All About Circuits Forum Sine wave with 555 timer and RLC circuit
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#1
05-22-2011, 05:46 PM
 peter_morley Senior Member Join Date: Mar 2011 Posts: 179
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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#2
05-22-2011, 06:02 PM
 Kingsparks Senior Member Join Date: May 2011 Location: Thaxton, Virginia USA (GMT -5) Posts: 118
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."
#3
05-22-2011, 06:24 PM
 DickCappels Senior Member Join Date: Aug 2008 Location: Thailand Posts: 847

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.
#4
05-22-2011, 10:36 PM
 peter_morley Senior Member Join Date: Mar 2011 Posts: 179
Filter

Would the filter look this this?...attached schematic and graph. A to D are assigned as larger amplitude to smaller amplitude of waveforms.
Attached Images
 RC filter.jpg (137.5 KB, 225 views)
__________________
"I think it's much more interesting to live not knowing than to have answers which might be wrong."
— Richard P. Feynman

The faster you move, the slower time passes, the longer you live.
#5
05-22-2011, 11:08 PM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,022 Blog Entries: 5

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.

My Cookbook

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#6
05-23-2011, 02:37 AM
 peter_morley Senior Member Join Date: Mar 2011 Posts: 179

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.
__________________
"I think it's much more interesting to live not knowing than to have answers which might be wrong."
— Richard P. Feynman

The faster you move, the slower time passes, the longer you live.
#7
05-23-2011, 03:19 AM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,022 Blog Entries: 5

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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"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
#8
05-24-2011, 12:07 PM
 THE_RB Senior Member Join Date: Feb 2008 Posts: 5,182

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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#9
05-24-2011, 06:38 PM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,022 Blog Entries: 5

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
__________________
..
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
#10
05-25-2011, 03:22 AM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

Quote:
 Originally Posted by peter_morley 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.

 Tags 555, circuit, rlc, sine, timer, wave

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