Simple variable frequency oscillator

This is a very simple circuit utilising a 555 timer IC to generate square wave of frequency that can be adjusted by a potentiometer.
With values given the frequency can be adjusted from a few Hz to several Khz.To get very low frequencies replace the 0.01uF capacitor with a higher value.
The formula to calculate the frequency is given by:
1/f = 0.69 * C * ( R1 + 2*R2)The duty cycle is given by:
% duty cycle = 100*(R1+R2)/(R1+ 2*R2)
In order to ensure a 50% (approx.) duty ratio, R1 should be very small when compared to R2. But R1 should be no smaller than 1K.A good choice would be, R1 in kilohms and R2 in megaohms. You can then select C to fix the range of frequencies.