Build a nearly 50% duty cycle, adjustable oscillator using D-FF
try this one:

two-inverter oscillator based around RS flip-flop

The ~Q output reinforces the R input via the capacitor, but the Q output opposes it via the resistor.The capacitor wins in the short term, but is soon over-come by the resistor. Thich cause the outputs 'Q' '~Q' to change state and the reverse to happen.

Adjusting the resistor adjusts the time taken for the resistor to win and thus the frequency. 

The mysterious 1meg resistor stops excessive current flowing into the R input. this is needed because the voltage on the resistor end of the capacitor exceeds the supply voltage and goes below ground.

By swithing the capacitor you can have two frequency ranges that differ by a factoir of about 10000.

The 10u resistor is just there to keep the simulator happy, in real life just use a wire.