Patent US 4,875,497

A capacitance liquid level or volume sensing and touch control system for use with a conductive liquid having some isolation from and a capacitance to ground in accordance with its volume, is characterized by a circuit for cyclically charging the liquid a predetermined amount and then discharging it for a fixed time through an impedance. The extent of discharge of the liquid at the end of the fixed time is sensed, and is used to generate a signal representative of the volume of the liquid and its level. The system also has a capacitance touch control circuit, which generates an output signal when a sensor probe is touched or closely approached by a user. The signals from the capacitance liquid level sensing circuit and the touch control circuit are applied to a discriminator circuit which, when a signal representative of a sensed minimum volume of liquid is present, enables outputs from the touch control circuit to operate peripheral equipment, such as a pump for recirculating the liquid. Preferably, a slow reacting feedback circuit maintains the sensitivity of the touch control circuit substantially constant despite relatively slow changes in the capacitance exhibited by the sensor probe due to naturally occurring conditions.