Press and hold the switch until the voltage reaches ~ 2.5V to start oscillation....
To vary the frequency alter the potentiometer resistance ... Don't increase the slider position above 45% the oscillation will stop due to a low Vgs of the transistors....
Here I used an NMOS and a PMOS transistors as a voltage controlled resistors, the control voltage taken from opamp square wave output. So when the opamp output is 15V the NMOS transistor starts to conduct and charge the cap, and when it is -15V the PMOS transistor starts to conduct and discharge the cap....
The frequency change is achieved by altering the Gate voltage of both transistors equally (by the help of the potentiometer at the right of the schematic) so that both transistors have equal resistance in their conducting region which yields a 50% duty cycle output.....
Suggestions are welcome
