LM555 as variable frequency oscillator

[xyz9915]

Hello!
I need a circuit in which the frequency of LM555 astable multivibrator can be controlled externally I mean by the help of transistor etc. So, please guide what modification may be done in the attached circuit

 

[bwaslo]

Look into replacing resistors with something like MCP4012.

If you want voltage control, don't use a 555, use a CD4046/74HC4046, a better chip for a variable oscillator, anyway. Use 555 only if you need to vary duty cycle. For most audio uses you'll want 50% duty cycle and something like the 4046 is more appropriate and has more versatile control.