displayed on the battery casing as mAh). If you're in doubt, try to choose two equally-sized batteries of the same brand. While two batteries with totally different storage capacities should be able to work, there is a possibility in those cases of accidentally overcharging a battery and causing damage to it. Take the two batteries and bind them together somehow (I used rubber bands).

Next, choose a piece of wire around 4 or 5 inches long. Being very careful, solder it to one of the battery's positive terminals. If you have a multi meter, connect one probe to the battery's negative terminal and the other to the end of the wire. If it registers a voltage, you're in business. For the sake of simplicity, this battery will be called the first battery.

Now, find a fairly short (1 or 2 inch long) piece of insulated wire. Strip about a quarter inch of insulation off both ends, exposing the metal underneath. Solder one end of this wire to the negative terminal of the first battery, and the other end to the positive terminal of the second battery.

Finally, find another 4-5 inch long piece of wire. Solder this wire to the negative terminal of the second battery. When all is said and done, it should look something like the completed battery pack in the photo. To check your work, connect a multi meter between the two long wires. If it displays a voltage reading, you've successfully frankensteined two cell phone batteries together.

If no voltage registers, there are a number of possibilities as to why. The batteries might both be flat, or you could have a problem with the solder joints. Try connecting the battery pack to a 6 or 9 volt battery for a couple minutes and then re-checking your multi meter. Should current register, your problem is simple: the batteries are flat and need to be recharged. But if you still don't get a reading, try re-soldering your connections.

The end result here is a 7.2 volt battery pack, which is more than capable of providing enough voltage to power the 6.5 volt LED bank. The funny thing about LEDs is that they're not at all like incandescent bulbs. They either light up or they don't. If we tried to use anything less than about 6 volts to power the LED circuit, the bulbs wouldn't light up at all.

Step 4: Putting It All Together

By now, we're already 80% done. We've got the battery pack, the LED array, and the spare components just waiting to be mounted on.

Begin by attaching the two far ends of the LED panel together with a single piece of copper wire. If you're good at reading circuits, the schematic will be a big help here. The goal of this is to tie the negative taps together, making one single, larger tap.

Next, install the 10 ohm resistor by connecting one of its ends to the positive tap on the LED panel. If you have perfboard, simply insert the resistor into the holes and connect the wires underneath. If you're using cardboard, puncture holes for the resistor's leads to fit through and do the same thing. After setting up the resistor, attach one wire protruding from your switch to the piece of copper wire that joins the two negative taps together. (Very Important: do not let the positive and negative wires cross unless they are insulated)

With our resistor, switch, and LEDs wired up, now's the time to put on the battery pack. Find the positive terminal wire leading off of the battery and connect it to the remaining lead on the resistor. This will complete part of the circuit. Next, attach the negative terminal wire to the free end of the switch. This connects the remaining part of our circuit together. All that's left to do now is to attach the final two wires.

Connect one of the wires to the same place you joined the negative battery wire and the switch. Then, attach the other wire to the joint where your positive battery wire and the resistor are connected. These wire leads will be used to charge the battery pack- using anything from other batteries to solar panels to wall-powered DC supplies. It is important to not let these two wires touch, or there will be a short circuit.

And now, we're done. Well, almost.

Step 5: Have Fun With It

Now that your flashlight is complete and ready to go, have fun with it! I decided to take a couple measurements right off the bat to see how it was doing.

I connected one probe from my multi meter to the negative terminal of the battery, and the other to the switch. When I turned on the switch, the current meter showed that the LED panel consumes around 140 milliamps. With my batteries having (at least) 600 milliamp hours of capacity, I should have around 4-5 hours of run time per charge. Not bad! The 10 ohm resistor limits the current just enough to stop the LEDs from consuming too much power, but not so much that they don't put out full brightness.

Taking it outside at night, I found that it has a viewing range of around 35-50 feet. This isn't all that great for a flashlight, but I sanded my LEDs in order to diffuse them before I used them in the project. One of the great things about a project like this is that it can be tweaked to fit almost any purpose- I diffused the LEDs because I'm using mine as a light for my work table, and it works pretty great.

Finally, I decided to show one of the multitude of ways in which the light can be recharged. I suppose this project could count as Eco-friendly because it takes used cell phone batteries (containing toxic chemicals), and puts them back into use and out of the landfills. In addition, I decided to use my 12 volt solar battery maintainer as a charger for the battery pack. I simply take it outside or set it in my window during the day and use it for lighting at night!