2016-04-02 LED Light Ideas For Autos

I bought a few dozen 1 watt LEDs when they were on sale last year at Electronic Goldmine.  They go on sale periodically, for well under a dollar each in a package of 5.  A while back they offered daylight but recently they have all been warm white.

These are surface mount, so they are supposed to be mounted to a PC board for heat sinking.  But instead I glue them to an aluminum heatsink with thermal adhesive called ?.  It works good, the heatsink takes away the heat and I can run the LED at full power, but I usually run them at a half watt or so.

I thought I should come up with some ideas on how to use them in a vehicle.  Earlier this year I bought a reel of flexible strip LEDs that are made for 12 volts, and these can be cut to length on the markers so just about any length can be used.  But they are not as good for replacing incandescent lights in point sources as 1, 2 or 3 LEDs on a round heatsink.

My Civic is just a few years old, but it came with incandescents for the lights.  I’ve replaced some with kits I got on eBay.  But I want to make my own for a few lights that need to be replaced. 

The main reason I want to replace the old lights is that I have had at least three occasions where the mirror or dome lights have been left on, causing the battery to be run down after a few days and then the car won’t start. This again happened recently, renewing my interest in replacing them. By doing so, the LEDs take less current and don’t run the battery down as fast, and the LED lights dim to a very low current quickly as the voltage drops, hopefully stopping the battery from being drained fully.

My plans were to put three 1 watt LEDs in series for a supply voltage of about 10 volts minimum. But I would also have designs for 2 and 1 LED. The three LED design is simple; just three LEDs in series with a small value resistor to limit the current. The 2 and 1 LED designs may also use just a simple resistor, but then the light will eventually run the battery down to 6 or 3 volts. To prevent this, I can use other methods of limiting the current. One would be to put a Zener diode in series with the LED to drop more of the voltage. A 6.2 volt Zener and single LED will give the same design as the 3 LED light, with the same resistor. A 3.3 volt Zener in series with 2 LEDs will give the same design as the 3 LED light, with the same resistor.

I thought about using a current limiter circuit. This gives a constant current as the battery voltage changes. This is good for constant light, but I want the current to drop to a much lower value as the battery voltage drops. Below about 12 volts, the circuit current should drop to a minimum value just enough to keep it running. So for now I will use the resistor for this current limiter.

The resistor is chosen to give the maximum design current at the maximum battery voltage. The LEDs are rated for 1 watt at about 3.3 volts, which is about 350 milliamps with good heat sinks. I figure that 250 mA would be a good choice, at 15 volts. So for 3 LEDs, 15 volts minus 9.6 volts gives 5.4 volts. Divide that by 0.25 amp gives 21.6 ohms. A standard value of 22 ohms is chosen. Multiplying 5.4 volts by 0.25 amp gives 1.35 watts power dissipated. A 2 watt resistor should be used. Or two 1 watt resistors, or four 1/2 watt resistors could be used. Using three 1 watt resistors is okay, but three 1/2 watt resistors is too close to their maximum power dissipation, so I would reduce the total resistor dissipation to 1 watt in that case. If I multiply 5.6 ohms by 4, I get 22.4 ohms, and 1/2 watt per resistor gives 2 watts total.

I can do it with four 91 ohm, 1/2 watt resistors in parallel, giving 22.75 ohms. Or else use four easier to get 100 ohm resistors, giving 25 ohms total. If you have to have the few mA lost with 25 ohms, then put a 180 ohm resistor in parallel with the four 100 ohms, to bring it down to 22 ohms.

The resistor’s values do not have to be the same. It is also easy to add another switch to turn on only 1 or 2 resistors to give a low and high brightness choice.

So with this, we have the basic simple design for three LEDs, or 2 LEDs or 1 LED and a Zener diode.


More complex, sophisticated designs can be used. Usually these are buck switching regulators. They do not have to be complicated. But these circuits don’t shut down when the battery voltage drops. One simple buck regulator is the Roman Black regulator, from http://romanblack.com. I found that this design does not start reliably with a load connected. If it does not start, it could damage the LED or burn itself up. If it does, it can fail shorted, putting very high current through the LED.

I also have a circuit from a 5V charger that plugs into the cigarette lighter socket. This circuit failed, so I’m not sure I want to use it! But it used a small transistor to pass a high peak current, so that may have been the reason it failed. I could use a higher power transistor to prevent this.

QS has a buck current regulated circuit at http://quantsuff.com. The nice thing is the current regulation. But I have found that you can buy circuits designed for powering LEDs from sellers on eBay or Amazon. These are adjustable for different LED currents.


One thing the typical buck regulator circuits have in common is an inductor or choke. These are more difficult to get than other components. I’ve thought about designing a circuit that doesn’t use an inductor. The circuit’s efficiency may be lower, but it still may be more efficient than a simple resistor. Another addition to design into this circuit is a timer. After a minute or so the light will start to dim and a pushbutton must then be pressed to add more time. The switch would be a three position switch: off, on and start timer. This circuit would be a simple analog circuit with a capacitor and resistor that discharge in a minute or so. I don’t see a need for a digital counter timer, but if a microcontroller is used somewhere else, it could be also used as a light timer. The pulse width could control the brightness.

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