At the heart of the Full-Cup Detector is a 555-oscillator/timer configured to produce a 15-Hz click, until
its probe contacts are bridged, at which time its output frequency goes to about 500 Hz. This circuit can be used by the visually handicapped to determine when a cup or bowl is full of liquid (coffee, soup, etc). Ul, an NE555, produces ticks at 15 Hz. A set of probes (wire, etc.) is placed in the container at the desired level. When the liquid level contacts the probes, the frequency of clicks increases to several hundred hertz, depending on its conductivity.