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Presence detecting system with self-checking

  • US 3,970,846 A
  • Filed: 08/15/1974
  • Issued: 07/20/1976
  • Est. Priority Date: 10/29/1973
  • Status: Expired due to Term
First Claim
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1. A presence detection system comprising a plurality of light source and photodetectors arranged in plural sets with a light source and a photodetector in each set, the source and photodetector of each set defining a channel and being disposed so that light from the source impinges upon the photodetector in the absence of an obstruction in the channel, each photodetector adapted to produce an electrical pulse when excited by a light inpulse, pulse generating means for energizing the light sources, a sender channel multiplexing means having an input connected with the pulse generating means and having plural channel outputs each connected with a different one of the light sources and having channel switching means with a channel address input, a clock connected with the channel address input for sequentially connecting the light sources to the pulse generating means, a pulse detection means having an input adapted for connection with one of the photodetectors, a receiver channel multiplexing means having plural channel outputs each corresponding with a different one of the photodetectors and having channel switching means with a channel address input, a switch connected between each photodetector and pulse detection means and being connected with the corresponding channel output of the receiver channel multiplexing means, said clock being connected with the channel address input for sequentially connecting the photodetectors to the input of pulse detection means, whereby said multiplexers are switched synchronously by the clock and said channels are sequentially activated with each channel being activated during an activation interval determined by the clock, said pulse generating means being synchronized with the clock to produce a pulse during the activation interval of each channel for transmitting a light pulse from the light source to the photodetector of the same set, said photodetectors thereby being sequentially excited by light impulses whereby said pulse detection means receives at its input a train of pulses including a pulse corresponding to each of the activated channels in the absence of an obstruction in any of the activated channels, said pulse detection means including detector means responsive to a missing pulse in said train of pulses.

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