LED Pen

I have always been fascinated by star trails at night. Its so amazing to know that there is a way to visualize movement of stars which seem to be stationary. When I got my new SLR that was the first thing I wanted to try out, but it took a while to know how to do it and then later living close to New York just made it impossible to see that many stars in the night.

So my friend Kunal and I decided to have fun with light and my dSLR, we came up with an engineering solution-make a LED pen. The initial idea was to make a single color LED pen but at the end of few hours what came up was a design where in we could change the LED as you wish- I am so glad we came up with that thought- it just has opened up hundreds of possibilities. Here are the first few images taken with the LED pen. Look below for how to make your own ultra low cost LED pen.Enjoy the pictures and leave me comments about any new designs or ideas you want me to try out.

Ohhh.. I almost forgot, I am already working on a new design for the LED light source. Look out.

How to make your own LED pen?

Inventors:

Kunal Doshi & Harish Damodaran

Things Required:

• Old dry eraser marker pen
• 2 AAA batteries
• Push button switch
• Wire jumper/bare wire
• Any small screw
• Glue gun
• Different colour Leds
• Any female connector which has solderable cylinders on one end.

Steps:

1. Prepare the Dry Erase Marker

Dry Erase Marker worked as the perfect exoskeleton for this project because it can hold 2 AAA batteries in it. First pull out the back lid of the marker using needle nose pliers. Remove all the contents from inside. A quick rinse with water is recommended. Leave it to dry.

2. Circuit Diagram

As you can see the circuit diagram is very straight forward. The only problem was to calculate R. For newbies, current through the LED increases exponentially beyond its forward voltage. The value of R is chosen such that the voltage across the LED is around its forward voltage Vf and forward current If. You can use this website to calculate R.

However, if you see all LEDs have different Vf and hence different R. So we decided to use a R value such that it lies somewhere in the mid range. This means there will be a high current for LEDs with lower Vf like red color led and less current (lesser brightness) for LEDs with higher Vf like blue color led the ease of using the same pen far surpasses the reduction in brightness and lower life of LED (10 cents each).

We selected R =33 ohms by measuring the current through it with different LEDs and it seemed to work well.

3. Building the Adapter

• The requirement is to use something that can hold the legs of the LEDs snugly we pulled out the cylindrical tubes from a RS -232 connector( I think) this can be accomplished by using needle nose pier, take care that you do not bend the tube.

• The next step was to solder the resistor to one end of the cylinder. Since the other end needs to be in contact with the +ve of the battery we wound the other end of the resistor on a screw whose head was large enough to not pass through the tip of the pen. Here's how it looks

• The other cylinder will have a small insulated wire that goes to one end of the push button switch. Next, the two cylinders were fixed in their place using a glue gun. Care was taken so that the glue does not seep into the cylinders itself. Here's how it looks:

• The switch was placed in place by drilling a hole on the side of the pen.

4. Bulding spring for negative end of the Battery

• Yes we did not have a spring for the end of the battery, so what to do??? Build your own, Isn't that's what engineers do? So we took a insulated wire and stripped the insulation off it.
• Next we wound this wire on a thin cylindrical pen ( you can use a small precision screwdriver) taking care that they are wound far apart. Now when you try to pull the wire down the pen you will get a spring like wire which was glued to the inner end of the bottom lid of the marker

Note: We used the lid upside down because it was only then that 2 AAA's will fit in it.

• Mark the +ve and -ve end on either end of the tip of the marker.
• Cut the legs of all the LEDs that want to use.
• All you need to change color is to swap out the led :)

Now that you are done with all the hard work. Pose with the new LED pen you have made. We did just that -

Phew, work well done. Time to have fun, here are some of the pics that we got using it. Nikon D3000 Digital SLR camera was used. ISO : 400. Exposure: 10 sec - Bulb settings and a standard tripod.

Thank You Kunal for the write up