Expert: cleggsan - 12/18/2006

Question
You may or may not want to answer this question, but let me give you a little background first. My roommate spends about 90% of his life playing online games. Seriously. Once in a blue moon I'll decide to download a large file overnight and, after I go to bed, my roommate decides to unplug my connection because his game might lag here and there. So here's what I want to do: tap into the low voltage DC that powers the Link light on our router and use it to trigger an AC relay. That way, if he unplugs my network cable, the Link light will go out and trip the AC relay and shut off his computer (I'm going to wire the relay into the test circuit of a GFI outlet). I'm in the IT field and have some background in DC electronics repair, but I need help with the AC relay. Is there an AC relay that will take a low voltage DC signal? I'm extremely hesitant to use a basic DC relay because I'm almost certain it can't handle even the split second charge across the GFI circuit. Can you point me in the right direction or even help me design a simple circuit? Thank you!

Answer
Relays with external contacts for switching will hand the ac just fine.  All you need is a very sensitive relay with dc coil OR an electronic switch to drive it.

If you use a double pole double throw relay you can wire the ac any way you want:  Relay ON,ac off - or Relay ON, ac on.

Here are some ideas:

http://www.axiomatic.com/relay-1a.html

http://www.hometech.com/brains/relays.html

You can find many types, sizes and applications by googling for relays of all kinds.

Cleggsan