View Full Version : Direct Drive Blue LED's

01-28-2009, 12:33 PM
Instead of using a Ren-16 to controll lights in a car i have decided to use a 595 with about 20 channels (2 LED's per channel) to control blue LED's. Now what i need to do is know how i could power this. I was thinking of using a 7805 to turn 12vDC into 5vDC and then direct drive the LED's from the 12vDC fromthe car. Will the 7805 be able to handle enough current to run 595 board itself. My next probem is how will i control 12vDC out with the 2803? In think there are a couple of copper traces i could cut in order to input 12v to the board and that will only go through the 2803 and not the rest of the board. Also will the uln2808 be able to handle enough current to drive 2 LED's on each channel? Im sorry that i have so many questions but i dont want to build the whole thing and then realize it wont work!
Thanks to any replies in advance.


P. Short
01-28-2009, 01:15 PM
Are you planning on operating this project while the car engine is running? The alternator may put spikes of several hundred volts (either polarity) onto the 12V supply, as well as increasing the normal voltage up to perhaps 14V. I don't know the answer, but if I were you I would do a lot more research on powering electronics in the automotive environment.

In addition, that 7805 is going to run very hot if the current for the LEDs is going through it. If the LED current is 25mA then the power dissipated on the 7805 would be 2.8W when all of the LEDs are on at full power (or even more if you are running off the alternator output).

01-29-2009, 01:52 PM
Ok first off here is what you do...If you wanna run these lights off a controller inside the car you ahve to a: modify the controller to run off of 12v dc.
Second you have to modify everything else from power supply on down to run off dc voltage...Well it's not as hard as it would seem...Voltage regulators in cars stop voltage spikes as long as they are connected correctly...Which again is pretty simple..
Find an unused 20 amp or 15 amp side in your cars fuse panel...Take 2 14 gauge wires and connect em up..White or red for positive and black or green for ground...Now get a lights plug for about $2.00 from an auto parts store..Then go get yourself a dc to ac converter for about $60.00...Hook your board up to that and run your lights till your hears content..Yesa this done work just be careful of the amperage as you will have to convert 300 watts at 12 volts to amps...
It's that simple altho I'd never do it...If you want lights to work there is another way the crazy light lady did...Using a ddc light controller which is posted on her site http:
//www.crazylightlady.com (http://www.crazylightlady.com)
Also to if you simply wanted a neat effect I used to do all the time when I was in car audio is simply hook lights up to the speaker wires off the stereo amplifer and use that to drive limited amounts of lights to the beat of the music depending on what frequencies you have it set to..If you use the bass only then everytime a bass note hits it will trigger the lights...Simple stuff that was done back in the 80's for show cars...

Hope this info helps.

01-29-2009, 03:03 PM
Unless this is going into a classic car, I’m not sure that this is so true any more. With the use of electronic regulation, control should be much faster and tighter. There are a lot of microprocessors used throughout cars now; can’t be blowing up these either.

I used to make electronic projects for a car when I was a teenager (long time ago) and didn’t know much about power filtering. The car had a mechanical regulator so the supply had to be horrible. I never had anything die.

BTW, make sure that it’s legal to do this in your state. Some states don’t allow flashing lights that are visible outside of the car. Some colors aren’t allowed either.

01-29-2009, 03:12 PM
Very true Ernie but for show cars anything is allowed....Back in ym day for the show van we had going at the time we had a power supply to power it at the shows that hooked up to 120 volts as they sound system and video game system ran off of 2 surrey locomotive starting batteries that were 6 volts each and chained together to make 12 volts...Not really the way to do it today but back then it was the way things were done..

01-29-2009, 06:21 PM

01-30-2009, 11:41 AM
Here is the circuit to do this trick.


01-30-2009, 04:13 PM

01-30-2009, 04:34 PM
To get lower voltages in a vehicle, I use an LM317 adjustable regulator, good for 1.5A when properly set up. I made a 4.5V regulator, which did dual duty for a portable CD player as well as a guitar processor/headphone amplifier.
Using various resistors will set the output voltage. Only requirement is input voltage must be at least 1.2 V above your required output voltage.
That would be easier than DC-AC-DC conversion. And it fit inside a 35mm film canister.

A Marchini
02-01-2009, 03:31 AM
First off they will not be on while the car is moving.

Second what I am actualy doing is making a knight rider type display.

Next what my plan is to do now is to buy a 12vdc to 120vac voltage converter, and pluging in a small computer power supply to that to give me 5vdc.
then I will directly drive the blue LED'S off of the ULN2803.

Acording to the datasheet for the 2803 it can handle .5A per chip (or 25 20mA LED'S) which is more than enough. I will connect 2 LED's per channel for a total of 32 LED's which will be (32x20mA=640mA)

Any input would be great!
And thanks for all of your replies.

So you have 2 ULN2803, with 2 LEDs each output , right? Are you connecting the LEDs in series? If so then its 20mA each channel, so 20mA x16 = 320mA total.
Two LEDs will probably drop nearly 5V by themselves, and the Vce of the ULN2803 is 1V so you will probably want to drive them from more than 5V.

Also, there are people that make automobile specific Dc/DC converters, takes all the power issues out of the equation.

Tony M.

02-07-2009, 08:31 PM
Mr Marchini is right in that if you are driving 2 LEDs per channel anyway, it makes more sense to put them in series rather than in parallel, that way you halve the total current draw. But you can't drive them from 5V, it will have to be from the battery.

Every discrete blue LED I've ever seen has a typical forward voltage of 3.4 volts and a nominal current of 20 milliamps. The maximums tend to be about 3.6V and 30ma, so you have leeway. If the car is stationary with the engine off then the nominal supply from the battery is 12V and the forward voltage drop across two diodes in series and the ULN2803 is 7.8V, so you need to connect a resistor in there as well to limit the current to 20ma. From Ohms Law, the resistor you need is (12-7.8)/0.02 = 210 ohms. The nearest standard (E12/24) value above this is 220 ohms and the wattage needs to be 4.2*0.02 = 0.084W. You could use a 1/8W resistor but a 1/4W one is better, it won't run hot.

Now taking the example of the engine running, a standard alternator will normally regulate the battery charge voltage to about 14V, so the calculation for the current drawn through the same resistor is (14-7.8)/220 = 28ma. This is too close to the maximum for comfort, so for a 20ma limit the resistor would have to be 310 ohms at 0.124W. The nearest standard value to this is 300R, 1/4W (= 21ma).

If you will never use the LEDs with the engine running then a 220 ohm resistor is fine. If you may have the engine running at some point, then a good compromise would be a 270 ohm 1/4 watt resistor. This will limit the current to 23ma at 14V or 15.6ma at 12V. This lower current draw at 12V isn't significant, the light output will still be OK.

Hope this helps.