Design and analysis of a low cost capacitive liquid level sensor

This paper presents a concept for a low cost, capacitive liquid level detector for use in under hood automotive applications. Current sensors for liquid level detection employ float switch technology. This technology is highly vibration sensitive, and limited in cycle life due to the contacting nature of the reed switch. The proposed sensor detects the presence of liquid by discriminating the change in dielectric constant of the media on the sensor probe. A solid state output is provided eliminating cycle life problems. The entire sensor is constructed from a printed circuit assembly and two plastic parts which are ultrasonically welded together. A patented two shot molding process is employed to manufacture these plastic parts. The two shot process enables selective plating of the surface of the plastic, thus allowing a very inexpensive way to construct intricate sensor and connector assemblies. All electrical interconnects are made using the selective plating, greatly simplifying the design and assembly process. The proposed low cost capacitive sensor uses far less parts, offers much improved useful product life and greater reliability at competitive cost.