Claulating the resistance for the IR Flame Photodiode sensor
An LM741 is severely limited in a lot of applications: -


Minimum recommended power supply rails are +/- 10 volts
Input voltage range is typically from -Vs + 2 volt to +Vs - 2 volt
Input offset voltage is typically 1 mV (5 mV maximum)
Input offset current is typically 20 nA (200 nA maximum)
Input bias current is typically 80 nA (500 nA maximum)
Input resistance is typically 2 Mohm (300 kohm minimum)
Typical output voltage swing is -Vs + 1 volt to +Vs - 1 volt
Guaranteed output voltage swing is -Vs + 3 volt to +Vs - 3 volt
Supply current is typically 1.7 mA (2.8 mA maximum)
Noise is 60 nV/sqrt(Hz) for LM348 (quad version of 741)
GBWP is 1 MHz with a slew rate of 0.5 V/us


So it looks like you are falling at the first hurdle by not supplying the 741 sufficient supply voltage.


  I need to fully control the output using the potentiometer.


I have no idea what that is meant to imply. Fix the power problems then decide what functionality you are desiring (important to specify this succinctly and with accuracy).