If I remember right (I am speed reading here) the 791 is an amplified block. That means that it needs a power supply. It has a jack for a sensor.

I think someone suggested connecting an IR output to the input of this block. This is correct, but do not take the elegant approach of using a mini plug into the sensor input to make this connection. That will short the power supply. Just use the screw terminals.

Also, I believe the tip of mini plugs is the plus, not the minus.