Introduction: RGB LED Light Pen

You've probably seen the numerous Instructables on how to make a Light Pen, which is used to make light drawings.

The techinque of Light Drawing is to place a camera on a tripod, turn off the flash and set the exposure time to the longest setting possible (or make use of a BULB setting on SLR cameras, which will hold the shutter indefinitely until you press the button again) and when you are in a totally dark room, you wave the pen around in front of the camera as it is exposing.  When you're done, the result is a drawing, of sorts.

Now, in the past the pens are clunky and unprofessional looking (modified flashlights and the like) and only have one color available to them.  So, if you want to draw in a rainbow of colors, you need seven different pens, batteries and LEDs.  That starts to get expensive.

My solution is to use a RGB LED.  They contain a Red, Green and Blue LED inside, using a common negative.  Its basically three LEDs in one.  By controlling the brightness of each primary color, I can make essentially any color, including white.

Step 1: Parts and Tools

For this build you need: 

- A tin or pen-like enclosure
- Three (3) Potentiometers
- Three (3) Knobs
- Two (2) AA batteries
- Rubber Grommet
- Wire
- Solder


- Drill and bits
- File
- Knife
- Hot glue and gun
- Allen keys
- Masking tape
- Ruler
- Pencil

Step 2: Assembling the Pen

I selected some small 200 ohm potentiometers, and the aluminum case from a fancy pen.  My plan is to cut a hole in the back of the case for the LED, and have the color knobs on the front, along with a button to turn the LED on.  One knob for each color, so they can be mixed.  A momentary button opposed to a switch make it easy to draw letters and segmented lines.

Take the masking tape and put a strip on your enclosure where you want to put the holes for knobs and buttons.  Measure and mark them with a pencil.  Using a drill bit that is slightly bigger than your potentiometer, drill your holes where you've marked them.  The potentiometers I am using are tiny ones meant for PCBs, and have a 1/8" diameter shaft.  I happened to have some knobs lying around that fit this size of shaft, so I decided to use them.  Also drill the hole for your button.

If your potentiometers have a nut and a threaded collar, then use that to attach them to the enclosure.  Mine have no nuts or threads, so I had to hot glue them in place.  I did this by putting the knob on the shaft, tightening it in place so that it would hold the potentiometer right up against the metal, and hot glued the pot in place.

For the LED, we need to diffuse it so that the light will be more visible from a non-direct angle.   I did this by roughing the outside with a file, but sandpaper would also work.  I decided I wanted to mount it inside a rubber grommet that would make it more attractive.   I found a rubber grommet that the LED squeeze-fit into.  I measured the inner diameter of the grommet and drilled a hole in the back of the enclosure a little bigger than that hole.  I ended up having to trim some rubber off the grommet with a knife to get it into the hole.  I then squeezed the LED into the grommet so the LED protruded out the opposite side.

Step 3: Wiring

The wiring is quite simple, and I am running the circuit of two AA batteries.  Essentially, the potentiometer is controlling how much of the electricity is going to the LED, and how much is going straight to ground.  The more the pot is turned to the left, the more power is diverted to the negative, and the more its turned to the right, the more power is being directed to the LED.

The schematic is below, and can be wired point-to-point with no PCB required.  The middle pin (the armature) of each pot is connected to the + of each LED, and the other two pins on the pot are connected to the + and - of the battery, respectfully.  The - cathode leg of the LED goes to ground.

Now, I did not have a battery holder that would fit inside my enclosure, so I just connected the wires directly to the batteries, wiring them in series.  Plan to have room for a AA battery holder, that will make life a lot simpler for you.

The button is wired between the + terminal of the battery and the three potentiometers, to make sure that there isn't any electricity available anywhere in the circuit when the button isn't pressed.

Step 4: Light Show!

When you're done, press the button and fiddle with the knobs to see what happens!  You can create pretty much any color in the rainbow by adjusting different levels of red, green and blue.

Set up your camera on a tripod, in a dark room, at night.  The room must be pitch-black or this won't work very well.  Set up the camera, and have someone press the button to begin the exposure.  When the camera starts exposing, draw a picture in the air with the light pen.  If you're on a timer, try counting in your head so you can get an idea of how much time you have left to draw.

Below are a series of pictures showing the rainbow of colors you can do with this pen.

Thanks for reading my Instructable.  If you have any questions don't hesitate to ask.