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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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  #1  
Old 05-22-2011, 05:46 PM
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peter_morley peter_morley is offline
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Default Sine wave with 555 timer and RLC circuit

A 555 timer creates a high low pulse. What I want to do is create a sine wave from the output of the 555 timer. I have looked online and it involves a bit of calculus so I wanted to see if it was worth my time. I realize the RLC circuit connected to the output of the 555 needs to be over-damped to create my sine waveform. I'm not doing any transmitting for radio waves at this point I'm just trying to learn how create oscillator circuits. Its a useful tool to have and most of all cool!
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Old 05-22-2011, 06:02 PM
Kingsparks Kingsparks is offline
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Default Sine wave from 555

I don't know about useful, that depends on you. As far as cool, well, I think so but then I like almost anything having to do with the 555. It won't be a true sine wave but you can get it pretty close. It is real easy to breadboard then "play with," "try it, you'll like it."
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Old 05-22-2011, 06:24 PM
DickCappels DickCappels is offline
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It can be pretty simple. Just set up the 555 to make a square wave (approx 50% duty cycle) at the desired frequency. Then make a RC low pass filter using a 1K resistor and a capacitor calculated to give you a corner frequency at about 1/2 the frequency of the square wave. After that, place another RC low pass filter with the same corner frequency, but using a 5.1k resistor (so you don't load down the first filter section too badly), then follow it by a third stage with the same corner frequency, this time using a 27k resistor. That will get you a pretty good looking sine wave. You can lower the corner frequency or add more stages to get better suppression of the higher order harmonics.
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Old 05-22-2011, 10:36 PM
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Default Filter

Would the filter look this this?...attached schematic and graph. A to D are assigned as larger amplitude to smaller amplitude of waveforms.
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File Type: jpg RC filter.jpg (137.5 KB, 214 views)
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Old 05-22-2011, 11:08 PM
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Pretty much. LC works much better, but RC gets the job done. One problem though, it still won't be a pure sine wave, just very close.

You could also use the input to the 555 (where it is a triangle wave) to remove one layer of RC circuits.

You can also use integrators to make a better approximation, instead of a sawtooth you would have a true triangle waveform for example.

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http://forum.allaboutcircuits.com/sh...ad.php?t=10936
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Old 05-23-2011, 02:37 AM
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Bill_Marsden - Ok so no matter what the output will not be a perfect sine wave. Can you show me how to make a sine wave the LC way with the 555 timer...and is it possible to make a sine wave with just an inductor, capacitor and resistor without the 555 timer? So two requests here.
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Old 05-23-2011, 03:19 AM
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There are lots of ways to make high quality sine waves. Pick a frequency.

Actually, with enough filtering, you can make a very good sine wave, one for all practical purposes is pure.

The old LC resonance formula is your friend, F=1/(6.28*(LC)^0.5). I'd LaTeXt except I don't have a lot of time for this post.

Just build a LC low pass followed by a LC by a LC band pass, not necessarily in that order. LC circuits are much more precision than RC versions.

Sine Wave Oscillators A PDF abstract by TI
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Old 05-24-2011, 12:07 PM
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The big question is; do you want to change the frequency?

It's much easier to generate (and filter) sine at a fixed frequency, if you want to be able to vary the sine frequency that changes everything!
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Old 05-24-2011, 06:38 PM
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Which is why it is better to use something like a Wien Bridge Oscillator or one of the other directives.

http://en.wikipedia.org/wiki/Wien_bridge
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Old 05-25-2011, 03:22 AM
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Quote:
Originally Posted by peter_morley View Post
A 555 timer creates a high low pulse. What I want to do is create a sine wave from the output of the 555 timer. I have looked online and it involves a bit of calculus so I wanted to see if it was worth my time. I realize the RLC circuit connected to the output of the 555 needs to be over-damped to create my sine waveform. I'm not doing any transmitting for radio waves at this point I'm just trying to learn how create oscillator circuits. Its a useful tool to have and most of all cool!
It needs to be underdamped.
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