Patent ID: 7830155

Claim:
A two-wire active sensor interface circuit comprising, a constant current circuit adapted to be coupled to a two-wire active sensor for receipt of a sensor current signal indicating one of two sensor states, the constant current circuit having a preselected constant current amount positioned between the two sensor states that varies the sensor current signal thereby generating a current level indicator signal; and, a digital interface circuit operably coupled to the constant current circuit for receipt of the current level indicator signal and operative to produce an interface output indicating which of the two sensor states is present; wherein the constant current circuit drains a current amount from the digital interface circuit whenever the sensor current signal is less than the preselected constant current amount; and, wherein the constant current circuit comprises a first transistor, wherein the digital interface circuit further comprises a second transistor comprising a control terminal that communicates with the first transistor without communicating with an intervening transistor, wherein the second transistor is a PNP transistor, and wherein the interface output is provided at a collector terminal of the second transistor; and a diode and a resistance that communicate in parallel with an emitter terminal and a base terminal of the second transistor.