Hobby Electronics Basics Looking to modify this circuit for 30Hz operation

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Looking to modify this circuit for 30Hz operation stephen 07-18-08
Posted by on July 18, 2008, 10:10 am
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I was given the parts to build this sine wave generator. As far as I
can tell it's a Wein Bridge with the two LEDs serving the function
originally reserved for the incandescent lamp.

http://www.stephenhobley.com/sinewave.gif

Unfortunately it's fixed frequency, the 5K variable only trims one
"side" of the wave, not selecting the fundamental as I'd hoped. It
looks liket the 68K resistors and 0.047uF caps determine the period of
oscillation (one pair for each half).

Does anyone know the mathmatical relationship between the values of R5/
R6 and C1/C2 and the frequency of oscillation? I'd like to get this
working in the 30Hz range.

Thanks,
Steve

Posted by John Popelish on July 18, 2008, 10:24 am
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stephen@stephenhobley.com wrote:
> I was given the parts to build this sine wave generator. As far as I
> can tell it's a Wein Bridge with the two LEDs serving the function
> originally reserved for the incandescent lamp.
>
> http://www.stephenhobley.com/sinewave.gif
>
> Unfortunately it's fixed frequency, the 5K variable only trims one
> "side" of the wave, not selecting the fundamental as I'd hoped. It
> looks liket the 68K resistors and 0.047uF caps determine the period of
> oscillation (one pair for each half).
>
> Does anyone know the mathmatical relationship between the values of R5/
> R6 and C1/C2 and the frequency of oscillation? I'd like to get this
> working in the 30Hz range.

To change the frequency, you have to (I think) change the
values of R5 and 6, simultaneously. Double both of these
and the frequency should fall to about half.

If you want to make a variable oscillator, you need a ganged
pair of pots that track each other well.

--
Regards,

John Popelish

Posted by on July 18, 2008, 10:27 am
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> step...@stephenhobley.com wrote:
> > I was given the parts to build this sine wave generator. As far as I
> > can tell it's a Wein Bridge with the two LEDs serving the function
> > originally reserved for the incandescent lamp.
>
> >http://www.stephenhobley.com/sinewave.gif
>
> > Unfortunately it's fixed frequency, the 5K variable only trims one
> > "side" of the wave, not selecting the fundamental as I'd hoped. It
> > looks liket the 68K resistors and 0.047uF caps determine the period of
> > oscillation (one pair for each half).
>
> > Does anyone know the mathmatical relationship between the values of R5/
> > R6 and C1/C2 and the frequency of oscillation? I'd like to get this
> > working in the 30Hz range.
>
> To change the frequency, you have to (I think) change the
> values of R5 and 6, simultaneously. =A0Double both of these
> and the frequency should fall to about half.
>
> If you want to make a variable oscillator, you need a ganged
> pair of pots that track each other well.
>
> --
> Regards,
>
> John Popelish

Thanks John,

Any idea of the approx values to get this near to 30Hz?
I could just make it up with sockets and keep plugging away, but I
wondered if the math was relatively simple.

Posted by John Popelish on July 18, 2008, 10:52 am
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stephen@stephenhobley.com wrote:

> Any idea of the approx values to get this near to 30Hz?
> I could just make it up with sockets and keep plugging away, but I
> wondered if the math was relatively simple.

If I recall, the basic formula is f=1/(2*pi*R*C) for each of
the RC pairs in the positive feedback loop, so with C being
4.7nF and f being 30 Hz, R would be 1.13 meg. It might be
better to use a larger pair of capacitors so that the
resistors could be proportionately smaller.

Any RC pairs that have an R*C product of 1/(2*pi*30)=0.00531
seconds would produce 30 Hz. For instance
1.13meg*4.7nF=0.00531 but so does 53.1k*100nF.

100nF=0.1uF.


--
Regards,

John Popelish

Posted by Phil Allison on July 18, 2008, 10:44 am
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> I was given the parts to build this sine wave generator. As far as I
> can tell it's a Wein Bridge with the two LEDs serving the function
> originally reserved for the incandescent lamp.

** Yep.


> http://www.stephenhobley.com/sinewave.gif
>
> Unfortunately it's fixed frequency, the 5K variable only trims one
> "side" of the wave, not selecting the fundamental as I'd hoped.


* It acts as a fine frequency trim only.


> It
> looks liket the 68K resistors and 0.047uF caps determine the period of
> oscillation (one pair for each half).


** The cap is actually 0.0047 uF.

Makes the frequency just on 500Hz.

F = 1 / 2.pi.R.C


> Does anyone know the mathmatical relationship between the values of R5/
> R6 and C1/C2 and the frequency of oscillation? I'd like to get this
> working in the 30Hz range.


** Change C1 & C2 to 0.082 uF ( 82nF)

Gives you about 29 Hz.



... Phil






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