Hi folks, I've already built a 12V DC relay keying interface (buffer) for a Yaesu FL-2100B amplifier and it works a treat. I've tried to build one based around a 4N35 optoisolator without much joy. I'm no whiz kid with electronics so please be patient with me, but I guess I'm trying to figure a way of using the optoisolator to switch the amp's negative voltage to ground (-16V/125mA). I don't know if adding a switching transistor at the output of the 4N35 would help, I've got a BC548B available. http://i6.photobucket.com/albums/y218/merlin100/1.jpg http://i6.photobucket.com/albums/y218/merlin100/Picture011-2.jpg http://i6.photobucket.com/albums/y218/merlin100/Picture009.jpg Any ideas would be most helpful. 73, Ian - MM0IMC. ps: It's my club's amp, so I can't get to test the circuit until I do my fortnightly visit.