Help needed finding oscillator values

[Kasterborus]

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

 

[neon]

duty=rb/ra+2rb
t1=,693[ra+rb]c
t2=.693[rb]c
f=1/tau=1.44/[ra+2rb]c
now you got it all