How to turn on a led with a transistor (simplified) *

Hi!

Today i want to show  you  how to turn on a led with a transistor. This action doesn´t have  a pratical use, but is for curious minds. I have an expression for this circuit:

we have to calculate the R1's value, that depends from BATT value. hen we can use the expression:

                                                            

beta is a transistor parameter, you can find it at transistor´s datasheets, but you will find it like hFE.

generally this circuit is for outputs with low current, for example a microcontroller port

But, if you find this circuit useless, then you have to see this circuit:

                                      

the microcontrollers don't support high current values ( some microcntrollers can drive only 10mA), and we want to drive two, three or more LEDs, then with previous circuit I purpose a new expression:

                                                    

you can se that is similar with previous circuit, but we have a new parameter: N

N is the leds number that you want to drive. but you have to check the transistor capability to drive current. In this case we have a BC547, that can to drive about 100mA. then, you can only drive 5 parallel LEDs.

I hope that this blog can help you

*Originally: "como encender un led con un transistor"

**PLEASE: I can't speak english very well, so, don't be evil with me :'(