Saturday, January 1, 2011

3$ LED Map Light Replacement

I needed to replace the map light lamp in my car this afternoon and I thought I would go for LED replacement bulbs.  The original incandescent bulb's orange glow looks dingy anyway and the last few times I needed to see by way of the map light it seemed rather dim.  I figured the local auto parts store would be teeming with LED's as it seems all of the futuristic cars nowadays are jamb packed with the things.  This, however, turned out not to be the case.  According to the worker at the parts shop, LED's are "high performance" parts and as such, must be custom ordered.  I was also told that "the internet has them".  Neat. Too bad "the internet" thinks LED replacement bulbs are more valuable than McDonald's orange juice.

Not to be out done, I walked to the dollar store next door to the auto parts shop, where I found these beauties for 1$ each.  A Trisonic 3 LED Push Light; batteries not included.  I bought three, driver/passenger map light and cargo area hence, the $3.  In case you were wondering, the text on the bottom front says "To uplift our character, begin with cultivating peace in mind, body, family, and activity.".

And on the back, the text "Cultivate a big heart-but a small ego", let me know these were the right bit of kit for the job because, that is exactly what I was trying to do.  No, this project wasn't "X to the Z" pimping any rides.  And, my car is neither empirically "faster" nor, objectively, more "furious" because of this upgrade.  Just humble LED's helping electrons change orbitals to emit photons for me to read maps by.

I wanted to verify the light worked as intended and would be bright enough first, before ripping it to pieces.

This is what I found inside.  3 LED's, a switch and a single resistor.

And just what do we have here?  The good people of Trisonic have designed their circuit to have the LED's in parallel.  This works for the design specifications of the product they made but it is not helpful to me so I first desoldered all of the components.  Then, I cut through two of the PCB traces to allow me to solder the LED's back in place in series instead of parallel.


While the LED's were desoldered from the board I wired one of them up to a 9v battery through a 1k0 resistor.  Measuring the voltage across the LED I got a forward voltage Vf=2.8v.  Previously, I had measured the current of all three LED's together to be 130mA before taking this thing apart so, I know each LED can handle at least ~43mA.  To select my current limiting resistor for the series LED's I have ((14vBatt-(2.8*3))/40mA = 140R or standard value 150R for 37mA.  Power dissipation 0.207W so, a 1/4 Watt resistor is fine.

Here, Vbatt = 14v because the car's 12v system operates around 13v with the car running at idle and peaks up to ~14v when the RPM of the alternator increases.  14v is kind of a design for "worst case" scenario with lower battery voltages resulting in LED operation towards safer limits. 

Here you can see the LED's soldered in the new series configuration and current limiting resistor in place.  In series, the LED's are hooked up anode to cathode of each other with a current limiting resistor on one end, the negative side of 12v in my case.


Here you can see the LED PCB hot glued into place in the light reflector housing.  You can also see the PCB traces I cut to allow the parallel/series modification.  The +/- 12v (nominal) power leads on the LED board are soldered to the wires that used to deliver battery voltage to the incandescent bulb.  The connector for the original lamp was left in place electrically and solder joints were insulated against short circuits.

A side by side comparison shows the difference in color of the LED's on the left and the incandescent bulb on the right.  The LED's are equally as bright and diffuse quite well so there is no "hot spot" of light.  I will perform the same LED retrofit to the right side of the light housing next.

This was a fun afternoon project and I am pleased with the results.  Average prices for ready made LED replacement bulbs for this style holder seem to be $15 x 3 lights = $45.  The method outlined above came in at ~$3 total. 

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