Re the above question: depends on whether it's an input intended to receive a signal meant for an LED, or it's an input from an IR sensor.

The IR sensor type input will supply power, and also have ground and IR signal. The emitter signal will just have signal and ground.

It's best to use a three pin plug because if you're trying to connect an emitter while the jack is set up to supply power to a sensor, you might blow the circuitry inside the component. Of course, you might just as easily blow the LED if the tip is DC intended to work a sensor.

The upshot is: if at all humanly possible, know what you're working with. And, admittedly, sometimes it's impossible to tell. All you can do is try something.