# Help With Op AMp Circuits

Discussion in 'Homework Help' started by avinashkris, Dec 16, 2012.

1. ### avinashkris Thread Starter New Member

Dec 16, 2012
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0
i need to design circuits using op amp and i m confused a bit..

i have to design a circuit which turns on an led when input is between 1.5 and 3 volt... is that a problem assiciated with monostable multivibrator ?

also a circuit to convert 50% duty ratio square wave into 305 duty ratio rectangular wave..

can some one xplain both of these

2. ### WBahn Moderator

Mar 31, 2012
18,089
4,917
You need to do a whole lot better at explaining your specs. We are not mind readers.

When you say, "... turns on an led when input is between 1.5 and 3 volt...,", do you mean that it is off if the input is <1.5V and also off if the input is >3V?

I assume you mean 30% duty cycle and not 305 duty cycle? If so, then what are the constraints of the frequency? Is it okay that the 30% duty waveform have a frequency that is one-tenth of the 50% square wave?

Does the frequency of the input waveform change, or is it fixed?

What technologies are fair game? Can you use digital parts?

3. ### avinashkris Thread Starter New Member

Dec 16, 2012
3
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sorry ..

yes led is off when input is <1.5 and >3v

yea 30 % duty cycle.. frequency of square wave is given as 1khz and no regards about rectangular wave frequency ..

what available to us are basic op amp circuits like integrator ,difrntr, astable ,monostable vibrators ,triangularwave generators etc

4. ### WBahn Moderator

Mar 31, 2012
18,089
4,917
Okay, so let's play with the LED one first.

What are your thoughts and ideas? They don't have to be complete, but just brainstorm a bit about what you think might work, or at least get you part way to the solution. Think in terms both of what you might do with what you know how to do and also what you could do if you only knew how to do something else (something else that you can clearly describe).

An example might be: I know how to make a circuit that outputs a high voltage if the input is >1.5V and a circuit that outputs a high if the voltage is >3.0V. Similarly, I know how to make circuits that output a low voltage if the input is >1.5V (or >3.0V). Now if I could just figure out a way to combine those outputs to make a circuit that does ....

Also, think outside the box. An LED simply needs a voltage difference across it in the right direction and greater than some minimum in order to light up (not forgetting the current limiting resistor, of course). We usually tie one end of the LED/resistor to a supply or ground, but that doesn't have to be the case.

5. ### avinashkris Thread Starter New Member

Dec 16, 2012
3
0
i think a comparator circuit with a reference volatge 1.5 can produce a high output but i dont knw what should i do to remain the output high till 3 v ..

6. ### absf Senior Member

Dec 29, 2010
1,493
374
You'd need 2 comparators and arrange them as what was called a "window comparator". Google for the term and see if you can understand the concept.

Ask some more questions if you still have doubts.

Allen

7. ### WBahn Moderator

Mar 31, 2012
18,089
4,917
Consider the following:

You have two circuits:

Let's say that Vin can vary between 0V and 5V.

One produces an output, Va, that is, say +15V when the input is above 1.5V and -15V when it is below 1.5V.

One produces an output, Vb, that is, say +15V when the input is above 1.5V and -15V when it is below 3.0V.

What is Va-Vb over the range of Vin?