Abstract: An ambient light responsive switch for street lamps or other loads, includes a thermostatic switch of the thermal time delay relay type, connecting a load to a current source and exposed to a heater resistor and snap opening and closing at relatively high and low temperature values. An ambient light exposed photoconductor, a first resistor and the heater resistor are series connected across the current source, and the first resistor and a second resistor are series connected across the switch. With the closing of the switch in response to a low light value the load is energized and the resistance in series with the heater resistor is accordingly lowered to increase the heater current. With the opening of the switch in response to a high light value the load is deenergized and the resistance in series with the heater resistor is accordingly increased to decrease the heater current and reduce the sensitivity of the switch.