Addressable transducer with a variable frequency oscillation for monitoring a physical quantity

An addressable transducer for use in a monitoring system of the type including a central station and multiple addressable transducers for monitoring the value of selected physical conditions at multiple locations with one of said transducers at each of said multiple locations,

• said system being characterized by a single pair of transmission lines extending from the central station to all of said multiple locations and having a voltage thereon which is modulated at controlled intervals to produce successive address signals for selectively addressing one of the multiple transducers to obtain a data signal therefrom,
• said transducer comprising:
• an address detector coupled with said pair of lines and adapted to produce a control signal in response to a predetermined address signal,
• a current sink oscillator having a frequency determining circuit for determining the oscillator frequency and being adapted to oscillate in response to application of an operating voltage,
• a sensor including a condition responsive impedance element coupled with said frequency determining circuit and adapted to modify said oscillation frequency according to variations in value of a selected physical condition,
• switching means for operatively coupling said oscillator to said pair of lines for deriving an operating voltage therefrom,
• said switching means being responsive to said control signal for coupling said oscillator to said pair of lines,
• a voltage supply circuit connected across said pair of transmission lines through said switching means,
• said voltage supply circuit being coupled with said oscillator to provide an operating voltage for the oscillator,
• and timing means operatively coupled between said oscillator and said switching means for actuating said switching means to remove said operating voltage at a predetermined time interval after it is applied,
• whereby said oscillator draws current from said pair of lines, said current having an undulating amplitude at the frequency of the oscillator whereby the frequency of the undulating current in said lines at said central station corresponds to the value of said physical condition.