Help needed finding oscillator values

Jul 09, 2008 1 Replies
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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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T

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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