Graduation Cap LED Mod – Rev 1

At most (high school) graduation ceremonies, it’s pretty common to see student’s caps with felt decorations proclaiming their college destinations. Rather than put a big Stanford “S” on mine, I decided to add some LEDs to make it more unique.

When I started, I intended to use something called “Light Pipe.” It’s like fiber-optic cable but it’s meant to shine through along the way not just at the end. Other than cosmetic uses, it can be used to show a recessed status light or other indicator. I would then use 4 RGB LEDs, like these, mounted at each corner of the cap.

These LEDs have a common cathode, so the diagram of the part looks like this:

1, 3, and 4 are the anodes for each color Red, Green, and Blue. 2 is the common cathode.

We have to remember that LEDs are not like resistors or incandescent lights. They require a consistent forward voltage across them at a certain current. This means that given a certain LED and supply Voltage, there is a specific resistor necessary for it to light up.

We can find this using Kirchoff’s Loop Rule:

I = LED forward current in Amps (A)

Vf = LED forward voltage drop in Volts (V)

Vs = supply voltage in Volts (V)

R = resistance needed in Ohms (Ω)

ΣV = 0

Vs − IR − Vf = 0

R = (Vs − Vf) / I

You just have to look up the values for Vf and I in the datasheet for the LED, and then choose R based on that info and whatever power source you have chosen.

In this model, I can change the color of the LED by varying the voltage across each anode. For example, if the Red and Blue lines were provided with their correct forward voltages while Green was give 0V, the color would be purple. The idea I came up with to do this was a triplet of potentiometers to change the resistance, and therefore the voltage drop, across each segment of the LED. Here is a sketch of the circuit using Eagle:

In this circuit, from Left to Right,Top to Bottom:

Two coin cells provide 6V to 3 potentiometers in parallel. As they are twisted, the resistance from the left branch to the bottom changes. Each is connected to a resistor to achieve the correct forward voltage, and then to the 4 LEDs with anodes in parallel for each color. The cathodes are in parallel and then go back to the negative terminal of the batteries.

The potentiometers I planned to use were these because of their small size. They vary in resistance from 50Ω to 10kΩ. Equivalent resistance is a sum in series, so the resistors I would need to use would be 50Ω less than originally planned. Also note that Red required a different forward voltage than Blue or Green so it is a different resistor.

I was very happy with this plan, but unfortunately I was running out of time. I wanted to prototype and try different potentiometers, but it would be a risky move to spend so much money if it didn’t work out. The only place I would be able to work on this was my school’s robotics team because I don’t have a decent soldering iron or breadboard at home, and I only had a week of school left.

So I decided to just go to Fry’s Electronics and try to buy everything. Suffice to say, when I tried out the LEDs on Wednesday they were pathetically dim. I scrapped this plan because I also found over a hundred small LEDs in an unsorted pile at my team’s lab. To Be Continued…

Additional notes:

Sparkfun has some great Eagle tutorials as well as what I’ve outlined above regarding LEDs. They also have tutorials and expertise in much more advanced stuff that I don’t know anything about.