if you want to control the speed of motor then follow the logic
use a LDR(light dependent resistor)
commet one end to Vcc and other end to a 10K resistor. other end of resistor to ground.
take out put from LDR and resistor interface
give it to an ADC
to drive the stepper motor use ULN2003 IC
read adc
motor port =0x01;
delay(adc value);
motor potr=0x02;
delay(adc value);
motor port=0x04;
delay(adc value);
motor port=0x08;
delay(adc value);