Modifying a Car Tail Light Circuit

I have discussed a car tail light circuit with chasing features in one of my earlier blogs. The system is able to transform into a full fledged brake light with all the red LEDs lit and also is able to perform the function of chasing turn signal indicator by switching the same sets of LEDs.
Recently some modifications were suggested by one of the interested readers of this blog, let's know more it.

Mr. Jefferson Furtado who is an electronic hobbyists and a keen reader of my articles suggested me some modifications for the above circuit which according to him would make the circuit more efficient with its operations. This is what Mr. Jefferson recommended:

"Just curious why you used the 4060 instead of two 4049s. Was also thinking I could do the same thing with a 556 and used the second timer to pwm the taillights instead of using the resister. What do you think? What are the advantages/disadvantages of the different oscillators?

Also I was thinking of using an inverter on the brake lead and nor gate it with the blinker lead so the blinker overrides the brake. How it is now if you stop at a left turn light, when you hit the brake the blinker effectively turns off. I could also do the same with the dimmed tail lights if they cause a problem with the sequencing. What do you think of this solution?

I'm a complete noob, I really just researched enough to be dangerous so any insights would be greatly appreciated."

My response:

The IC 4060 section can be replaced by any suitable oscillator, there's no specific reason behind using the IC4060, any similar oscillator will do.

Your suggestion looks interesting, would be nice if you could provide it in the schematic form."

Jeff = I'm not sure how to add images to a reply, but here are links to my schematic

I think I need to change my schematic so that I'm running the ICs from a 5 volt regulator and use the 12 volt regulator to run the LEDs to maintain a consistent brightness level. Can I piggy back the 7812 to increase the AMP capabilities or should I just use one 7812 for the left and another 7812 for the right?


My analysis about the above circuits:

The first circuit looks perfect to me, the second circuit concept is also OK, however the transistors are wrongly configured.

Correction= The transistors are NPN types, so all the diodes and the LEDs in line should be reversed, the resistors common point goes to the positive instead of negative as wrongly shown in the diagram

Thanks Jeff, that was indeed a great effort, appreciate it.

The 7812 IC need not be enhanced because it has sufficient current for driving all the connected LEDs.

9 thoughts on “Modifying a Car Tail Light Circuit

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Haha I realiazed I had the transistors wrong after I posted, I'm working on a new update. I did swap the 7812 out for a 7805 becuase the chart I used to figure the resisters and pots used with the 556 used a 5 volt source. I then added two 7812s to power the LEDS, one for each side. I know the 7812 will support up to 1A, but each of my taillights will actually have 27 70mA LEDs which is 1.89A. I could run half of each taillight off of a seperate regulator but I thought piggy backing would be easier. I did have some confusion about the transistors, I started out with PNP transistors but if I understand correctly they close on a low signal instead of a high so I stuck with the NPNs. My thought was I needed to switch on the V+ for the LEDs so I could use the regulated power supply, but I see now that I can still connect the LEDs to the regulator and switch on the V0 side. Thank you for your comments and I really appreciate the help!

    I'll post again when I have my new schematic done.

    • I dont understand, am I missing some fundamental? Am I doing the math wrong because I have LEDs in series and strands in parallel? My thinking is; If I have a strand of 3 LEDs and each LED consumes 70mA, my string draw 210mA. Then with the brake light and all 9 strands lit up my draw will be 210*9=1890 or ~1.9A.

    • LEDs in series always divide the same current across them, so each string with 3 LEDs in series will consume only 20 mA (not 70 mA) because 20 mA is a healthy amount of current for keeping the LeDs bright. so finally 20*9 becomes 180mA

    • I see that I was adding everything and that is wrong so I came up with this; I have 3 LEDs consume 70mA at 2.6 volts. Basic Ohm's Law says R=V/I = 2.6/.07 = 37. So my circuit is R1, R2, R3, R4. R2-4 (the LEDs) are all 37ohm/.07A/2.6V. For R1 R=V/I= 12-(2.6+2.6+2.6)/.07 = 12-7.8/.07 = 4.2/.07 = 60. Now if I=V/R = 12/37+37+37+60 = 12/171 = .07A or 70mA. 70mA*9strings = 630mA.
      | 4.2v 2.6v |
      | |
      12v/.07A |
      | |
      2.6v 2.6v

      This tells me the 7812 can easily run one taillight with a good safety margin and might be able to run two for a short time (I'll stick with 1 for each side)

      It makes sense that the current is constant accross the circuit, so please explain how/why the current would divide accross the LEDs and which law that is? Am I still doing something wrong or we just using different spec for the LEDs

      The actual LED specs are:
      Forward Voltage VF (Volts) @ IF = 70mA
      Min Typ Max
      2.19 2.6 3.03

    • You might have read about 3 LEDs in parallel, which would have certainly consumed 70 mA, that simply means each LED would be consuming 70/3 = 23mA

      Here the LEDs are connected in series for each string so each string will not consume more than 20 mA….. that's the basic spec for most LeDs.

      From where did you get 70 mA as the actual spec, can you please show me?

    • I got the specs from Phillips. Their LUXEON Rebel Color LEDs are max 1A and typical 350mA or 700mA. They also make a 1A 5-15vdc LED suitable for headlamps. I was looking at thier SuperFlux LEDs;

      I was also toying with the idea of using red, green, and blue Rebel LEDs so I could play with the color. If I did that I would use your design for a 12A car voltage regulator using a lm317 and your prgrammable timing circuit to PWM the different LEDS for color mixing. Starting to think beyond taillights, maybe under car illumination.

      Another thought I had was to replace the 556 with a single 4049, that would give me enough NOT gates to time two circuits and invert the blinkers leads before going into the 4001.

      I've been programming for nearly 20 yrs, with electronics and micro controllers I can finally bridge the gap between my code and the physical world. Why didn't I get into electronics a long time ago!?!! Love it! I'm also working on a servo mechanism with a stepper motor for throttle by wire with electronic cruise control. I did the first prototype with an Arduino, it works good for throttle control, but its not fast enough for the quadratic position sensing for cruise control. I'll share when I'm further along 🙂

    • OK, if you are planning to use the 1 watt LEDs then your calculation is right, I had recommended the ordinary 5mm LEDs for the proposed taillight circuit, that's why I got confused with the current ratings.

      Great job, appreciate it, carry on:)

Leave a Comment