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 @
and pics uploaded on flickr:
Step 1: Things Needed & Circuit Diagram
2. Circuit Diagram
Step 2: Prepare the dry erase marker & circuit diagram explanationDry 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
Step 4: Bulding spring for negative end of the Battery
Step 5: Results
More pics here:
Enjoy!!!! Have Fun with Light :)