Led Pen With Adapter for Connecting Different Color LEDs

This project was conceived to do some fun with LED long exposure photography. However, most of instructables I saw spoke about creating different pen's for each color LED, this led to the creation of the new ultra-low cost led pen with changeable LED port.

I have more detailed explanation of building on my website along with other projects @

http://www.kunaldoshi.com/home/projects/led-projects/led-pen-with-adapter-for-connecting-different-color-leds

and pics uploaded on flickr:

http://www.flickr.com/photos/12039238@N05/

Step 1: Things Needed & Circuit Diagram

Old Dry erase marker pen
2 AAA batteries
Push Button Switch
Wire jumper/bare wire
Any small screw
Glue gun
Different Color Leds
Any female connector which has solderable cylinders on the other end

2. Circuit Diagram

Step 2: Prepare the Dry Erase Marker & Circuit Diagram Explanation

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 stuff from inside. A quick rinse with water is recommended. Leave it to dry.

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 middle of the range, though this means high current for LEDs with lower Vf like red led and less current (lesser brightness) for LEDs with higher Vf like blue 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.

Step 3: Building the Adapter

Since the requirement was to use something that can hold the legs of the LEDs relatively snug we pulled out the cylindrical tubes from a RS -232 connector( I think) that I had using needle nose plier taking care that you dont 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.

Step 4: Bulding Spring for Negative End of the Battery

Yes we dint 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.