Problem with Transistor

Thread Starter


Joined Dec 9, 2019
Hello, i'm a beginner in eletrônics and i'm having a few problems with a basic schematics, i'm trying to connect a transistor, i'm trying to connect it to my arduino but the led won't power up in the simulator, but if i connect it on the protoboard it goes just fine ... I don't know what is wrong. the sourcers in the schematics are of 5 and 10 VWhatsApp Image 2019-12-09 at 09.50.54.jpeg


Joined Jan 29, 2010
hi C,
Welcome to AAC.
Try connecting the LED and the 1K in the transistor collector, connect the emitter to 0V.
Let us know how it goes.


Joined Mar 14, 2008
Your circuit is an emitter follower, so the emitter voltage across the resistor-LED can be no more than the input base voltage minus about 0.7V.

Audioguru again

Joined Oct 21, 2019
You did not say the specified forward voltage or color of the LED. A red one is about 2V and a white, blue or modern bright green one is about 3.2V. Since your transistor is an emitter-follower, not as a common-emitter switch then the 4.7k base resistor is not needed.

With 5V into the 4.7k resistor then the base voltage will be a little less than 5V, maybe 4.8V. Then if a 3.2V LED is used, it and the 1k resistor get 4.8V - 0.7v= 4.1V and the current in the LED will be only (4.1V - 3.2V)/1k= 0.9mA which will be fairly dim.


Joined Mar 10, 2018
You can drive an LED in the emitetr, although that is not the best way to do so,
as Eric points out.

Here is a sim driving in emitter. 3 1N4148s were used to sim a 2V LED. As you can see
LED current is not very good.


Regards, Dana.