The npn Transistor is used to control the current flow through the LED. The resistor R_s can be chosen using the formula below:

I_LED = 0.7 V / R_s

Note that this type of current limiter is far from perfect. You actually have to expect base voltages of 0.5...0.9 V, and they will vary greatly with temperature and depend on the exact type of transistor used. Consider this as a mere starting point for digging deeper into the subject of transistor current sources...

The npn Transistor is used to control the current flow through the LED. The resistor R_s can be chosen using the formula below:

I_LED = 0.7 V / R_s

Note that this type of current limiter is far from perfect. You actually have to expect base voltages of 0.5...0.9 V, and they will vary greatly with temperature and depend on the exact type of transistor used. Consider this as a mere starting point for digging deeper into the subject of transistor current sources...

For more detail you can refer the below link http://www.physics.unlv.edu/~bill/PHYS483/current_lim.pdf

Transistor is used to control the current flow through the LED. And resister Rs can be set with the below formula.

Iled= 0.7/Rs

The npn Transistor is used to control the current flow through the LED. The resistor R_s can be chosen using the formula below:

I_LED = 0.7 V / R_s

Note that this type of current limiter is far from perfect. You actually have to expect base voltages of 0.5...0.9 V, and they will vary greatly with temperature and depend on the exact type of transistor used. Consider this as a mere starting point for digging deeper into the subject of transistor current sources...