Time delay

Discussion in 'The Projects Forum' started by isk, Dec 22, 2009.

  1. isk

    Thread Starter New Member

    Dec 17, 2009
    Hey guys, I'm new in this forum. I have a final year project that have to be handed in soon. I have a mercury reed switch that outputs highs and lows at a constant rate. This reed switch would be worn on an elderly, if he/she was unconscious, the mercury reed would be stagnant, meaning lows all the way.
    I would like to create a circuit where something monitors the lows for approximately 5 minutes, then it would send a signal to the other corresponding part of my project. I've look into the 555 monostable state but from what I've learnt, it needs a single trigger pulse to activate. But how can i create a circuit to disregard a train of pulses(when the elderly is moving) but instead acknowledge the lows(when the elderly is unconscious)?

    Thank you,
  2. Duane P Wetick

    Active Member

    Apr 23, 2009
    The 555 IC can be used to detect a missing pulse in a train of pulses. Extending things out to a long term, maybe 5 minutes, without any pulses says the person is un-conscious...or dead.

    Cheers, DPW [ Everyone's knowledge is in-complete...Albert Einstein.]
  3. isk

    Thread Starter New Member

    Dec 17, 2009
    Hi Mr Duane P Wetick, can you elaborate more regarding your answer? :)
  4. SgtWookie


    Jul 17, 2007