LOL good one... yeah it is VERY loud ... you'd think there was a semi in the house .... my electrical background is very poor it's been ages since school and I'm not even sure I know what a "common ground" is... the relay has 5 pins 30, 85,86, 87 and 87a (in the middle). Every article I read on relays tells me something different and then people post saying that the article is wrong, I have yet to find two articles that agree on which pin is what. From what I gather the 30 in the signal input for the Arudion and 85 and 86 are the coils pos/neg I'm using an old ATX power supply (the yellow and black wires, 12volts) to sound the horn. Don't know where to hook both grounds, one coming from the power supply should go to the horn, would I connect the 5v Arduino ground to the horn also? Forgive me for being so confused, just not sure how everything goes.