Only a few years ago, the choice of LEDs was limited to IR, red, yellow, and green. The LED manufacturers have been busy extending the spectrum, and filling in the gaps. The latest generation of organic LEDs (OLEDs) has added some dazzling new colors to the spectrum. This circuit uses a set of 13 differently colored LEDs
to generate a full color spectrum. The photo does not fully represent
the colors generated due to camera limitations. The real-world display
is very eye-catching. If you want to “trick out” your PC, this circuit
is for you. Forget about those boring blue PC light displays.
Operating Voltage: 6-12V DC
Operating Current: 145ma at 12V DC
The LM2940T-5.0 low dropout voltage regulator converts the 6-12V DC
input power to regulated 5 Volts. It was chosen over a standard 7805
regulator so that the circuit could maintain regulation while operating
on a 6V battery. The 1N4001 diode protects the circuit from reverse
polarity, if a battery or power supply capable of generating over 1 amp
is used, a 1 amp fuse should be installed between the supply and the
circuit. The 5 Volts is used to drive each of the LEDs through individual current limiting resistors.
The resistor values were determined experimentally for equal
brightness. Values are given as examples only, different sources of LEDs
will require different resistor values. Resistor selection turns out to
be the most difficult part of the circuit’s construction. A 100 ohm
resistor in series with a 1K pot could be used in place of each resistor
if individual brightness adjustments are desired. The table below lists
the LED colors and wavelengths.
LED Color Wavelength Description
Deep Red 700nm ————-
Red 660nm traditional red
Orange Red 635nm “high efficiency” red
Orange 623nm also called red orange
Amber 594nm ————-
Yellow 588nm & traditional yellow
Yellow Green 567nm traditional green
True Green 523mn ————
Cyan 501nm verde green, blue green
Aqua 495?nm ————-
Deep Blue 470nm ultra blue
Powder Blue 430nm first generation “powder blue”
Violet 410nm ————-
The circuit was built on a prototype perforated board with printed
solder pads. The circuitry is hand-wired on the back side of the board.
Care should be taken when soldering to the LEDs, a clip-on heat sink should be used while soldering the leads. Care should be taken to avoid zapping the LEDs
on the violet side of the spectrum, they are sensitive to static
electricity. The circuit board can be mounted on a piece of white
hardboard, the white paint reflects the colors nicely.
Apply power to the circuit and enjoy the colorful glow. Do not stare
directly into the array at close range for extended periods, some of
the LEDs are extremely bright.
Taking The Circuit Further:
The spectrum could be extended on both the IR and UV sides. A brief
scan through the Mouser catalog indicates the availability of these IR
wavelengths: 940nm 880nm, 875nm, 870nm, 850nm. UV LEDs at 400nm, 395nm and 380nm are also available. There are also many LED
colors available with wavelengths between the 13 colors shown, the
colors selected were chosen for an evenly spaced color spectrum.
An open-collector LED driver circuit could be connected to the negative LED
leads for computer control.The circuit could be used in conjunction
with a photo detector for characterizing optical filter curves.
Typically, the photo detector output is sent to a logarithmic converter,
the log-ratio of the direct light versus the filtered light
characterizes the attenuation at a given wavelength.
Most of the LEDs were purchased from Digi-Key,
Jameco, and Mouser. All of the parts were T1-3/4 size, clear packages
were used wherever possible. LEDs from different manufacturers may have different focus characteristics. All of the resistors are 1/4 Watt parts. LED
part numbers are not available, the rainbow was assembled from parts
that were accumulated over several years. Beware that different LED manufacturers use different names for their colors, the wavelength is the best indicator of the color. The Aqua LED is the most difficult part to find, All Electronics carries them, although the wavelength is unspecified.