A device includes an antenna configured to be disposed within a cavity of an appliance. The appliance includes an electrode and the antenna includes a sheet of conductive material having a surface area that is equal to or greater than a surface area of the electrode. The device includes a voltage sensor coupled to the antenna, an output device, and a controller coupled to the voltage sensor and the output device. The controller is configured to generate an output at the output device. The output is determined by a voltage of the antenna.