Kids are familiar with subtractive color mixing, which is when you combine filters or absorbing pigments (paint): red and yellow give orange, red and blue give purple, blue and yellow give green. But additive color mixing, of light, is less familiar: red and green give yellow(!), blue and green give cyan (light blue), and red and blue give magenta (a pink-purplish color). Additive color mixing has a 1-to-1 correspondence to the 3 color receptors in our eye. In fact, the ‘red’ and ‘blue’ from subtractive color mixing should really be magenta and cyan.

Additive color mixing can be convincingly demonstrated with three torches, that emit red, green and blue light, either by using coloured LEDs or by putting a filter in front of each of the (white light) torches. Here I propose and alternative, 3 LEDS powered by USB on solid-core wire that allow to demonstrate color mixing and invite play with colors and light. It can be built in an hour or so and is sturdy enough to survive major abuse.

Step 1: Materials

3 5mm LEDs: 1 red, 1 green, 1 blue

3 resistors: two of 220Omh, one of 470Ohm

1 USB-A male connector

60 cm of solid-core hookup wire

12cm of 2mm shrink-tube

6cm of 6mm shrink tube

Hot glue

Step 2: Component Selection

5mm LEDs have a built-in lens, but the beam angle and quality very strongly, so best is to test and select suitable LEDs that give similar-sized spot. On a breadboard, light up the 3 LEDs, in parallel, but each with a current-limiting resistor, anything from 150Ohm to 1kOhm is fine. Project the light on a white surface and check that circular spots of similar size are produced by the LEDs.

The efficiency varies strongly between LEDs of different color, and on top of that our eyes also have different sensitivities. We can tune the amount of light of the LEDs by choosing the value of the current-limiting resistor. We also want to stay within the 20mA maximum current of a LED. I found that the green LED was brighter than the others: when all 3 LEDs had the same resistor, and the light of the three was superimposed, the spot was pale green instead of white. This was resolved by using a larger resistor (470 Ohm) in series with the LED, instead of 220 Ohm for the red and blue LEDs.

Step 3: Build Instructions

Prepare the materials: cut the solid hookup wire to 3 strands of 10cm and 3 of 9 cm and strip 5mm at both ends. Cut the shrink tube to pieces of 2 cm. Cut the leads of the LED to 1 cm. Cut the leads of the resistors down to 5mm.

Solder 1cm-pieces of the resistor leads to the + and - of the USB connector. Solder the 3 resistors to the ‘-’ and the 9cm wires to the end of the resistors. Solder the 10cm wires to the +. Pass the thin heatshrink over the wire. Solder the LEDs to the wire and cover the leads with the thin heatshrink and heat to shrink it. Pass the wide heatshrink over the LED so that it covers the bottom, and heat to shrink.

Finally, use hotglue to encase the resistors near the USB plug, and twist the wires in pairs.

Step 4: Usage

Plug the device into a USBpower bank and the 3 LEDs will shine up brightly.

The solid-core wire allows to give a position and direction to each LED such that it will stay in place. The LEDs can shone on a whie surface and be oriented to overlap partially and see the effect of additive color mixing. It can also be used for quick improvised decorative lighting: consuming 50mA or less, it easily lasts 24h on a small powerbank.