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Modifying Car Turn Signal Lights, Park-Lights and Side-Marker Lights

Modifying Car Turn Signal Lights, Park-Lights and Side-Marker Lights

The post explains an innovative circuit modification which allows a single common lamp to be used as a parking light, turn signal indicator light, as well as a side marker light on the relevant positions. The idea was requested by Mr. Chris

Technical Specifications

I have a project that is slightly related to this one and could use your help.

I have 2 single filament bulbs that I would like to use as parking lights as well as turn signals.

They would be wired to stay on anytime the car is running, and each light would turn off and on when the corresponding turn signal is activated.

The wire from factory the turn signal harness controls the +12 side. I also have 2 single filament bulbs that I would like to use as side markers anytime the parking lights are on, and I want them to flash opposite the turn signal, so when the turn signal bulb is lit the side marker is off, and when the turn signal bulb is not lit, the side marker bulb is.

The side markers will not be lit unless the parking lights are on, but I do still want them to flash anytime the turn signals are activated. I prefer not to use led's as my project is striving for a retro look, but I might if it's not possible with the incandescent bulbs.

The Design

According to the first requirement a single lamp needs to perform the dual function of a turn signal lamp as well as a park light, also the lamp should switch ON as soon as the turn signal switch is turned ON.

The following modified turn signal cum park lamp implements the features exactly as per the above specs.

When the park light is in switched ON position, the TIP122 switches ON the lamp through its base trigger via the upper diode and the series 1K resistor.

Now, if the turn signal switch is triggered, the TIP122 responds by flashing the lamp, while the lower BC547 transistor which also gets triggered with the turn signals makes sure that the signal received from the park light switch is grounded and inhibited from influencing the lamp.

The second requirement demands another lamp which may be positioned as a side marker lamp to respond to the above side indicator or the turn signals but flash with an opposite switching. Also the same lamp is also supposed to light up when the park lights are ON.

The diagram of the enhanced or the modified side marker lamp as shown above satisfies the conditions by flashing the connected lamp with opposite switching, and also responds to the parking  switch toggling during normal operations.

The TIP122 is responsible for triggering the lamp when the park lights are switch in.

However when the turn signal switch is toggled, irrespective of the park light input the BC547 begins oscillating in response to the turn flash signals causing the TIP122 to also blink the lamp with the corresponding flash rate.

The 100uF capacitor makes sure that the TIP122 is fed with its stored positive feed for sustaining the illumination on the bulb during the flashing actions.

As per the suggestions from Mr. Chris, the first diagram has been slightly modified with the following couple of improvements:

1) The transistor has been upgraded to TIP142.

2) The park-light input trigger has been replaced with +12V trigger from the ignition key so that the lamp works as fog lights and not as parking lights.

The above circuit could be enhanced further by adding a PWM dimming control feature as shown below. The feature enables dimming of the fog lights when the park lights are turned ON, but ensures that the effect is inhibited as soon as the turn signals are switched ON.

The 100k preset could be set appropriately for achieving the desired reduction in the fog light illumination



About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

19 thoughts on “Modifying Car Turn Signal Lights, Park-Lights and Side-Marker Lights”

    • I assume your hazard light is getting the supply from the motorcycle battery right? so yes in the circuit too the supply will be from the battery and will cut off each time the turn signals are switch ON.

  1. Wow nice project! I already search your website but I cant find what I'm looking for. I know this is off topic and not the right place to request. I hope you can help me. I have a motorcycle raider 150 or satria F150(other international name). I'm from the Philippines.

    My motorcycle have a hazard lights and all the bulbs are LEDs. I'm using aftermarket Electronic Flasher relay. I have a minor complain with my hazard lights. When the hazard light is switched ON, the turn signal switch is useless. I have to turn off the hazard switch so that I can make a turn signal and its irritating.

    I already try using 5pins relay, mosfets and diodes and simulate the prototype circuit into my circuit simulator software but no luck.

    Please make a circuit that temporary cancel or switch off the hazard light when you toggle the left or right signal switch of the motorcycle and goes back to hazard light mode again if the turn signal switch is set to OFF state.

    Thanks and hoping a good response from you!

    • I am glad you liked the above project!!

      I think the second circuit from top can be applied for your requirement.

      the indicated side-marker lamp now becomes your Hazard lamp.

      This design will allow you to keep the hazard lamp shut off as long as the turn signals are working.

  2. Great. I'll change up the Capacitor to shorten the delay.

    I definitely would like to try the 555 circuit when you have time. I already have the 555's. PWM dimming is not something I have done before, so I get to learn something new.

  3. I will add a better heatsink to the TIP142

    The astable circuit is interesting. I haven't seen that before. I don't think it will work however because the turn signal function also operates when the parking lights are on. This would mean that the bulb would also dim when the turn signals are on if the parking lights are also on. I'm not sure how you could separate the two. I might have to leave this option out. Also, currently the circuit is connected to the parking light, but only for the side marker function in the second diagram.

    Did you see my question in the 2nd paragraph about the delay? I assume lowering the capacitor value will shorten the delay, but I want to make sure before I change it. Thanks again for all your help. I'm almost there.

    • I think t can be tackled also, meaning the 555 circuit will respond only when the park lights are ON and not when the turn signals are ON. If you decide to go for it, we can discuss it in the course.

      I just forgot to address the second issue in my previous comment, yes the delay on the fog lights can be corrected by reducing the 100uF capacitor to may be 22uF

  4. Ok. Finally got everything working. I made a mistake on the light bulb rating. I thought it was a 45w bulb but it is only 24w. I originally used a 55w for testing and the TIP142 got very hot very quick. I then found the correct bulb and added the heatsink you suggested, with some thermal compound and after 1 hour of continuous operation it reaches 137 degree F. I am really surprised to see it get this hot. I am only pulling 2A total for both bulbs. The TIP142 should only have about 1.8A on it. My testing was done in the house at 70 degrees F. My concern is when I put the circuit under the hood of the car especially in the summer. Is the TIP142 going to be ok or do I need to find another way to cool it further?

    Everything in the circuit seemed to work as expected. One thing I noticed is that when the turn signal is turned off, it takes the fog light bulb about 4 seconds to come back on. Can we shorten this delay by reducing the capacitor value?

    I thought of one more addition that would be a nice feature to this project if it isn't too much trouble to add. It would be nice if we could dim the fog light bulb somewhat when the parking lights are turned on. Maybe we could reduce the voltage to around 9V to achieve this. I haven't experimented with any lower voltages yet. I thought about using an IC 7809 but it won't handle enough current and I don't want the flashing turn signal function to dim, or the side marker, only the main fog light bulb and only when the parking lights are on. If you have time, let me know what you think about this extra function. Thanks

    • yes that's quite a mystery how these devices become so hot despite of their datasheets indicating formidable amp and watt specs for these devices.

      Anyway, we don't have any other choice than making the heatsink larger in size.

      The parking lights are not included in the above designs I believe, and therefore the trigger would need to be taken from the parking light switch for initiating the dimming action over the shown fog lights.

      There isn't an easy way of implementing this, one way could be to use a 555 IC astable circuit as given here:


      Once you build it the integrations won't be difficult.

      Connect pin3 of the IC to the base of TIP142 via a 1N4007 diode (cathode to pin3 and anode to the base of TIP142)

      now, the IC555 circuit should get powered when the park light are switched ON, it could be simply done by hooking up the supply lines of the circuit with the parking lamp terminals.

      R1 could be a 22k resistor while R2 = 100k preset and C1 = 0.1uF

      optimizing the above preset would help you achieve the desired dimming effect over the fog lights whenever the park lights are switched ON.

  5. Great. Thank you. I ordered the tip 142's. What size aluminum plate or heatsink do you think is necessary? I will probably have all 4 transistors for each side on a single plate.

    • I don't think heatsinks would be absolutely necessary since the TIP142s are rated to carry 15amps comfortably, however it'll need to checked practically how much warm the deices get during continuous operations, if it feels significantly hot probably then we'll have to resort to the aluminum plates on them, the optimal size will need to be fixed with some trial and error.

      I guess the following type will be good enough:


  6. You are probably correct. I like this circuit as it would be more compact, but a large heatsink would negate that. One problem I see with relays is I don't think we would want a relay switching on and off anytime the turn signal is on. I think it would wear the relay out pretty quick unless you had a different idea. One thing I probably should have mentioned, in case you decide to draw up another diagram, is that the fog light will act more like a daytime running light than a parking light except that it will stay on at night also. It will not be controlled by the parking light switch like the side marker. That's my fault for calling it a parking light. I think the only change that would need to be made is to replace the parking light switch input in the first diagram with key 12v +. I'm anxious to hear what you think our best option is. I apologize if you get multiples.

    I hope you just get this once. I sent from my phone and had to retype it several times as I kept loosing it somehow.

    • Although modern relays are pretty tough and won't wear out so easily unless their contacts are misused with wrongs specs, still a solid-state option is always desirable if it's feasible. So I think we should quit the relays and revert to transistors.

      For avoiding large heatsinks we can opt for a heavier transistor in terms of current (amps), so we could probably replace the TIP122 with TIP142, I guess it won't require a heatsink for 45 watt load, however a small aluminum palte attached to it will make things much efficient so it can be tried.

      I'll make the required changes in the first diagram soon.

      yes the presented comment got posted many times, but that's not an issue, i have deleted all the repetitions comfortably.

  7. I do have one question. The bulb for the side marker is a small 194 wedge bulb that shouldn't draw much power. The other bulb is a 45 watt fog light bulb. Will the TIP122's be able to handle the current draw of these bulbs?

    • If we go as per the datasheet of the device, we find its collector capacity to be at 5amps, which means it would be capable of holding 45 watts quite comfortably, but would require a large finned heatsink which could make it a lot bulkier

      I think we should rather go for a relay circuit instead? what do you think

      I'll try to update a relay alternative in the above article soon.

  8. Thank you for posting the diagram. Looks pretty straight forward. I'll order the parts today and post my results or questions as soon as I get everything together. Thanks again.

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