Sorry if my title sounds confusing. To elaborate further. In my garage I have a junction box with three sets of Romex wires tucked into the box with a flat cover plate. (no outlet at this box). After removing the cover plate and when using my voltage tester two of the black wires light up when an in-wall timer they are connected to in another location is in the "ON" position. The third one does not. This in-wall timer turns on our outdoor entryway light and outdoor garage lights at the same time. I was hoping to install an outlet at this junction box in my garage so that I can plug in a transformer to turn on my landscape lights at the same time the in-wall timer turns on the entryway and garage lights. When I turn on the in-wall timer manually, the wires become hot. And when I turn them off there is no detection of voltage at this junction box with three Romex wires.
I tried using a GFCI outlet with the third prong hole facing down and installed the black wires (that were hot when I turned the power on at the in-wall timer) to the brass screws (one wire per hole). And the two coordination white wires at on the side with the silver screw. I then used the bottom brass screw to connect the wire that wasn't hot when the power was on along with the white wire to the silver screw at the bottom. It's not working and all I see is the bottom red light (which should be green to let you know it's good to go and has power. Otherwise the outlet does not provide power.
How do I wire this correctly in order for this outlet to serve as a power source that only turns on when my in-wall timer sends power through these three wires?