changes required for the PWM circuit (*)

[itsmyturn]

I am using the PWM circuit of the below link to control the speed of a line-following robot. Here i have replaced the 100K pot (VR1) with an LDR(light dep. resistor) of the robot. But it is not giving any speed variation. Instead , it is giving some unusual results like: when the supply is given, the drop across the resistor R8 is zero but the motor is running. But i have one confidence that the resistance of LDR is not being compatible with the circuit. Can anybody help me out how to DEBUG this circuit.......



http://www.cpemma.co.uk/pwm.html