Need help on mercury wetted switches

Discussion in 'The Projects Forum' started by isk, Dec 17, 2009.

  1. isk

    Thread Starter New Member

    Dec 17, 2009
    Hi everyone, I'm a student trying to complete a final year project. I have a problem regarding a section of my project. This involves the mercury reed switch. I would like to create a circuit that will cause a delay of approximately 5 minutes. I would like to send a signal out when the mercury reed sends a constant signal to a delay circuit that will wait for 5 minutes before sending a final signal out. Can anyone help me out?
  2. awright

    Well-Known Member

    Jul 5, 2006
    You are describing the function of a common "delayed on" time delay relay. You haven't provided much detail regarding available voltage, the current rating of the mercury-wetted relay, or the required degree of precision in the time delay, but you should be able to find a compatible time delay relay. TDRs are moderately expensive (for a student project) but depending upon your location you might be able to find one for a few dollars on the surplus/salvage market. I'd guess about $50 new or about $5 salvage.

    If you want to do it yourself, google "555." This is the most popular timer integrated dircuit and there are very many tutorials on the web with example time delay circuits.

  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Amprex did or does make thermal time delay relays. Accuracy around +_ 10%; wild guess $ 5 US.
  4. isk

    Thread Starter New Member

    Dec 17, 2009
    hey awright, thanks for your reply. yes I've done my research regarding the 555 timer. From what I've known, it only need a single trigger signal to turn on the monostable function of the 555 timer. As I'm dealing with a mercury reed switch that will give a constant highs and lows, depending on movement, is there anything that i can connect the constant outputs to, so that when the mercury reed goes low for a period of time, it will only give a single trigger signal output so as to activate the 555 timer?