1.) First, pick up a charger and look for a small sticker on the adapter part of the charger (sometimes it is molded into the adapter instead). On the sticker look for the word "output". Look to the right of "output" and the charger should say something like 12VDC 500 mA. This charger is 12 volts DC or direct current. Keep looking through the cellphone chargers until you find the voltage you want.
2.) After you have selected the right charger for your experiment, cut off the end of the cord that plugs into the phone.
3.) Next, peel the cord apart and strip the ends of the two wires. For some cords there may be just one piece of cord that contains two wires inside. For this type of adapter, just strip the outer wire to expose the inner wires, and then strip the ends of the two wires inside.
Wasn't that easy? Now you can attach the two wire leads to your circuit and then plug the charger into the wall. Make sure that the two wires don't touch each other. The phone charger should put out around the same voltage it said it would next to "output." Now you won't have to stop experimenting just because your batteries go dead. These cell phone power supply's plug into the wall and supply voltage as long as you like.
Since cell phone chargers plug into the wall, they are dealing with high voltages and amperage's. Although the adapter converts the voltage and amperage to a much lower number, be extra careful at all times. Complete and experiment with this project at your own risk.