Instructables

Interior Auto Lighting

Flip the switch and make sure it your inexpensive auto interior lighting works.

Sorry for lack of images of the finished product lighting up the floormats but the camera for some reason wont pick up the light and gives me black images.

The final product looks really good in person, if you arent satisfied with the brightness you could just add some leds.

bikeNomad7 years ago
(1) LEDs must be connected in the right direction! If you look in the LED, the lead with the larger internal structure is usually the cathode, which should be connected to the negative side of the power source. (2) You *DO* want to have a resistor in series, or you *WILL* probably burn out the LEDs. Unlike incandescent lights, an LED will maintain a fixed voltage (at a given temperature) across itself. Which means that (generally) you can NEVER attach LEDs directly to a power source. You have to use resistors or some other means of limiting the current! The only reason that the little keychain lights don't fry the LEDs is that that internal resistance of the watch batteries that they use is so high. And you can typically expect a nominal 14.4V from the alternator when running, but that can vary up to maybe 15V or so. White LEDs tend to have a forward voltage drop of between 3 and 4 volts. So you should be able to hook 4 or 3 of them together with a resistor in series to have a light that won't burn out but will still work when the car is turned off. What size resistor? Typical white LEDs are good for 20 or 30 mA or so. So if your forward voltage for the LED chain is 12V, you'd want to have a resistor no smaller than (14.4-12)/0.03 = 2.4/0.03 = 80 ohms or so. 82 ohms is the nearest 5% resistor value. For 20mA, you'd use 120 ohms. But this depends on the forward voltage of your LEDs. For testing the forward voltage of your LEDs, start with (say) 220 ohms, and measure with a meter across the LEDs. Then calculate the desired resistor as above. If your LEDs don't come on at all with the 220 ohm resistor, take one LED out of the chain and try again.
AMEN! ive been using leds on my car for a while now and i SUPPORT EVERY WORD this guy said, most led instructables have the same error... you have to limit the current going through the LED if you want it to last.
ve2vfd7 years ago
You'll probably want to add resistors as a car's voltage is 12-13 vdc when the engine is off, but when it's on the alternator spits out 14.4vdc... That extra 2.4vdc is enough to kill or seriously shorten the life of your LED's.
jumpfroggy7 years ago
Great writeup. Both of my interior lights burned out and I have a 100 pack of bright white LED's. I planned on doing this, but I'll probably put 3-4 in a row and wire them directly to the bulb sockets (since I'm replacing existing lights). This would make for good glow lighting near the feet, etc.