# Need help in a small project please

Discussion in 'Electronics Homework Help' started by shubham123, Nov 11, 2012.

1. ### shubham123

3
0
Nov 11, 2012
I have to make a project on 'Transistor As A Switch' and draw a graph between the input & output voltage showing the cut-off region,saturation & the active region.
I have attached the circuit diagram.The problem is that I dont know what values to take for the various components.

I tried making it with BC547 or BC 548 transistor,100k and 1k as Rb & Rc respectively,3volt for Vbb and Vcc(varied the voltage using a 10k potentiometer),Multimeter as Vi and Vo to measure the input and output voltage.Could not get any significant result,the votage kept varying a lot.

From what I understand,when Vbb will be less than 0.7V then multimeter Vo reading is equal to Vcc.As Vbb increases above 1V,Vo decreases.But as I said there was irregular voltage in the multimeter Vi.
Thanks a lot

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2. ### Harald KappModeratorModerator

10,810
2,438
Nov 17, 2011
1) this is homework, so I moved it to this section of the forum.

2) Is that picture all you were given? I would have expected to have some more detail in the task description. Please give us the full task description.

3) A quick Google search turned up this tutorial. I guess it will answer your question. If not, you're welcome to come back here.

Harald

3. ### shubham123

3
0
Nov 11, 2012
The link given is not working
This is the only diagram that was given.The basic idea is to measure the input and output voltage and draw a graph marking the active region,stauration region & the cut off region.

I made the circuit again using the following components:
BC547 transistor
Vbb = 1.5volt cell (100k potentiometer for varying Vbb)
Vcc = 3volt cell
Rb = 680k+680k+120k resistors in parallel
Rc = 10k resistor

I got some reading at Vo & Vi.As I increased Vi,Vo decreased by a small amount(approx. 0.1volt decrease in Vo with every 0.2volt increase in Vi).I think this lies in the active region.
Now I want to know how do I increase the range of Vi so as to get 0.5volt as min. voltage(currently getting 0.9volt to 1.6volt using the above components)

I was given to understand that Vo would near zero volt as Vi becomes greater than 1volt
and Vo will be equal to Vcc when Vi is equal to or less than 0.6volt.
I have attached my reading of Vi & Vo.

Last edited: Nov 12, 2012
4. ### Harald KappModeratorModerator

10,810
2,438
Nov 17, 2011

The other link doesn't work on my computer any more, sorry, it did work at the time I posted it.

I regret but currently I'm too busy to do an analysis. Someone else on the forum?

Harald

5. ### shubham123

3
0
Nov 11, 2012
Just got what I think is the correct readings.I had connected the potentiometer
& transistor wrongly.I have attached the new readings.Let me know if they are correct.Thanks a lot for answering.

6. ### Harald KappModeratorModerator

10,810
2,438
Nov 17, 2011
Make a graph and compare it to the graphs in the transistor's datasheet.