Abstract: A photocell unit in combination with a programmed microprocessor which samples the output of the photocell and detects day or night by sampling over a predetermined period of time, e.g. 80 minutes to establish a right reference value and a predetermined number of samples of increasing light values to establish a "sunrise detect value" to turn lights off. A further predetermined increase in light values is used to establish a "sunset detect" light value. This value is retained throughout the day until sensed light values again diminish to the "sunset. The sampling techniques makes the system immune from errors due to transient lights and also avoid unwanted daylight operation of lights or undetected light outages.