BMW E36 Blog

Turning Your BMW Lights On Automatically At Night . . . Buzzing Relay

1st May 2008

Turning Your BMW Lights On Automatically At Night . . . Buzzing Relay

posted in Electrical, Technical Info, Tips & Tricks, Uncategorized |

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I wrote seven articles until now about how you can make your BMW lights turn on automatically at night. I will summarize these posts for you in case you have missed them. In the first post, I proved that this should be something easy to implement inside a BMW E36 car. In the second post, I presented a circuit diagram that will help you create the electronic circuit that is supposed to control your lights and turn them on automatically when it gets dark. Of course, if you don’t know how to implement such a circuit or you simply feeling too lazy to do it, you can ask some electrician to do it for you. I’m sure he will agree if you paid him $20. In the third post, I wrote about my first attempt to implement this circuit and told you that it’s partially working but it needed some enhancements. In the fourth post, I created a video with the circuit implemented on breadboard and connected directly to my brother’s car lights. In the fifth post, I soldered the circuit on permanent board and showed you exactly how I intend to use it inside my car. In the sixth post, I posted 4 videos that I shoot while testing the auto light system. In case you didn’t see these videos I suggest you check them first before proceeding. In the seventh post, I talked about a problem in controlling the headlights of the car (low beam lights). Until that time, I was able to get my dashboard lights and the angel eyes lights to turn on automatically at night, but not the low beam lights.

When I connect the circuit to control the low beam lights, I hear a buzzing sound that comes from the relay which is supposed to work as an electronic light switch. Normally the buzzing sound indicates that the relay is connecting/disconnecting really fast and that what causes the buzzing sound. So, I tried to analyze the problem to find out what is really happening.

It’s all explained in the electronic diagram below:-

I think that when the Autolight circuit sends a signal to the relay, the relay simply closes the switch which turns the headlights on. The problem occurs when the switch is closed which makes both the relay and the headlights to become connected in parallel and since the headlights resistance is much lower than the relay resistance, the relay simply disconnects which makes the headlights turn off. When the headlights turn off, the electric current passes again through the relay which makes it connect again and thus turn the headlights on again. Of course, this happens very fast which results in a buzzing sound from the relay because it’s connecting/disconnecting very fast. I think I will need to make some change to the circuit to isolate its power source somehow from the headlight power. But how can I do this? this is the big question 😀 . If you have any ideas, please feel free to share them.


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There are currently 4 responses to “Turning Your BMW Lights On Automatically At Night . . . Buzzing Relay”

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  1. 1 On May 5th, 2008, ricky said:

    You are suspecting that with the current going thru the headlight and the relay in parallel, the relay isn’t able to pull enough current to make the relay close. One wouldn’t think the headlights would be that much less resistance than the relay. Perhaps the current is not going directly through the headlight beams but goes to another control module. Add resistance to the headlight side?

    Optocoupler can work as a solid state relay..

    Maybe you want to replace the light-sensor module with a simple switch to get the relay working first?

  2. 2 On May 5th, 2008, Tony Sticks said:

    Hi Ricky,

    Thanks for the suggestion. It seems the resistance of the headlights is very very low (almost zero) when they’re turned on, then after a moment, their resistance goes higher. The problem is that when they’re turned on, they will consume all the current momentarily which will make the circuit turn off and so the relay is turned off too which cuts the power off the headlights. When the headlights are turned off, the circuit will run again and this will make the circuit go in a loop.

    Some people suggested that I should use a totally separate wires to power the circuit itself (from the battery for instance) instead of getting them directly from the hot wires in the light switch. This will prevent the circuit from being affected by the headlights turning on.

    I’m thinking of using the wires that power my radio as the source for the circuit because as we all know, the radio doesn’t stop when we turn our headlights on. This means that the current that is powering the radio is constant.

    I’m going to try this idea next week and hopefully it will work.

  3. 3 On May 7th, 2008, hanipon said:

    bro… this project is cool… hmm.. i know how difficult will it be when we try to implement some electronic stuff.. have to calculated everytthing.. current, resistance, voltage and so on… i would like to suggest.. here i attach attach link which maybe can help u… how if u try change the sensor circuit from what u have done to this one.. only the sensor circuit part.. it is quite similar to what u have done.. just add a few resistor.. the relay, i think it’s better to use same spec as headlight relay..

    good luck bro.. looking forward to this project.. hope can implement this thing to my ride.. tq…p/s:sorry..bad english

  4. 4 On May 8th, 2008, Tony Sticks said:

    Hi Hanipon,

    Thanks for the comment. I’m not sure if the problem in my car is the electronic circuit itself. I believe, after discussing this issue with some experienced users, that the problem is in my installation of the circuit inside my car, not the circuit itself. besides, my circuit is simpler than the one that you’ve pointed out :p.

    Anyway, I have couple suggestions that I’m going to try this weekend. I hope it’ll work.

    thanks again for the comment.