A program to measure the output from a photocell sensor.

photocell.py

import RPi.GPIO as GPIO, time, os

DEBUG = 1
GPIO.setmode(GPIO.BCM)
BLUE_LED = 23
GPIO.setup(BLUE_LED, GPIO.OUT)

def RCtime (RCpin):
    reading = 0
    GPIO.setup(RCpin, GPIO.OUT)
    GPIO.output(RCpin, GPIO.LOW)
    time.sleep(0.1)
    GPIO.setup(RCpin, GPIO.IN)
    #This takes about 1 millisecond per loop cycle
    while (GPIO.input(RCpin) == GPIO.LOW):
        reading += 1
    return reading

try: 
    while True:
        lightlevel = RCtime(18)
        print lightlevel
        if lightlevel > 1000: 
            GPIO.output(BLUE_LED, True)
        else:
            GPIO.output(BLUE_LED, False)

except KeyboardInterrupt:
    GPIO.cleanup()
    print "Clean program exit."