Here is a simple variable frequency circuit based on timer NE555. Here NE55 is wired as an astable multivibrator, whose output frequency can be varied by varying a potentiometer. This circuit is a must in the work bench of a electronic hobbyist. Frequencies ranging from several Hz to several KHz can be obtained using this circuit. For very low frequencies (few Hz) replace C with a higher value electrolytic capacitor. The low frequency oscillator schematic is shown below.
Circuit diagram with Parts list.
The values of R and C can be obtained using the following equations.
1/f = 0.69 * C * ( R1 + 2*R2).
% duty cycle = 100*(R1+R2)/(R1+ 2*R2) .
The good option is to select R1 in K Ohms and R2 in M Ohms.