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Sci.Electronics.Basics -> Help needed finding oscillator values
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Author: KasterborusDate: 09:24 09-07-08
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I would like to build 2 square wave oscillators from a 555 timer using
the classic R1/R2/C1 configuration. I would like them to have a 50%
duty cycle (or as close as can be) and operate at 30 and 60Hz
respectively.
Can someone show me how to calculate R1/R2/C1 values so that I arrive
at components close to 'regular' values. So far my math has been
arriving at weird values.
Better yet, if one of the fixed resistors could be replaced with a
variable that would give me the 30-60Hz range.
Any help is greatly appreciated.
Dave
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Author: Tim WescottDate: 10:54 09-07-08
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Kasterborus wrote:
> I would like to build 2 square wave oscillators from a 555 timer using
> the classic R1/R2/C1 configuration. I would like them to have a 50%
> duty cycle (or as close as can be) and operate at 30 and 60Hz
> respectively.
>
> Can someone show me how to calculate R1/R2/C1 values so that I arrive
> at components close to 'regular' values. So far my math has been
> arriving at weird values.
>
> Better yet, if one of the fixed resistors could be replaced with a
> variable that would give me the 30-60Hz range.
>
> Any help is greatly appreciated.
>
> Dave
You have to play tricks to get a 50% duty cycle. You can parallel the
discharge resistor with a diode so that the charge current goes through
the diode and the discharge is alone; this gives you about as good a 50%
duty cycle as you'll ever get.
You can vary the frequency with a pot, but it'll vary the duty cycle as
well. You can fix this with a dual pot, but that limits your capabilities.
Consider using the 555 to drive a divide-by-two counter made from a J-K
flip flop. Run the 555 at 60-120Hz, with a relatively small discharge
resistance value to give the frequency set pot more authority. Then use
the J-K flip flop to give you a perfect 50% duty cycle.
You could even use a 4020, 4040, or 4060 counter and _really_ divide the
555 output down; this would let you run the 555 in the high-audio range
where it's happiest, and it would let you use a reasonably small
capacitor instead of the monster you'll need at 30Hz.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
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