| 摘要 |
An automatic switch (3) for an electric domestic appliance (1, 13, 33, 36) is provided. The switch comprises a base (4) and associated electronic circuitry (8) for inclusion in an electric power supply circuit that includes a heating element whereby liquid in a container may be heated to boiling temperature. The switch includes a sensor (7, 18) for monitoring radiation emanating from the surface (6) of liquid to be boiled in a container and preferably also a radiation source (5, 19) to produce radiation that is reflected off the liquid surface. The sensor is configured to provide a first output associated with radiation sensed whilst a generally quiescent state of the surface prevails, and to provide a second output associated with radiation sensed when a generally turbulent state of the surface of the liquid prevails that is associated with the onslaught of boiling of the liquid. The change of state from the first output to the second output is operative to trigger a switching action in the electronic circuitry to open the electric power supply circuit.. The electronic circuitry may include a first time delay arranged to open said electric power supply circuit a predetermined time period after said change of state from the first output to the second output and may also include a second time delay adapted to open said electric power supply circuit after a predetermined period of time in excess of a predetermined maximum time period permitted by the timer for heating liquid in the container. |
