I always had some issues finding money in my wallet to pay taxi fare when I come back home late night and most of the time, the indoor lamp of the taxi is either out of service or is simply too dim.

So, I came up with this simple circuit which uses SMDs and a button cell battery. The whole thing together is very small and hence, won't take up much space in your wallet but yet, will give you a decent lighting.

Step 1: Things You'll Need;

- Small piece of PCB(mine is a left over from a previous project)
- SMD components(LED + resistor)
- Mini Drill + 0.8mm diameter drill bit
- Sharpie(to draw tracks on the PCB)
- Tweezers
- Sewing kit(it you're planning to sew your LED to your wallet)
- Flat cable(or any other pair of wire)
- 30W soldering Iron
- Hot air soldering tool(optional, but will make things much easier and faster)
- A wallet(obviously)
- Small switch
- Button cell battery holder(optional)
- Paper Cutter
- Button cell battery(CR2032)

Step 2: Determining Resistance Value;

We can't just connect the LED directly to the battery as it may burn out if the current is too high. To calculate the resistance value of the resistor, we use this simple formula:

Rled = (Vin - Vled ) / Iled

we know that the forward voltage drop for a white LED(in my case) is around 3 to 3.3V. Since we are using a 3V battery, the equation for the resistance is simply

Rled = Vin / Iled

We don't want the LED to be too bright. So, we will assume the forward current to be around 20mA - 25mA (will still give a decent lighting)
Therefore,

R led = 3 / 20mA
= 150 ohm

Step 3: Making the PCB;

I won't be going through the process of making a PCB. There are many nice tutorial that will guide you through.

The PCB is a very simple and small one. However, you should take in consideration the amount of space you want to leave to drill a hole for sewing.

Step 4: Soldering Components;

After making the PCB, its time to solder and drill the holes. Whether you want to drill before or after soldering is up to you. I found that it was easier to solder first and then drill the holes but however, you'll have to be very careful so as not to damage the components.

To start with, tin the copper tracks with some solder. Be careful not to add too much.
-If you are using a hot air soldering tool, apply some solder paste on the track, place the components and heat it.

else if you don't have a hot air soldering tool, you can still use your soldering iron but is quite tricky to solder it.
- 1st, using your tweezers, place your component on the pre-tinned copper tracks.
- while holding it firmly there, remelt the solder on the track.
- while still holding the component, remove your iron and give it a few seconds to cool down.
- check whether the component is correctly soldered to the board and won't come off.
- repeat the above steps for the other component.

NOTE:
-Before soldering the LED, make sure to check which pin in the anode and the cathode. This can be done using a multimeter set to continuity/diode test. Anode of LED is connected to resistor which in turn is connected to +ve wire and cathode is connected to -ve wire.

-If you are using more than 1 LED, make sure both LEDs are connected in PARALLEL. (see picture 3 and 4)

Step 5: Drilling;

Next step is to drill the holes. I drilled a hole in the centre and made some notch on the edge so that when sewed, the PCB won't move or rotate in all direction which will break the solder point between the PCB and wire on the long run.

Note:
-If you drilled after soldering, dust may get between the components and are pretty hard to remove by just blowing on it. In this case, just use an old toothbrush to clean the PCB.

Step 6: Soldering the Switch;

Depending on where you plan to put your switch , it might be a real pain if you don't have much practice in soldering since the space as well as cable wire length is limited/small. You should get it after a few tries though.

Make sure to solder on the top part on the switch leg since we want to use the lower part of the leg to fix the switch to the wallet.

Note:
-The switch must be connected/soldered on the wire connected to the anode(+ve wire).

Step 7: Pre-sewing Steps;

Before starting the sewing process, decide where you want to put the switch and battery holder first. Position and attach the switch or sew it if your wallet is too thick/hard to get the legs through. I attached mine since it was fairly easy to get the legs through.

Step 8: The Actual Sewing!!!

Time to get your needle!!! If you don't know sewing, you got a reason to learn now :P

This step is straight forward and mostly depends on where you want to place your LEDs.
The main steps are:
   -Sew the LED closer to the switch first followed by the second LED.
   -Then sew the wire to your wallet to avoid accidentally stretching/pulling it.

Colour of the thread used is up to you. You can choose depending on your wallets' colour if you want to make it discrete.

Caution:
         -Beware of your fingers!!!

Step 9: Connecting to the Battery;

I ended up making my my own battery holder since the the other one was too big. I will not be posting how to make the battery holder unless requested. Its straight forward.

I had one 'pocket' with a zip in my wallet and decided the battery will go there. I made a small hole using my cutter and had the wire go through.

Step 10: Hiding the Whole Thing;

Finally, hide in the battery holder. You can also paint the wires to which ever colour you want using a permanent marker. I don't have a black marker, so the cables will remain grey until I find one.

Step 11: The Final Product;

Following the above above steps, you should successfully be able to light up your wallet

Note:
-This is my first instructable. Please be nice in your comments, also, any suggestion to improving this instructable will be the most welcome.

-If you have any questions, please feel free to ask. I'll try my best to answer all questions as soon as possible.

Thank you all for reading...