Simple voltage controlled oscillator
With the component values shown, the oscillator has a frequency of 8 kHz. When an input signal is applied to the base of Ql the current flowing through Ql is varied, thus varying the time required to charge Cl. Due to the phase inversion in Ql the direction of output frequency change is 180 degrees out of phase with the input signal
The output may be used to trigger a bistable flip-flop.