# Amplification Question (Op Amp?)

Discussion in 'The Projects Forum' started by silverslik1, May 11, 2010.

1. ### silverslik1 Thread Starter New Member

Apr 30, 2010
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I built a circuit where the LED's respond to music by lighting up when the music plays (I believe its a low pass filter because it intensifies with bass and not treble)

http://www.instructables.com/id/Music-LED-Light-Box/ - there is the circuit I used and for the LED I bought on of these off ebay - http://cgi.ebay.com/ebaymotors/LED-...r_Truck_Parts_Accessories&hash=item1e5a4eebea (blue specifically)

When I hook up the LED's straight to the 2 9V I have in series, it lights up very bright obviously, but when I put it in the circuit, the lights barely light up due to the lack of current it needs I'm guessing?

I know if the signal were louder from the music source, they would light up but I don't have an amplifier after my music source or transistor I'm using. Can I use an op amp to boost the signal after my transistor or something? If so how would I do this? I have a 741 chip and resistors I can use, but I'm not sure what values or if I'm doing an inverted or non-inverted configuration.

Thanks

May 26, 2009
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3. ### silverslik1 Thread Starter New Member

Apr 30, 2010
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Attached is the schematic with some simple LED's that is the same circuit from the website

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4. ### ELECTRONERD Senior Member

May 26, 2009
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What is the output voltage from the TIP31?

5. ### silverslik1 Thread Starter New Member

Apr 30, 2010
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Around 13.45V to the first LED...the batteries are producing around 16.5V in series

6. ### ELECTRONERD Senior Member

May 26, 2009
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That is a fairly poor design for what you're trying to accomplish. I would try to do something like in the attachement.

A Low Pass Filter and High Pass Filter seperates your frequencies. Certain LEDs that you choose will only light up according to the low bass notes. Likewise, the higher frequencies that may be the melody would only light up LEDs that you select. The op amp utilizes positive feedback, so that with any slight perturbation of audio, it will trigger the op amp to turn on the LEDs depending on the frequenices.

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7. ### Audioguru New Member

Dec 20, 2007
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Instructables are designed by people who know nothing about electronics.

The circuit shown does nothing until the input signal level tries to exceed 0.7V. When the input signal level reaches 0.7V then the base-emitter of the transistor is forward-biased and short-circuits the music source as the transistor tries to turn on. If the music source is an MP3 player or amplifier then it might be destroyed and the transistor also might be destroyed since the transistor is missing an input current-limiting resistor.
When the signal swings negative then the emitter-base of the transistor might have avalanche breakdown which also might destroy the music source and transistor.

The transistor must conduct 640mA to light the LED rope and its base current must be
64ma. The music source might not be able to supply a current as high as 64mA.

An opamp with positive feedback is a latch. Its output saturates at one of its supply rails and stays there. Maybe you are thinking of using a comparator with a small amout of hysteresis. Then it quickly switches on and off.

8. ### ELECTRONERD Senior Member

May 26, 2009
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In saturation the output will hit one of the rails, under any perturbation in the voltage input which is defined by the feedback, and it should go back in its linear region when the input conditions are adequate once again. What makes you think it would stay on one of the rails once initiated there?

9. ### silverslik1 Thread Starter New Member

Apr 30, 2010
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This sounds great, but I'm not sure I follow all of it. I understand the high and low pass stuff but I like it being a low pass so it really lights up when there is more bass in the song.

Is there anyway to make it work in the above configuration I have? Would the op amp work or would I have to do something else?

10. ### Audioguru New Member

Dec 20, 2007
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An opamp with positive feedback does not have a linear region.

If the output goes slightly positive then its positive feedback increases its positive input and its output goes to the positive supply rail and stays there.

If the output goes slightly negative then its positive feedback increases its negative input and its output goes to the negative supply rail and stays there.

If the positive feedback is only a very small amount then it is hysteresis which might be overcome by the input signal to cause its output to switch high and low very quickly at the frequency of the input signal.

11. ### ELECTRONERD Senior Member

May 26, 2009
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An op amp with positive feedback does have a linear region, although it isn't large or stable. Have you ever seen the graph that depicts Vin and Vout with positive feedback?

Hypothetically, let's assume that at anything greater than 6V, it will trigger the op amp to go to the positive supply rail. Also, anything under -6V will trigger the op amp to go to the negative supply rail. Between 6V and -6V the op amp acts like an amplifier in its linear region. As told in this lecture video:

http://ocw.mit.edu/OcwWeb/Electrica...2Spring-2007/VideoLectures/detail/embed21.htm

I admit that I didn't know that once triggered, the op amp output would remain on one of the supply rails, but I'm still skeptical about that.

12. ### Audioguru New Member

Dec 20, 2007
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No!

The low frequency voltage gain of an opamp is 200,000 to 1 million. Positive feedback makes it higher. Nobody has seen an amplifier output that has so much gain.

No.
The opamp output latches to the positive or negative supply voltage.

I did not see the long video and did not read the long blah, blah, blah.
I did not see a schematic.
So maybe there is negative DC feedback and positive AC feedback plus some phase shift that makes an oscillator.

13. ### ELECTRONERD Senior Member

May 26, 2009
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Audioguru, around 14 minutes into the lecture, the professor stated that theoretically if my input was exactly steady and with no perturbation, that the following equation would be true:

$V_O_U_T = -V_I_N(\frac{R_2}{R_1})$

Now that I realize that nothing is in practicality is perfectly steady, I agree with you.

How would you go about using a comparator with a slight amount of hysterisis? Could you please define hysterisis?

14. ### Audioguru New Member

Dec 20, 2007
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Ho, hum that lecture is boring.

A comparator with a slight amount of hysteresis has a small amount of positive feedback so when the input voltage is slightly more or less than the threshold voltage then the output will very quickly "snap" to the positive or negative supply rail.

May 11, 2010
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What is meant by "linear region"? I think of an op amp using negative feedback as a linear amplifier, because it's definable as an equation, not just always railing like a comparator.

16. ### ELECTRONERD Senior Member

May 26, 2009
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That's exactly what I meant. In that case, once the output hits the positive or negative rail, does it stay there?

May 11, 2010
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No, it changes again if the voltage crosses the reference... but the positive feedback gives it a little "snappiness" so it doesn't fluctuate wildly when the reference and comparison voltages are nearly equal.

18. ### ELECTRONERD Senior Member

May 26, 2009
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I take it that you were referring to Audioguru's post? Next time use quotes so that your remark is easily understood.

May 11, 2010
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It changes again if the comparison voltage crosses the reference... but the positive feedback gives it a little "snappiness" so it doesn't fluctuate wildly when the reference and comparison voltages are nearly equal. This is a Schmitt trigger.

20. ### silverslik1 Thread Starter New Member

Apr 30, 2010
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...soo....uh....should I use the op amp or not I'm still really confused. I don't understand most of this at all

Could someone help me make this work, I'd hate to have the parts go to waste