Abstract: An amplifier system includes an operational amplifier having a reference applied to one input terminal. A voltage above or below the reference voltage is applied to the other input terminal by way of an interface circuit. The interface circuit adapts the input for use with a switch or, alternatively, the interface may be disabled to permit use of a variable resistor input such as a thermistor. The output of the operational amplifier may be a switching circuit. The amplifier system is adapted for use in a fluid level system to permit the alternate use of a switch type or thermistor level sensor.