This is my first instructable, so please bear with me for my lack of experience. It uses a 3.7V 18650 li-ion battery, which you can recharge using a wall charger. I made it with parts that I had lying around, so you will probably need to modify the design a bit to fit your needs and/or parts. BTW, if you enjoyed this instructable please vote for me in the first time authors contest!
Step 1: Find Your Parts
I wanted a low power consumption flashlight, and I had to use only the parts I had. What you will need:
A piece of PVC pipe in the desired width (or just a piece like me)
Diode ( If you want to make the design I did)
Some flashlight reflectors and lenses
2x 11 ohm resistors
A two way switch (click once, pin 1 on, click twice, pin 1 off&pin 2 on, click a third time all pins off.
A battery spring and other connector(flat)
A soda bottle top (or something else to cover the battery)
You will also need some tools such as:
hot glue gun (optional)
Heat gun or propane burner
Drill and bit
Step 2: Plan Your Flashlight
I wanted one mode where one LED is on, and another mode where all the LEDs are on. So I had to put a diode in (as seen in the picture) to make it work. I have included a much more simple circuit (the one in white), which you can use to base yours on. You will have to make your own circuit, but DO NOT FORGET the resistor. Without it, the bulbs will get hot and eventually burn out much faster the they would have otherwise. If you want to use AAs or AAAs, than just make the flashlight wider, or longer, to accommodate more batteries. Use your imagination!
Step 3: Make the Body
So I had a big scrap of PVC leftover from another project, so I had to melt it and form it around the right sized object. I used a propane burner to melt it, but you can try an oven, heat gun, or normal stovetop. Make sure you have good ventilation, and use gloves or just be careful handling it because it can get really hot. If you are using the right sized pipe, than just melt the end to install the soda bottle cap (or whatever you are using to hold the battery in). Then you will need to glue it in.
Step 4: Prepare It for Wiring
First measure, then drill the hole for the switch (It needs to be far enough from the edge the the LEDs can fit in). After that, you need to put the positive terminal inside the tube. First you need to find something to hold the terminal. I recommend something like a small piece of wood, or something that fits in the pipe very snug. After that, glue the terminal to the wood (make sure you have soldered your wire to it). Then you should measure (From the endcap) very carefully where it should be placed, then glue the it in. The battery should be touching both terminals at all times.
Step 5: Wire It
Now you have to assemble the circuit that you have designed. Solder everything together, and then glue the switch in. You also need to make the negative terminal. I glued the spring terminal to a piece off flattened PVC pipe, so that the wire which goes to the front of the flashlight (along the side of the battery) does not get wound up while screwing the cap on. Also make sure that there is some slack in the wire that comes out of the back, because it will be removed a lot. Then you should be able to put the battery in and test the circuit to see if it works. If it works, then tape all the bare wires so that it does not short circuit.
Step 6: Finishing It
After wiring and taping it, you should pack all the wire and resistors into the flashlight body. Then glue the bulbs in place. After that you need to fill in the empty space at the front of the flashlight. I cut small pieces of PVC and glued them into place, but you can use whatever you have. Thank you for reading, and I hope you enjoyed this Instructable.