Colour Mixer

Control your Tri-colour LEDs

Johnny “Meccano”

Issue 45, April 2021

A fun and colourful little project that the kids will just love.

We receive project submissions regularly from makers all over the world. Some projects are quite complex and solve real-world problems, and other projects are much simpler and intended to entertain or just inspire young makers. The following project from our contributor, Johnny Meccano, is a good example of a smaller project to entertain. We caught up with Johnnny to find out more.

Thanks for the project submission, Johnny. What was the idea behind this project?

I came across a mixed bag of parts in a clearance bin at Jaycar a few years ago. In amongst all the goodies was a square shaped LED with 2 lugs on top and 3 lugs on the bottom.

Oh, we just love those bargain bags that Jaycar does every now and then. That is a curious-looking LED that we haven’t seen before. How did you test it?

Since I had no idea what it was, other than an LED, I hooked up a 1.5k ohm resistor to the positive lead of my benchtop power supply at 9V and started fiddling. It was a fair bet that the large double width lug was the cathode so I hooked the negative crocodile clip to it and tapped each lead with the resistor lead. I got red, blue and green. No idea what the centre lug at the bottom was for.

Interesting. Perhaps our readers who are familiar with the LED could let us know what that lug is for. What did you do with it next?

I had an idea, what if I could dim each colour separately. After browsing some Short Circuit books (aren't they totally excellent!), a simple circuit was devised using a P100 NPN transistor and a B10K potentiometer. See attached circuit diagram.

We are very familiar with those Short Circuits books from Jaycar. They have been inspiring kids to get into electronics well before the likes of Arduino, RPi, etc. came along.

After I got one colour to dim successfully I did the same for all three colours and built it into a transparent jiffy box. I put corresponding coloured knobs on each pot and an on/off switch.

Simple! We also notice an unusual-looking diffuser on top of the LED. What is the story behind that?

The LED illuminated OK but not that easy to differentiate between the colours. Then I remembered all the Christmas lights I bought from Kmart on Boxing Day. They were practically giving them away. One set had rather large prism effect clear plastic baubles so I glued one on top of the LED and it worked perfectly.

You’re sounding like the MacGyver of the maker world being able to repurpose parts like that. Ha ha. A good skill to have for us makers. If our readers want to make one for themselves, what do you suggest?

For the purposes of this build, I have drawn the circuit with a 5mm round LED from Jaycar as I still have no idea what my square LED is. I'm sure they'll be equally spectacular.

You may wish to play with the resistor values but mine works just fine with the 1.5k resistors.

Great. Thanks for sharing the project and story behind it, John. For our readers who are interested in building something similar, we have provided a wiring diagram and parts list.

Parts Required:JaycarAltronicsCore Electronics
1 x Tri-Colour RGB LED Common CathodeZD0270-COM-00105
3 x 10K Linear 9mm Potentiometers or EquivalentRP8510R1976CE07791
3 x PN100 NPN Transistors or EquivalentZT2283-CE07792
3 x 1.5kΩ Resistors*RR0576R7562FIT0119
1 x UB5 Jiffy BoxHB6005H0155AH0155
1 x Small Toggle SwitchST0335S1310ADA3221
1 x 9V BatterySB2423S4970BCE05337
1 x 9V Battery SnapPH9232P0455CE05205

Parts Required:

* Quantity shown, may be sold in packs. You’ll also need hook-up wire and heatshrink.

Johnny “Meccano”

Johnny “Meccano”

14 year old student and Meccano builder.