Question about a simple transistor switch??

[samy555]

Hi

The first image shows the situation when the transistor works as a switch

In the second photo I connect D2, the result: LED is no longer shine
My question is why?

Thanks alot

 

[andre_luis]

Because available 6mA current ( 4-0,7/510 ) to share between Q2/D2 net is not enough to place Q2 into saturation region.
Try replacing R3 from 510ohms to somewhat like 47ohms.


+++

 

[Audioguru]

A red LED is about 2V.
Your LED current is (4V - 2V)/330 ohms= 6.1mA.
Your base current is (4V - 0.7V)/510 ohms= 6.5mA which can be as low as 0.6mA by using a base resistor that is (4V - 0.7V)/0.6mA= 5500 ohms.

 

[samy555]

Thank you for fast reply
In the emergency lighting circuit

When the main 220Vac is exist the transistor off

When the main 220Vac is OFF the transistor ON

I know that D1 and D2 control that process, but I can't imagine how?

- - - Updated - - -

A red LED is about 2V.
Your LED current is (4V - 2V)/330 ohms= 6.1mA.
Your base current is (4V - 0.7V)/510 ohms= 6.5mA which can be as low as 0.6mA by using a base resistor that is (4V - 0.7V)/0.6mA= 5500 ohms.

Hello sir Guru
Thank you very much for trying to help
As you know, I can not formulate my questions as required
My problem with that emergency light above started when I tried to repair
I am amazed by the simplicity and complexity of the circuit at the same time.
I am now trying to understand.
I'm glad you are here these moments
I repeat my thanks