Abstract: The accumulation of ice on a surface such as an evaporator fin in a refrigerator unit is detected by a mechanical probe which is moved by a bimetal bender element upon periodic timer-controlled energization of a bimetal heater so as to pass over the surfaces of an evaporator fin. If the movement of the probe is obstructed by ice on the evaporator fin surfaces a switch is operated by a bistable mechanism, causing energization of a defrosting heater and switching off the refrigerator compressor motor. If no ice is detected the probe returns to its original reset position without operating the defrost control switch.