I used a red LED for this instructable, because it is easier to see than a clear one and I didn't have a small clear one at hand. If you make one of these using the instructions, it will be much brighter than the one in the photo, its just easier to see whats going on when its not too bright.
I have seen similar instructables to this but thought this was worth putting up as it is very simple and uses an old battery for the main assembly. This is my first instructable, I saw something very similar to this on a website a while ago and thought it was a nice idea.
Step 1: Things You Will Need
You will need to remove the top section from a dead 9 volt battery. They are easy to remove by bending back the aluminium around the edge. The positive terminal is usually connected by a flat metal strip, this one has brown paper covering the strip. You can bend it back and forth close to the terminl to snap it off. The other black piece of plastic at the bottom of this picture is from the base of the battery.
Just for interest: Another use for the terminal is to make your own battery clips. Just solder a length of insulated hookup wire to the backs of each terminal and glue the base piece from the old battery to the back. I know battery clips are cheap, but it's a good way to recycle some of the old battery. I also hang onto the aluminium battery case. It is useful as a small enclosure or a source for thin flat aluminium.
Step 2: Drill Hole for LED and Choose Resistor
Drill a hole between the terminals for the LED, I cheated and melted a hole wth my soldering iron.
You need to determine the value of resistor you will need. If you connect to the battery without one, too much current will flow through it and it will burn out. You can calculate the resistor value required using an online calculator such as
If you bought a new LED chances are it came with some specs and you can enter these into the calculator. If you don't have the specs, most clear or blue LEDs will need a resistor about 270 ohms, and red LEDs will need a resistor about 390 ohms
Of course this is a guess and based on 20mA forward current, the 9 volt source voltage, and 4 volt forward voltage for clear and blue LEDs, 2 volt forward voltage for red and other colour LEDs,
A 1/2 W rated resistor is fine and small enough for the job.
Don't worry about exact values, use something close.
Step 3: Soldering in the LED
You must connect the LED the correct way around. The cathode connects to the negative battery terminal. (anode connects to positive).
If it's a new LED one of the connecting leads is usually shorter, this is the cathode. Sometimes round LEDs also have a flat section on the cathode side. If you can see inside the LED, the cathode is usually the larger of the two electrodes. To be sure connect to the battery temporarily before soldering in place. Be sure to connect your resistor in series with the LED though or out comes the magic smoke.
Solder the anode lead to the outside edge of the large battery connector. This will connect it to the
positive terminal of the battery when you snap it on.
Step 4: Connecting the Resistor
Solder one end of the resistor to the cathode lead of the LED and make a small hole near the small terminal to push the other end of the resistor lead through.
Don't put the hole right next to the small terminal, the small terminal has to go inside the large terminal on the battery you will use to power this. If you put the resistor lead right up against this terminal it will get in the way.
(you might notice in this configuration, the resistor is connected to the negative side of the circuit, In a series circuit such as this, it doesn't matter and in this case it is easier to hook up this way. I would suggest putting the resistor between the positive battery terminal and the LED though if you start hooking up multiple LEDs etc.)
Step 5: Connecting Other End of Resistor
Solder the other end other resistor to the back of the small terminal. (the back of the terminals are actually the front of the 'torch'.)
You can leave this solder joint out and just bend the resistor wire so it's sitting over the back of the terminal. You can use this like a momentory switch, press to turn on, release and the wire should lift enough to break contact.
Step 6: You're Done
That's it. Just clip it to a 9 volt battery to turn it on. You can get fancy and add an actual switch to the circuit of course. You can even buy a flashing LED and then use this at night for a saftey light on your bike or whatever.