Abstract: A sensor timer is provided for automatically turning on and off a lamp or alternatively for allowing the lamp to be manually turned on and off using the switch normally associated with the lamp. The sensor timer automatically senses the arrival of nighttime despite seasonal variations in the actual time of nightfall and automatically turns on the lamp a first selected time period after nightfall. The lamp is then automatically turned off a second selected time period after the lamp has been turned on or alternatively is turned on and off randomly during the second selected time period. The sensor timer can also dim the lamp. The sensor timer converts from automatic mode to manual mode in response to the turning on of the lamp through its manual switch thereby to allow the lamp to be controlled directly by the manually operated lamp switch. To convert the sensor timer from manual mode to automatic mode the lamp switch is turned on, off and on in rapid succession.