How to Make Led Lights Work Off Car Electrical

Every year I put a wreath on my sprintervan.  I decided at the end of last XMAS to buy a plastic wreath and figure out a way to wire led lights into my sprinter electrical system so that when the head lights come on the wreath strapped to the front of the bus would come on.

Using a coupon I bought a string of qty 20 led lights that runs off a battery pack (qty 3 AA batteries).  It cycles through colors and patterns.

Tools and supplies needed:
1) string of battery powered LED lights
2) multimeter that can read voltage and current.
3) soldering iron with solder
4) some extra red and black wire
5) wire strippers/crimpers/wire cutters
6) shrink tube & heat gun
7) small connectors (male - female) type
8) resistors (you need to make a calculation to figure out what kind to use)
9) plastic wreath

http://www.mackconsult.com/?p=2272

Step 1: LED Light String

Get a battery powered string of led lights.  They have to be led lights because strings of lights that plug in to a wall electrical socket runs on AC, led runs on DC.

Carefully disassemble the battery pack.  If you can save the batter pack this will help you later when you figure out what size resistor to use.  I then clipped off the battery pack (leaving the in line resistor already implemented). 

Then wind the light into the plastic wreath.

Step 2:

One needs to calculate what size resistor is put in place to drop the voltage down from the car voltage to what the battery pack does.  If you don't the led's will be over powered and burn out.

I used an adjustable power supply that I was able to set at 4.5 volts (qty 3 AA's) and 12 volts.  But one can use batteries to mimic the power supply of the battery pack (if you were careful you can re-use the battery pack for this test) 

Then put an amp meter in line on either the positive or the negative side and turn the lights on using your improvised battery supply that is mimicking the original battery pack.  Record down the max current seen, in my implementation is was 55 Milli-amps. 

So using V=I*R, the voltage drop needs to be 7.5 volts from the vehicle electrical system, so the resistance needs to be about 140 ohms. 

Know you have to take into consideration the power rating of the resistor so W=I^2 * R.  Which is about 1/2 Watt.  So I went to radio shack and spent a whole $1 on a 4 pack of 1/2 watt 150 ohm resistors.

Step 3:

Solder this extra resistor in series with the existing resistor and then put a couple of small connectors in so its easy to take the wreath off and on. 

The two power wires run behind my grill to the running light on the front right of the vehicle.  Just spliced the positive into a "live" wire when the parking/marker lights are on.  Then the ground wire goes to a bolt in the engine compartment.

Use the heat gun and shrink tubing to cover all the soldered connections.

LED's are not known for lasting a long time in harsh environments, but we will see how long they last on the front of my vehicle.