We are planning the modification of a Paradise P1 SLSA for Deaf Pilots. Because pilots who are Deaf will be flying mainly into uncontrolled airports we think a collision avoidance system is important. The audio alert of course will do no good for a Deaf pilot. What kind of LED could we plug into the audio output to give visual signal?

There is no "indicator light" output on the XRX (although it is a nice idea). When the XRX detects a real close target, that triggers the voice announcement, the display also flashes. Since the display is LCD, I admit that the flashing is easy to miss, unless you are looking at the display, at the time.

Not sure what kind of voltage level comes out of the audio line when there is an alert, but you should be able to harness it and turn it into a light. You could put the "audio line out" on the base of a transistor and when there is a voltage increase, it would forward bias that transistor and provide a ground for a light through its collector/emitter. This page shows how to use a transistor as a switch, and if you have someone in your group that's handy with electronics, this could be done in a few hours for less than $5, including case, light, and electronic components.