Switching using BC557

Concept of this circuit is..
When the circuit is powered on the LED will glow continuously.Similarly when the BC557 is switched ON by RB0,RB1 will go to high state.I used voltage divider to get 4.9V from 12v supply and given to RB1.I tried this circuit practically but when I connect the BC557's emitter terminal to voltage divider means,I can't get 4.9V,it dropping to 1.7V...

My doubt is why voltage dropping occur at BC557 and is this circuit is correct..?or have to make any changes in this circuit..?

Hi,

RB0 = ON --> RB1 = ON.
Why not simply connect RB0 with RB1?

What´s the benefit of your circuit? Tell us the true idea behind your circuit.

****
But maybe you want this:
Make R1 = R3 = 100k. Try it.

****
The problem wit your circuit is, that the function depends on 5V precision and 12V precision.
If one of both is out of a valid range... it won´t work anymore.

Klaus

KlausST said:

Hi,

RB0 = ON --> RB1 = ON.
Why not simply connect RB0 with RB1?

What´s the benefit of your circuit? Tell us the true idea behind your circuit.

****
But maybe you want this:
Make R1 = R3 = 100k. Try it.

****
The problem wit your circuit is, that the function depends on 5V precision and 12V precision.
If one of both is out of a valid range... it won´t work anymore.

Klaus

Click to expand...

The LED will goes to off status after particular time period.
This circuit is used to check whether their is power supply given to LED or not.

If I connected RB0 and RB1 means I can't check the status of LED.So at particular time period the BC557 will be switched ON to check is their power supply to LED is available or not...?
According to the status observed from the RB1(which operated by BC557) an relay will be switched to control an motor.

Hi,

The LED will goes to off status after particular time period.

Click to expand...

Since the LED is ON all the time, as long as 12V is available..You don`t check LED status, but 12V supply status.

The LED circuit has nothing to do with the transistor circuit. The transistor circuit will operate the same way... with or without LED.

If you truely want to check LED status: use a photodiode.

***
Why not simply connecting the voltage divider R5/R6 to RB1? ... and forget about RB0, Q1, R1, R2

***
did you try the 100k/100k ?

Klaus

KlausST said:

Hi,

Since the LED is ON all the time, as long as 12V is available..You don`t check LED status, but 12V supply status.

The LED circuit has nothing to do with the transistor circuit. The transistor circuit will operate the same way... with or without LED.

If you truely want to check LED status: use a photodiode.

***
Why not simply connecting the voltage divider R5/R6 to RB1? ... and forget about RB0, Q1, R1, R2

***
did you try the 100k/100k ?

Klaus

Click to expand...

I tried with 100K also,but facing the same problem...
I have to check at particular time only.
If I switched ON transistor means only I want to check the status.Transistor is mandatory for my project.