# LED Ground effects kit for your car

9 Steps

## Step 7: Circuit time!

If you're at all savvy with circuit boards this should be cake. This circuit creates the pulse effect, if you want just a continuous light, you can skip this.

The idea is the 555 timer generates a square wave which charges up the capacitors in the on phase. In the off phase, the capacitors light the LEDs, depleting over time and creating the fade out effect. Theoretically, during the on phase, the current used to charge the capacitors should detract from the LEDs, creating a fade in effect, but often the current is so large that this is nearly instantaneous. Alternatively, you can put in a switch that bypasses the pulse circuit (so that the ground from the LEDs goes straight to the mains ground) so you can go from pulse to steady on.

I more or less used capacitors and resistors that were lying around. You can customize the frequency using the 555 timer calculator. Alternatively, you could sub in a potentiometer for RB to get a variable rate.

The 12000 uF capacitor is just a bunch of large 12V capacitors wired together. You may need more or less depending on the number and rating of the LED strings to get the right effect.

Test out the circuit on a breadboard connected to the car's electronics before putting it onto a PCB. And then test it when you've soldered it before boxing it up.
Remove these ads by Signing Up
farzad_sh_2020 says: Aug 12, 2010. 3:04 PM
Hi i do all of these works on the board but there is no pulse on led. what can be wrong ? all the things is correct
HYPER_piccolo says: Apr 5, 2010. 2:43 AM
hi there, i know its been 3 years from the last reply, but i allways wanted to add the pulse affect on my undercar neons.
And that's my question, i have installed neons and not leds. can i somehow connects your "pulse generator" to my neons to have that effect?
thanks
DeusXMachina (author) in reply to HYPER_piccoloApr 8, 2010. 3:40 PM
Umm, I don't think neons would work with a pulse setup like that, because they need a constant voltage. It would have to be built into the setup, I think. But LEDs, lights, and certain fluorescents work.
HYPER_piccolo in reply to DeusXMachinaApr 9, 2010. 11:16 AM
yes, i asked a friend who is very good at electronics and he told me the same thing, so i guess i will make something with leds :)
thanks
capricorn111 says: Aug 4, 2009. 10:55 AM
i want to do same experiment with my car. and i need bit help.. i have a basic knowledge about electronics .. and few questions i want to ask.. that how many LEDs should i use in single (side ) strip? and 2nd question is should i connect LEDs in parallel ? and another question is if i want to attach 2 strips left and right under bottom of my car. can i use one circuit ? or i have to made two individual circuits for strips??
afifiaffiq says: May 30, 2009. 5:30 AM
Hi guys, if i just want a continuous light, i can skip this step right??how about step 8?? do i need to do that??thanks
eet_1024 says: May 19, 2009. 5:59 PM
I think your schematic needs some revising. First, there's no path for the current to flow to the 12,000uF Cap and LED banks. Second, there is no current limiting resistor between the output of the 555 and the bipolar transistor; though, that may be the only way to get the NPN to turn on enough without using a second transistor between the 555 and power transistor. If it were a N-MOSFET, then this wouldn't be much of a problem. Is the "pulse" effect just full on and full off (i.e. flashing)? Or, is it supposed to ramp up and down in brightness? I'm trying to figure the purpose of the capacitor bank.
DeusXMachina (author) in reply to eet_1024May 20, 2009. 7:47 AM
Sorry, the positive end is supposed to be hooked up to the car's 12V mains. I'm not really sure why I would need a current limiting resistor. The purpose of the capacitor bank is to convert the square pulse of the 555 to a ramp up/ramp down effect.
eet_1024 in reply to DeusXMachinaMay 20, 2009. 5:36 PM
The resistor limits the current through the 555 (which is typically rated for sourcing 200mA). For this application, I would use 68 ohm 3 watt or larger, depending on the current needed to saturate the power NPN. Personally, I would use a power MOSFET with low on resistance (such IRLML2502) to minimize heat dissipation. That would also let me uses a small wattage resistor. I'm guessing that wire resistance and inductance in conjunction with the 12mF caps form the time constant for the ramp up and down.
justrelax says: Jul 27, 2007. 12:54 PM
i was going to kill you if you wouldnt put the 100 ohm resistor there.=)i was trying but the 555 timer was overheated so many times. i am going to try it again..so i have to ask have many leds can i use for this proje?i was thinking over 100 ? <br/>
DeusXMachina (author) in reply to justrelaxAug 4, 2007. 2:37 PM
Because you're basically putting all the load through the transistor, you could use as many LEDs as your car's aux can supply. I think most cars can do at least 100 W, which is a ridiculous amount of LEDS. You could literally cover your car with LEDs and convince people it's a spaceship :-P but I guarantee you get pulled over. @TrumpetNeel - heh I was wondering who would notice it's an RC car. I couldn't find any good pictures of real cars.
I have another question: If I don't want it to flash or blink, and I just want a steady light, do I use resistors? If so what size should I be looking for? I just want to have the power go from the battery, through the switch, and into the lights. Should I put resistors in?
If the LED string is 12V, you can go directly from your 12V supply.
KronoNaut says: Jul 11, 2007. 2:07 AM
Just wondering. Would it be possible to use a tap from the coil for the pulse? If you could do that, then you would flash faster as your RPMs increased. -g
DeusXMachina (author) in reply to KronoNautJul 11, 2007. 10:37 PM
Directly off of the ignition coil....very unlikely as it uses really high voltages. I've thought of this same situation myself, the pulse rate determined by the RPM, and never really figured out a good way to do it. Perhaps if you made some sort of induction setup around one of the spark plug wires....basically there is no easy way to do it.
KronoNaut in reply to DeusXMachinaJul 12, 2007. 1:57 AM
I'll see if my mechanic can think of anything. The negative tap to the tach on th primary coil can jump from zero to 100 volts, then levels off at 12-14 as the points close. If that can be leveled off... Induction from the 10,000 plus volts off the secondary coil would work, but I'm hoping for more of a plug and play approach using that negative tap. If I find a solution, I'll post it.
sanity in reply to KronoNautAug 18, 2007. 8:29 AM
I like your idea, I think it would be sufficient enough to put a resistor of high enough ohms between the primary of the coil (not the secondary) and capacitor, and also with a ceramic capacitor decoupled to an RCA cable for your subwoofer!!! yeah cool, sub bass REV and blazing REV leds! :)
KronoNaut in reply to sanityAug 18, 2007. 2:18 PM
Here is some voltage info for the negative terminal from http://www.physicsforums.com/archive/index.php/t-40278.html.
'Points' closed, voltage will be close to zero. Period of time goes by and the points open. Voltage immediately spikes to 100 or more volts. This is due to the same reason the voltage on the secondary suddenly jumps to thousands of volts. The magnetic field in the coil collapses. A turns ratio difference causes the spikes to be lower on the negative side of the coil than the secondary. The spike drops back down as fast as it rose. There can be some steps in this waveform caused by the breakdown of air/fuel charge on the spark plug. The voltage finally levels out at about 12 to 14 volts until the points close again.

Perhaps you could attach a voltage regulator to the negative terminal to keep it kicked down to a lower voltage? http://www.allaboutcircuits.com/vol_6/chpt_5/7.html

Or even easier. use the negative terminal to to kick a relay that would deliver the 12+ volts to the lights.