In Lieu of a Breadboard: PCB LED Test Harness
Here is a project idea for some of you. If you are anything like me and you are experimenting with an Arduino or TI Launchpad for the purposes of your model railroad, then you likely find yourself adding and removing LEDs and associated resistors to/from breadboards over and over again. So it dawned on me: Why not solder some LEDs and appropriate resistors for a 3.3V source onto a spare PCB project board and slap some headers on there? Well, since I was waiting for some parts for other MSP430 projects, I did just that!
It is a very simple design, actually. All the cathodes (negative side) of the LEDs are tied together while the anode leads (positive side) are kept electrically independent. Each anode then has a appropriately sized resistor for the color of LED. This is called a ‘Common Cathode’ design. Note that there are some applications that would require a ‘Common Anode’ design. e.g. LEDs being driven by a bank of Darling Drivers (ULN2803)
Now, a bit of math:
Blue drops about 2.9Vdc, so a 47ohm resistor will allow about 8.5ma when connected to a 3.3Vdc source:
(3.3 - 2.9) / 47 = 0.0085
White drops about 2.6Vdc, so a 47ohm resistor will allow about 14.8ma when connected to a 3.3Vdc source:
(3.3 - 2.6) / 47 = 0.0148
Red drops about 1.8Vdc, so a 120ohm resistor will allow about 12.5ma when connected to a 3.3Vdc source:
(3.3 - 1.8) / 47 = 0.0125
Using a simple PCB, (I has some spares from RadioShack) solder on a header pin onto each of the resistors, and one pin attached to the common cathode trace and voila, you have a compact board that you can use for quick testing (and easy setup/teardown) of your LED based launchpad projects!
Enjoy! Leave comments if this spawns other ideas that would be useful to model railroader electronics hobbyists.