This seems to be some easy thing someone figured out already, but here I am after an hour of searching - and fearfully asking the question.
My aim is to build a relatively simple 555 circuit using as basic elements as possible (suitable for newbies like me) that provides a square wave with duty cycle of either >90% or <10% (adjustable duty cycle is a bonus, if it doesn't affect the complexity too much), and with the frequency adjustable in a range of about 1-200Hz by a potentiometer. (It would be used in a simple strobe, the output drives a transistor that turns on/off some 1W LED)
In the standard 555 astable oscillator circuit (see below), R2 determines the off-time, and R1+R2 the on-time. Adjusting R1 thus alters the duty cycle that I would like to avoid.
The only idea I found so far is this question, where the frequency controlling capability of a voltage applied on pin 5 is discussed. I, however, tried this with pin 5 connected to a 470k potmeter as a voltage divider from 0-Vcc in an f = 1Hz, duty cycle = 50% configuration: adjusting the voltage on pin5 only disturbs the oscillation, but does not affect the frequency noticeable.
So: What is the minimal circuit for adjusting the frequency of the 555 without affecting duty cycle?
Update: At the end I turned to the most trivial road and swapped both resistor with a suitable logaritmic potmeter... :)