Light pen construction.



Parts List for this blue light pen
Plastic Tubing - 5/8" outside diameter - 1/2" inside diameter
Plastic Tubing - 1/2" outside diameter - 3/8" inside diameter
1 LED
1 Normally Open Switch
1 20 ohm Resistor - size is determined using Ohm's law
3 1.5 volt Button Batteries
Heat shrink tubing
24 gauge wire
Electrician's tape



LEDs, switch, resistors, heat shrink and electrician's tape purchased at local electronics store.



This information is used to determine how many batteries and which resistor to use.
This blue LED requires a 4.0 Forward Voltage Drop (Vf) to light.
It will pass 25 milliamps of Current (If).
3 1.5 volt batteries in series will supply 4.5 volts.
Obeying Ohm's Law and using this Current Limiting Resister Calculator of LEDs, a 20 ohm resistor should be placed inline in the circuit.



A Normally Open switch will allow you to turn the light on and off instantly.



Here is your 20 ohm resistor.
Read the package carefully when buying. A 20K ohm resistor is not the same!



Any combination of batteries that add up to the required voltage will do.
AAA batteries are 1.5 volts and 3 in series will give you 4.5 volts.
I found these tiny 1.5 volt button batteries inside of an A23 battery.
See this 12 Volt Battery Hack Instructable for more about that.



Cut a hole in the larger 5/8" OD plastic tubing close to one end to accomodate the switch.
Since we hold this light like a pen, I place the switch where we can easily push it with the index finger.
It is important to ask the intended artist what works best for them and design the pen accordingly.



Solder the LED, the resistor and the wires in series.
The resistor can be placed anywhere in the circuit.



I place heat shrink over the exposed wires and...



Heat the shrink with a lighter, protecting against short circuits.



Fold the wiring up and slide inside of a 1" piece of the smaller 1/2" OD plastic tubing.



Feed the wires through the 5/8" tube, one through the hole and solder the switch inline.



Continue feeding all wires through the tube.
Squeeze the switch into the cutout hole. (I bent the leads to fit.)
Squeeze the 1/2" tube into the 5/8" tube.



Continue feeding the wires out the end of the tube.



Time to add the power source



This is very low-tech, but I have yet to find or build a battery holder that suits my purpose.
Strip the ends of the wires and wrap the bare wire and a small bit of aluminum foil together into a ball



Use the electrical tape to hold the batteries together and the wire ends in place on the correct battery terminals.
Polarity is important here. Test the light at this point and try reversing the connections if it does not work.



Stretch and wrap the tape tightly, insuring a postive connection.



Fit the battery pack into the end of the finished pen.



It works for me!
Now drawing is another story.