transistors
I started playing with transistor earlier and i have problems i must have miss understand transistor.
simulate this circuit – Schematic created using CircuitLab
My problem is the current from base will flow to the emiter and light up the led. however if i reduce the voltage of the base and test the base dosent draw enough current from collector to the emiter, or am i miss understand the function of the transistor?
Okay, looking at the picture I think you may have the transistor the wrong way round.
Try turning it round.
See this picture for reference:
As you can see the collector is on the right with the flat part facing you, so you have the collector connected to the LED in your circuit (if the 2N2222A part you are using has the same pinout)
I got the picture from here.
EDIT - It's actually a 2N222A, but the above advice still goes as the pinout appears to be the same from the picture posted.
As Russell mentions the more standard way is to connect the LED to the collector, but your circuit should work if set up correctly.
With 0 on Q1's base, Q1 will saturate to probably within 100mV of the 5V rail i.e about 4.9V on the collector. If it were an NPN emitter follower and you applied 5V to the base, the emitter would be at 4.3V - maybe that's where you are having a minor bit of confusion?
With Q2's base sitting at 4.9V and its collector being at (say) 3.3V you'll have a heavily forward biased base-collector diode (un-natural operations) and may indeed damage Q2 due to excessive current flow.
Iif you had (say) a 10k resistor in series with Q2's base this would protect the device and you would see a voltage of nearly 3.3V on the emitter because Q2 would be saturated-on.
Best Answer
There are so many things you just got wrong.
(1) Voltages -
For an NPN (silicon) transistor to turn ON the base needs to be at least 0.6V more positive than the emitter.
For a (red) LED to turn ON you need at least 1V8 across it.
The minimum voltage (V2) that needs to supply the base is 0.6 + 1.8 = 2.4V
The voltage at the collector needs to be higher than the base, when fully turned ON this is at least another 0.2 volts so the minimum in this circuit should be 2.6V (V1)
(2) Controlling currents
Your circuit doesn't limit any current. You haven't destroyed the transistor and/or LED because your voltages are too low to do any damage (i.e. its not working). This is bad design.
Adding series resistance will prevent damage due to excessive current as the voltages are increased.
(3) Circuit configuration
The circuit you have used is called an emitter follower it is not using the transistor as a switch. The voltage at the emitter follows the voltage at the base (but about 0.6V less.
A transistor used as a 'switch'
Typical values for R2 would be in the range of a few hundred Ohms to a couple of thousand Ohms depending upon supply voltage. The value of the resistor depends upon the curent needed - more current , smaller value. Typical values for R1 would be in the thousands to tens of thousands of Ohms. (usually about 100 times R2 which assumes a minimum gain of 100 for the transistor).