The following circuit of a DC cell phone charger using a single BJT is probably the simplest in its forms and may be built very cheaply and used for charging any standard cell phone from a DC 12 volts external source.
The circuit diagram illustrates a rather straightforward design incorporating very few components for implementing the proposed cell phone charging actions.
Here the main active part is an ordinary power transistor which has been configured with another active part, the zenet diode for forming a nice little DC to DC cell phone charger circuit.
The resistor is the only passive component other than the above couple of active parts which has been associated in the circuit.
So just three component is to be used and a full fledged cell phone charger circuit is ready within minutes.
The resistor acts as the biasing component for the transistor and also acts as the "starter" for the transistor.
The zener has been included to inhibit the transistor from conducting more than the specified voltage determined by the zener voltage.
Though, a cell phone ideally requires just 4 volts for initiating the charging process, here the zener voltage and subsequently the output voltage has been fixed at 9V, because the current releasing ability of this circuit is not very efficient and assumably the power should be dropping to the required 4v level once the cell phone is connected at the output.
However the current may be decreased or increased by suitably increasing or decreasing the value of the resistor respectively.
If the cell phone "refuses" to get charged, the resistor value nay be increased a bit or a different higher value may be tried for making the cell phone respond positively.
Kindly note that the circuit was designed by me based on assumptions only and the circuit has not been tested or confirmed practically.