# DC and Low Pass Filters

Discussion in 'General Electronics Discussion' started by Muhasaresa, Feb 1, 2012.

1. ### Muhasaresa

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Jan 2, 2012
Hi all

I made a circuit featuring two 555s and the outputs connect together so when I run the signal through my ipod earphones I can hear two notes at the same time (yey!). I also made the DC signals go through a low pass filter there is a clear change in in the tone (which is good as it sounds more 'smooth').

But how can a low pass filter work with DC? Everywhere I looked on the internet AC was always used.

I would appreciate any help offered,

Muhasaresa

2. ### Laplace

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Apr 4, 2010
DC is as low in frequency as you can get. So DC has no problem passing through a low pass filter.

3. ### Muhasaresa

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Jan 2, 2012
Thanks! But how come there was sill a difference in the wave output? There was a video I watched on youtube that said that a DC square wave going through a low pass filter would make the voltage average out. So I think it meant that if the duty cycle was 50% and the square waves went from 0V to 10V the low pass filter would make the output a constant 5V. Is this right?

Muhasaresa

4. ### Resqueline

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Jul 31, 2009
That's correct, if the filter has a cutoff frequency below the square wave frequency. If it has a cutoff frequency around the square wave frequency you'll get a sine out.

That's what a low pass filter does, passing low frequencies, cutting off high frequency overtones, thus rounding/smoothing off any sharp corners of the input wave.

5. ### BobK

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Jan 5, 2010
A square wave going from 0 to 10V for example, is not DC. It is AC with a DC offset of 5V.

Bob

6. ### BobK

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Jan 5, 2010
Let me try once more. A square wave is not DC. DC is a constant voltage. A square wave is a changing voltage. A changing voltage is AC, possibly with a DC component.

A square wave consists of many sine waves. One at the fundamental frequency of the square wave, and at all odd multiples of that frequence (3, 5, 7...) in specific ratios. A low pass filter will reduce the content of the higher frequencies, which makes it no longer a square wave. That is why it sounds different.

Bob

7. ### Muhasaresa

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Jan 2, 2012
So if the waves are AC how come it works with a buzzer. I thought buzzers only worked with DC because of the electromagnet and contact?

8. ### BobK

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Jan 5, 2010
A buzzer, fed a pure AC signal, would act like a very poor speaker, reproducing the frequency of the signal. But a 0 to 10V square wave is not a pure AC signal, as I have said twice before, it is a signal containing both and AC and a DC component. In this case the average voltage might be enough to make the buzzer operate somewhat like it would on 5V DC. Also the coil of the buffer is a low pass filter, removing some part of the AC signal.

Why would you want to put a square wave through a buzzer? You will get sound if you put it to a speaker (through a capacitor to block the DC part of the signal)

Bob

9. ### Muhasaresa

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Jan 2, 2012

Thanks! I did a mini test with what you suggested (a speaker) and it worked . I used a capacitor to filter out the DC component so I get only AC. However, when I draw up a diagram, I remembered I was sill using a batter as a power source. So does that mean the capacitor that blocks the DC overpowers the battery and sucks electrons back from ground?

Muhasaresa

P.S. Please say if you want my diagram and I will attach it.

Last edited: Feb 3, 2012
10. ### BobK

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Jan 5, 2010
When the output is high, The capacitor is charging as current goes thorugh the speaker to ground. When the output turns low, the side of the capacitor that is connected to the speaker has become negative and the thus the voltage on the speaker is negative and the capacitor discharges through it. So you get a true AC signal, going both positive and negative even though the battery only produces a positive voltage with respect to ground. It is magic!

Bob

11. ### Laplace

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Apr 4, 2010
My first thought on reading that was, "No, it's not magic; it's just physics." It also illustrates the difference between engineers and physicists. The linear circuit analysis textbooks that engineers study introduce the capacitor as an ideal circuit model along with the differential equation governing the operation of the capacitor model. That is all an engineer really needs to know. But my physics textbook introduces the capacitor as parallel conductive plates in an electric field and describes the movement of electric charge to the plate surface. The net charge in a capacitor never changes, just the distribution of charge from one plate to the other changes. It is a surface phenomenon only, because an electric field cannot exist inside a conductor. Then it introduces a dielectric material between the conductive plates and describes the creation of induced dipoles for non-polar dielectrics, and the molecular realignment of permanent dipoles for polarized dielectric material in an electrostatic field. ...Maybe it is magic.

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Maybe it is.

And for his next trick, Laplace transforms himself!

Last edited: Feb 4, 2012
13. ### Muhasaresa

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Jan 2, 2012
Yey !!! Thank you

14. ### BobK

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Jan 5, 2010
The magic comment was intended as humor after I had just explained how you can get a pure AC signal from a DC source. The OP was having trouble with that concept.

Bob

15. ### Muhasaresa

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Jan 2, 2012
I would disagree. I think magic makes up 34% of electronics. Capacitors are responsible for 15% of magic.

Muha