I changed my turn signals to LED light bars the other day. They are completely legal, but the issue I'm having is I can't use my parking lights and turn signals. The bulb that I took out and replaced with the light bar had two filaments, I want the turn signal to work as follows: When the parking lights are on and I apply the right turn signal, I want the right light bar to flash while the left one stays on. And Vise Versa. Except when I do this now the light bar dims a very small amount while the parking light and the turn signal is applied. My Question would be, what do I need to do to get the light bar instead of dimming a little bit to turn off and on while my parking lights are on. I'm not sure if this little explanation made sense, but you can ask if you need more info. Thanks!


closed as off-topic by brhans, Transistor, Marcus Müller, winny, Finbarr Jan 8 at 16:17

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  • \$\begingroup\$ is it possible , this can never work without details? \$\endgroup\$ – Sunnyskyguy EE75 Jan 7 at 17:49
  • \$\begingroup\$ I can't find anyone who has done it before. But i know that in the rear of the vehicle some cars have brake lights or running lights that are LED that can also act as turn signals, so I am assuming I can do it in the front. \$\endgroup\$ – Alex Finch Jan 7 at 17:56
  • \$\begingroup\$ If the second filament of the bulb you took out was acting as a side lamp, it needs to go bright/dim/bright as the original did. To do this you need either a light bar that has this function built in (they have 3 wires), or find a controller that will do that, and some of these light bars won't respond well to the PWM that is used for the dimming if they've got their own drive electronics built in (they'll interact with the PWM and flicker). \$\endgroup\$ – Phil G Jan 7 at 18:00
  • \$\begingroup\$ hmmm okay, where could I find one of those controllers? Do you know what they're called? \$\endgroup\$ – Alex Finch Jan 7 at 18:50
  • \$\begingroup\$ "They are completely legal ..." Why do you say that? And where? (You have no location in your user profile and it can be a help in location specific questions.) \$\endgroup\$ – Transistor Jan 7 at 19:08

This can be done with three relays, single-pole, changeover type.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Parking light and turn signals.

How it works:

  • With parking lights off the blue wire is energised. If either turn signal is switched on RLY2 or 3 will click on and off illuminating the lightbar on that side when the normally open (NO) contact is closed.
  • With parking lights on the orange wire is energised. Both lightbars will turn on. If either turn signal is switched on RLY2 or 3 will click on and off disconnecting the lightbar on that side when the normally closed (NC) contact is opened.


simulate this circuit

Figure 2. Timing diagram.

Note: It is important that the turn signal changes state immediately the switch is moved. This avoids the half-second or so delay that would occur if the turn signal logic is not inverted when the lights are on. This design achieves that requirement. 0.5 s at 72 kph is a travelling distance of 10 m when one car is stationary and 20 m if both are moving towards each other at that speed. That half-second can make a big difference in avoiding a crash.


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