Details of the circuit

For those who like to know more detail, the 555 timer is operating in Astable mode.

This is the typical circuit for 555 timer operating in astable mode. This means the output will be turning on and off. When it's on, it will supply the same voltage as the supply voltage to power the 555 timer chip.

Duration of output on (sec) = 0.693* (R1 +R2) * C
Duration of output off (sec) =0.693* R2 * C
Frequency = 1.44/ ((R1+R2+R2) * C)
Duty Cycle = Duration of output on / (Duration of output on + Duration of output off)

The most important thing in my application is the duty cycle. As the duty cycle increases, my heater will be hotter.  As you can see, the capacity value get cancelled out when calculating duty cycle. so the size of the capacity will not affect the duty cycle, only the frequency.

One has to vary the resistance of both R1 and R2 to get the widest range of duty cycle, from 0 to 100%. and the easiest way to do this is replacing both R1 and R2 with a single variable resistor.