Hoangson said ‘the voltage drop on the LEDs = 2 volts’. LEDs commonly run 1.4 to 3 volts depending on color and type. I use one type of LEDs that use 5 volts. This voltage is very temperature dependent.
You used 4 LEDs in series. Series is a good way to use LEDs, not parallel. When using parallel LEDs do not use one resistor. “why did you not use only a resistor 200 ohm for this .” You did the right thing using two resistors!
Transistor= 2N2222. I could not find that data sheet because everyone makes 2N2222A.
For the 2N2222A (On-Semiconductor) the DC Current Gain at Ic=150mA and Vce=10 volts is 100 minimum, 300 typical and ? maximum. (probably 600) With a Vce of 10 volts this test is unusable! I just wanted to show that Hfe can vary 6:1 from part to part. Hfe at Ic=150mA Vce=1 volt has a minimum of 50 and typical about 150.
You are using the transistor at 50 mA so Hfe will be a little higher. If the transistor is cold the Hfe will be lower. 25C/-50C will (at 50mA) cause ½ the gain.
Because you want low Vce (Vsat) you need more base current. The gain will be lower with 0.2 volts of Vset.
Never use typical numbers. In this case use minimum numbers. Use numbers over a temperature range your project will be used at. Look at what condition the transistor will be used at. Example: Hfe at 0.1mA is 100, at 50mA Hfe=220, at 500mA Hfe=40. Look at the graph of Hfe and Ic and know this is typical not minimum. Look at the table of Hfe to see min/typ/max.
Look at the graph of Vce and Ib to see how well the transistor closes. Useing the 10mA curve we can see that with Ic=10mA and Ib=1mA the transistor is very ‘on’. At Ib=0.1mA the Vset is 0.1 volt but at 0.05mAIc the transistor starts to open up. Remember this is typical, at 25C.
I know your teacher will say use a Hfe of 100 and you need to pass the class. In the real world we look at min and max specs and use much more base current than the teacher said to.