# Need help on mercury wetted switches

#### isk

Joined Dec 17, 2009
5
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?

#### awright

Joined Jul 5, 2006
90
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.

awright

#### Bernard

Joined Aug 7, 2008
5,228
Amprex did or does make thermal time delay relays. Accuracy around +_ 10%; wild guess \$ 5 US.