Electronics Forum / Basics / July 2008

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.