Tuesday, February 21, 2012

LEDs Inside a Trailer

White LEDs are a good choice to light the inside of a boy scout trailer. They are low power. They do not produce too much light. They are inexpensive. They can be operated from a 12V battery. They are physically resilient.

The schematic shows four units of four LEDs and the associated circuitry. The unit can be repeated as many times as needed (four in this schematic). The LEDs are arranged in two parallel stacks of two series LEDs (2S2P). A stack of two LEDs will always operate with a battery voltage as low as 11.1V (4.4V for each LED, 1.3V across the resistor, 1V dropout). Two stacks will match close enough for this application while keeping the total current through the regulator below half it's rated capability. The LM317 is a nice adjustable regulator that I am using as a linear current regulator. Using a 33Ω resistor across the 1.2V reference results in 36mA of current. 

Schematic of four four LED units.

To implement this design, I obtained the following from Mouser. I bought 1000 resistors, so they are essentially free per piece. Each four LED unit costs $1.41, 16 LEDs costs $5.64.

Manufacturer Description Part Number Price
Cree White LED CP41B-WGS-CK0P0154 $0.30
Fairchild Adj. Regulator LM317LZ $0.17
Xicon 33Ohm 1/4W, 5% 291-33-RC $0.009


I breadboarded this circuit to get an idea of the brightness and so I would have a reference to drop LEDs into during construction to verify which direction they are pointing. I find these square LEDs difficult to orient.

One four LED unit on a breadboard.

The troop's adult quartermaster and I decided one light placed approximately every 30cm would be enough illumination for unloading the trailer at night. When I started to wire up the 16 or so LEDs that would be needed, I discovered this approach is a headache. Lots of branching and back and forth wiring is needed. Not a bad solution, but I can do better.

Next week I do better. Anyone for a boost switch mode DC-DC converter?

I've been listening to the Digikey/DesignNews "180 days of Education" For the last several weeks. The microcontroller lectures (the first two weeks) were worth the time. They are basic to intermediate level lectures well presented. The sensors lectures of the third week just didn't seem as useful to me. Probably worth listening to, but not necessarily. I will find the time to hear the next lectures.

Bruce McLaren