What could be cooler then  LEDs, Bicycles and Safety?  Just when it is combined together in a cool looking project!
The Idea is to take a basic cheap bike helmet and make it interesting and safer.  I did this by adding 4 Superbright LED's and some reflective material.  This Instructable is a very good one to attempt if you are just getting started in electronics and soldering.  This is because it is not a very costly instructable (as far as electronics go) and the skills needed are very basic.  It is also a great application of Ohms law!

## Step 1: The Theory

This Step is all about basic LED wiring and Ohms Law.  Some of you may not need to read this and will just jump past it. However, I recommend those new to electronics to read this first.

Ohms Law states that I = V / R . In this, I is Current in amps (or mA as it is in LED's), V is Potential Difference in Volts, and R is Resistance in Ohms

Since this equation is used for the whole circuit, it isn't very useful to us the way it is now.  We have to change it so it will give us the resister value we need.  This Equation is:

R= ( Vs-Vl ) / I

Vs
is the supply voltage; what come out of the battery.  In this project it is 9 Volts.

VI is the LED voltage.  It is the voltage the LED should have.  It is stated on Data sheets you can find usually at the web site you bought your LED's from.  In my case, the white LED's I have were 3.5 Volts.

I is the Amperage the LED operates at.  It is usually 20 to 30 mA ( 0.02- 0.03 Amps).  It can also be found on the data sheet for your LED.  Mine was 20 mA.

So if I was to attach 1 White LED to the battery I would have
R= ( 9 - 3.5)/ 0.02
R= 275 Ohm

This means that you want to have at a minimum a 275 Ohm Resister, however to sustain battery life it would be smarter to use 300 to 330 Ohm Resister

Now, because one LED is kinda wimpy for a head light we will put the LEDs in not only in series but in series and parallel!

The math gets a little more complicated from here on.

If we put 2 of these white LEDs in series we have to use the equation
Vl = Number of LEDs x LED Voltage
Vl = 2x3.5
Vl = 7

R= (9-7)/0.2  *IMPORTANT* Number of LEDs does not change the Amperage in the equation*
R = 100

ok, so we can do 2 LED's in series, but can we do 3?

Vl = 3 x 3.5
Vl=10.5

R= ( 9 - 10.5 ) / 0.02
R= -75

We cannot have a negative resistor so we have to keep with 2 in series, but we want to have 4 LED's.  The way to do this is to have two sets of two LEDs series in parallel. Okay, that is really badly worded, but the picture below should clarify it a lot.

Now that you understand the process go and try it out with your LEDs.

If you are not good at all this math stuff you could just enter your info onto
This Site
However if you know how and why your circuits work it makes everything much easier when you get into more complicated electronics.

The Illustrations were created on Yenka and edited on Photoshop Elements

## Step 2: Materials & Tools

The needed materials are:
-9V battery
-9V battery clip
-4 LEDs-- I used white but any other colour would work, too.
-2 resistors (Value will change based on LED's, more info in next step)
-a switch of some sort, not a momentary; a flip or slide would work well here
-a good amount of wire, solid core works best but stranded is fine
-helmet (bike)
-foam for cushioning

Tools:
-soldering iron
-solder
-hot glue gun
-hot glue
-needle nose pliers
-Dremel  tool (not needed but makes it easier)
-pen knife (not shown)

Bread-boarding a circuit is a common practice to find potential mistakes in your design (or math).  I only bread-boarded half the circuit because the 2 parts are identical.  If your circuit 2 parts are not identical, then you should bread board both parts.

If you need to know how to bread-board or you would like to know more about how they work I would recommend to check out Cew27's Instructable on Breadboard Basics

## Step 4: Soldering

The first thing you want to do is solder two of your LED's together.  Make sure you solder the positive of one to the negative of the other.  It makes them fit better in your helmet if you solder a wire in between them so there is little of the original lead left (about 1/4"-1/8").  This makes them fit much easier into the helmet.  Then you are going to make a 2nd one with your other 2 LED's. (Picture 1)

Now you want to solder the resister to the negative lead on each LED set.  It doesn't matter if you solder the resister on the positive or negative lead, but if you keep your resistors on the negative leads as a general rule, then it makes everything easier because you don't have to keep in checking the polarity of your LEDs. (Picture 2)

Next, add about 3"-4" of wire to the + and resister.  This makes connecting everything together easier later on. (Picture 3)

## Step 5: Attaching LEDs

In most helmets there is 3 or so air vents in the front of the helmet.  We are going to place the LED's into the 2 outside ones.  You can experiment with different spots if you like.  I positioned mine at the bottom facing just below horizontal and at the top just above.  They are secured with hot glue.  At this point you should have the positive and negative wires of each side unconnected to anything.

## Step 6: Add Battery and Switch

At the back of most helmets there is a space  were you can place the 9 volt battery. (picture 1)
I hollowed this space out with the pen knife.  The Dremel tool works, but it takes longer.  You also need to keep the new hole space snug to the battery.  This is because the battery is not held in with anything but the elasticity of foam, so don't go over board with the knife.

Then solder the + from the battery clip to one of the connections on the switch. Next solder a  wire to a different connection.  In other words, solder it so it works as a switch.

You will want extensions on the wires so they reach all the way to the front LED assemblies.

## Step 7: Connect It All Together

At this point you should have 6 wires with unconnected ends.  Two from the back battery area and two from each of the LED units (total of 4 from the LED's).  The 3 positives (One battery positive and 2 LED positives) will need to be connected together and the 3 negatives (One battery negative and 2 LED negative)  will need to be soldered together.  The connections should end up in the hollow area in between the 2 LED units

To keep the helmet comfortable it is best to use the Dremel  tool to carve a trench between the LED units and the middle space  for the wires to rest.

Next, secure the wires to the helmet with electrical tape.  This keeps the connections secure and makes the helmet more comfortable.

## Step 8: Finishing

Now  it is time to make this helmet feel even better on your head.  In the study groups 3 out of 7 Walrus' prefer a soft helmet to a rough one.  (JOKE)
We are going to add foam to the inside of the helmet so you  will not feel any of the new electronics we have added.  Minimal addition of foam will make a difference.  There is too much foam in the first few picture.  I later cut it down to 1/2 to 1/4 of a inch.  You can see it in the final picture

If you don't have foam it isn't really needed.  Foam just makes the helmet a little more snug and comfortable.

I also added some reflective material to the back so I would be visible from the front and the back.  It is sticky on one side.  You could buy some similar products at
This Site*
*I do not get a kickback, it is just a good site.

## Step 9: Go Out and Be Seen

Other cool ideas that could be expanded on:
-Red LED's on the back
-Flashing LED's with a 555 timer

Thanks for reading, and if you think this is a good instructable throw a vote my way in the Light Up the Night Contest.

Four LEDs at the front are ok but I'm sure you'd be just as visible with two at the front and then put two red LEDs at the back. I like the way the lights are integrated into the helmet, but I'm afraid with everyone else regarding the batteries and modifying the helmet structure.
Got to agree with the others here, <br /> <br /> Nice idea and explanation, but you shouldn't modify your helmet structurally like that. In fact that would be illegal here in Australia, I wouldn't be suprised if it was too in your area.<br /> <br /> The manufacturers don't put that foam in for no reason - they put the minimum in to provide the required amount of protection. If you take some away, there isn't the required level of protection anymore.<br /> <br /> Maybe a re-think and come up with another solution? You could use some self-adhesive velcro to &quot;stick&quot; the battery on the outside.<br />
Good idea. Pretty slick looking.&nbsp; But I'm a little leery of the way you've hollowed out a space for the battery--the foam that was there isn't protecting your noggin anymore.&nbsp; An impact on the back of your head might drive that battery into your skull.&nbsp; Maybe a flat cell-phone battery stuck to the outside of the helmet in back?<br />
&nbsp;Li-Po Batterys (lithium polymer) are thin, light, last longer than that 9V battery, and they're&nbsp;rechargeable.<br /> <br /> <br />