Capacitive sensor

An electrographic printing machine of the type having an apparatus for detecting the level of material in a container, said apparatus comprising:

• a power source;
• a first electrode, positioned adjacent the container, electrically connected to said power source;
• a second electrode, spaced from said first electrode, positioned adjacent the container;
• an electrical amplifier, electrically connected to the second electrode, for amplifying a current induced in said second electrode and for generating a voltage signal, said electrical amplifier maintaining said second electrode at a virtual ground to minimize environmental impedance effects;
• a rectifier, electrically connected to said amplifier, for rectifying the voltage signal;
• and
• a comparator, electrically connected to said rectifier for comparing the voltage signal to a predetermined voltage signal corresponding to the container being full with material, said comparator including a fail safe circuit that, in response to the voltage signal from said rectifier being of a voltage less than a first preselected reference voltage and less than a second predetermined voltage, generates a signal that the container is full.

A container of the type having an apparatus for detecting the level of material in the container, said apparatus adapted to be connected to a power source, said apparatus comprising:

• a first electrode, positioned adjacent the container, electrically connected to said power source;
• a second electrode, spaced from said first electrode, positioned adjacent the container;
• an electrical amplifier, electrically connected to the second electrode, for amplifying a current induced in said second electrode and for generating a voltage signal, said electrical amplifier maintaining said second electrode at a virtual ground to minimize environmental impedance effects;
• a rectifier, electrically connected to said amplifier, for rectifying the voltage signal;
• and
• a comparator, electrically connected to said rectifier for comparing the voltage signal to a predetermined voltage signal corresponding to the container being full with material, said comparator including a fail safe circuit that, in response to the voltage signal from said rectifier being of a voltage less than a first preselected reference voltage and less than a second predetermined voltage, generates a signal that the container is full.