i made some speaker cables and i got paranoid about having a broken speaker cable by accident between my head and cab. not so much at home, but on tour or something. i thought something like this would be handy and it cost pretty much nothing to make. the 9v battery clip i had lying around, same with the tin, and the jack sockets. the cable is cloth covered cable from a guitar, and the stripboard, LED's and 2K resistors are the only things i bought.
oh yeah. the inside of the tin in insulated otherwise there is a permanent sleeve connection.
now i still need to drill the holes in the tin!