Transistor Switch -help with explaining circuitry needed

Transistor Switch

Im reading as eletronic engineer but begun. I cant figure out this circuit.

**broken link removed**

Can somebody explain to me why 2 transistors are needed? I could see this work with 1 transistor.

I really appreciate your help.

Re: Transistor Switch

Where is the input for this circuit?
--
Amr Ali
www.embedded-systems.blogspot.com

Re: Transistor Switch

Hmmm sry new at this. The input should be the 5v, which then will switch on the 12v when on.

Transistor Switch

So,
When the input at the 5V is 5V the output should be 12V.
When the input at the 5V is 0V the output should be 0V.
Am I right?
--
Amr Ali
www.embedded-systems.blogspot.com

Re: Transistor Switch

Yes. The problem is. Why use 2 transistors? If you look at it and think about it, isent it possible to do this with only 1 transistor?

I could make a drawing if you want?

Re: Transistor Switch

I guess the Logic behind this is as follows:
1. When the 5V input is high, Q1 is like a short circuit.
2. The base input for Q2 is ~ 6V, which lead to the working of Q2 and it will look like a short circuit. The final output is ~ 12V.
3. When the 5V input is low, Q1 is like an open circuit.
4. The base input for Q2 is ~ 12V, Q2 will look like an open circuit. The final output is ~ 0V.
Using 2 transistors, is to get the output swing between 0V and 12 V.
--
Amr Ali
www.embedded-tips.blogspot.com

Re: Transistor Switch

Oki i try to undertand.... When you say 5v input is low. What does that mean?

What whould happen if I used 1 transistor? Just so i can try compare a bit. Would help to know

Re: Transistor Switch

I mean 0V is connected instead of the 5V input.
Using 1 transistor will not switch the complete voltage swing. In addition, the output will be inverted. In other words, connecting 5V (0V) input will lead to 0V (12V) output.
--
Amr Ali
www.embedded-tips.blogspot.com

Re: Transistor Switch

Oki I have not the time now to look at it but thx alot. I try see if I can figure it out now. Thx alot. I may return

Re: Transistor Switch

Mikethk,

This is usual problem when it is required to control higher voltage load from microcontroller or some other logic circuit that operates on 5V or even lower. Since output of microcontroller or logic circuit can swing only from 0V to 5V, it is needed to extend swing of control voltage 0 to 12V in this case.
If one transistor is used, NPN with grounded emiter, negative line to load will have to be interupted to control load, which in some cases is not accepatble. Same thing can be achieved with PNP transistor with emiter at +12V and interupting positive line to load ( as shown in your circuit) and without NPN level shifter, but control logic would have to float below +12V to achieve that, which also might not be acceptable in some cases. So yes, same thing can be achieved with one transistor if you can accept either negative line to load being interupted or you can float control logic bellow +12V.

Re: Transistor Switch

Hey they have used 2 transistors to get the full swing from 0V--12V.
Its a complementary structure.