12VDC transformer / relay / alarm panel -- need a capacitor and resistor!

I'm holding a relay coil shut with a 12VDC / 500ma transformer (wall wort). The output of the relay is connected to a normally closed zone in my alarm system. I use this to tell me if the power has gone off while I'm not at home. However, with the transformer and relay only, I get no "power off" delay before the... show more I'm holding a relay coil shut with a 12VDC / 500ma transformer (wall wort). The output of the relay is connected to a normally closed zone in my alarm system. I use this to tell me if the power has gone off while I'm not at home. However, with the transformer and relay only, I get no "power off" delay before the zone goes into alarm. Ideally, I'd like to get about a 30-60 second delay so that it won't go into alarm for short power outages. I don't have any way to do this through the alarm panel, so I need an electronic solution. I'm aware of a device which already does this (http://www.smarthome.com/7154.html) but I think I can do it cheaper with a capacitor (2, 3 .. ?) and a bleed resistor. Does anyone know the math and/or can you provide an answer to what size caps I need?
4 answers 4