This is my first electrical project ever, so I'll also try to make this instructable nice for other beginners to follow.

I live in a small house where the laundry machine couldn't fit into the bathroom so it had to stay in the hallway. The water pipes have to go from the bathroom door, therefore it can not be closed or locked. I'm about to have a party and I don't want anyone walk in on someone else in the bathroom, so I'm attaching a hallway curtain and this traffic light sign near the door so the light will indicate when the restroom is in use. People will operate the switch themselves as they come and go.

This sign either has a red or green light on so when I'm not using it, I'll simply take the battery off. Feel free to use this idea for other similar signs, or if you're handy, add a separate on-off switch to the circuit.

I'm not going to live in this place for very long and I want this to be a very cheap project so I don't need the housing to be fancy, I just need it to work. So I grabbed a cheap milky white freezer container because it doesn't require serious tools to make holes into. At first I thought that I'd just want some milky material to diffuse the LED lights inside and only the switch would be attached to the surface, but as the LEDs are very close to the plastic, their light looked quite spot-like and I didn't like it so I ended up having the LED heads pierce through the plastic altogether. If you choose an opaque container, be prepared to drill correct sized holes for the LEDs to come through. If you choose a somewhat translucent one, the LEDs can be inside there. Be sure to pick a hard enough material so it doesn't bend all too much or break when the switch is operated, the point of the housing is to crucially hold that component in place.

Also, one thing I thought of but wasn't bothered with for now was, if you don't like how the LEDs look, get some acrylic door knobs or acrylic "diamond beads" to install in front of the LEDS, to the housing. They'll spread the light around into a beautiful and impressive mosaic.

Step 1: Requirements

Materials you'll need:

Standard 5mm LEDs, one green and one red
One 9V battery
A holder for the battery
A three-prong rocker switch
One resistor, 470Ω ±5% (striped gold, brown, purple and yellow)
Some wire
A tiny soldering board
Epoxy glue and/or Blu-Tack
A container box of choice
(Depending on the container) the appropriate accessories you'll need for fixing the container to a wall.

Tools you'll need:

Wire cutters
Box cutter knife
Soldering iron
Some soldering tin

Not mandatory but very nice:

A multimeter for testing current
Electrical tape
A few crocodile clip cables for testing the circuit before soldering stuff into place
(Acrylic beads, door knobs or such to create a more interesting looking light lens)

Step 2: Creating the Circuit

Before I soldered anything, I used crocodile jump wires to see that this circuit actually works and the LED bulbs are alright. At some point I did something wrong and burned one green LED dead, so it was good that I had spares and did this testing, it would've been a nasty surprise after everything had been soldered!
If you're certain that you can do this without testing, go on.

Think about the housing of the sign. Make markings to it to indicate where you'd want the switch and LEDs to be.

Once you know, plan what'd be a logical juxtaposition for the components to be on the soldering board so you won't have too much wires crossing and the project will be easy to handle. Before things get messy, it's a good idea to take notes of your plan so you can follow through. My soldering board had numbers on one edge and letters on another, so it was easy to talk about the holes with their code names in my notes.

This image is how the circuit should be connection-wise.

Step 3: Putting It Together

I placed the circuits into their places on the soldering board. With wires, it helps to leave some excess so you can sometimes entwine the copper to attach to something temporarily, makes it a lot easier to handle and sometimes even eliminates the need to solder any tin. It can also help to use electrical tape to hold stuff in place when you work. I left it there.

When everything's ready, you can start soldering. I prepared the board first and soldered in the resistor, then the battery cables.

Before soldering the LEDs, I measured the rocker switch, drew its shape to the housing and carefully cut it out with a box cutter.

I did the same for the LEDs later on as I realized that I wanted them to actually stick out.

My rocker switch prongs had holes in the middle so I didn't solder it at all, I just twirled the copper around them as loops. I placed the switch to the housing where it belongs.

Then I just soldered the rest of the wires and the LEDs.

BE CAREFUL WITH THE END!!! The wires do visually connect quite counter-intuitively: If you have a green button on the right side, you have to have a green LED on the OPPOSITE side! And same for the red. If you have greens on one side and reds on the other, your switch will work the wrong way around. It doesn't affect the use of course, but it's just counterintuitive and a bit annoying.

For now, I used Blu-Tack to hold the circuit temporarily in place inside the housing, but I will apply some Epoxy glue somewhere there because it's crucial that the LED bulbs stay in their holes in the housing.

I'm going to need some properly covering black paint or ink to write "WC" to the sign.

Lastly, fix it to the wall.

Light-weight and cheap!