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Analog photosensor operating on the power from a standard 4-20 ma instrumentation current loop

  • US 5,254,846 A
  • Filed: 02/24/1992
  • Issued: 10/19/1993
  • Est. Priority Date: 06/01/1990
  • Status: Expired due to Term
First Claim
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1. Photoelectric measuring and indicating apparatus, comprising:

  • light emitting means, coupled to a current source, for transmitting light having an intensity which varies with a current supplied thereto;

    light responsive means for receiving light and producing a signal indicative of the intensity of the received light;

    control means for controlling a current from said current source supplied to said light emitting means as a function of the intensity signal produced by said light responsive means, so that the current supplied to said light emitting means is at a maximum magnitude when the light received by said light responsive means is at a minimum magnitude, and the total current supplied to said light emitting means is at a minimum magnitude when the light received by said light responsive means is at a maximum magnitude, and the total current drawn by the apparatus is an inverse analog function of the intensity of the received light;

    said control means comprising a timing circuit operable in response to the voltage of the intensity signal such that said timing circuit draws a DC current from said current source having a magnitude that varies inversely with the magnitude of the voltage of the intensity signal; and

    said control means further comprising means, coupled to said timing circuit, for selecting a first resistance to draw a DC current through said apparatus having a maximum magnitude when the intensity signal has a minimum magnitude, and for selecting a second resistance to draw a DC current through said apparatus having a minimum magnitude when the intensity signal has a maximum magnitude.

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