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Dome Light Relay Problem.

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  • Dome Light Relay Problem.

    Hi, ive been comming to mp3car for advise on my carputer project for some time, finally though ive come up with a problem that im yet to find a solution to, My car dome light works on some sort of relay i suspect.... Foolishly i assumed that the dome light in my VW Bora 2004 turned on by a +12v when key fob was pressed, it would appear infact it is turned on by some sort of a relay, as there is a +12v constant and a +12v that drops to 0v when the keyfob is pressed to open the doors which i assume is what causes the relay to cut out and the light come on.

    My question is how am i going to get a 12v to turn the PC on off of this dome light, do you think its best to put another relay in the roofing behind the dome light that acts the same as the current relay then run the switched 12v to the PC? or is there a more simple way.



  • #2
    Dome lights are often ground switched - ie, they are always connected to +12V.

    Complications with dimmers also occur.

    You want the PC to turn on the dome? Just interrupt the path to whatever switches the dome on - eg, door switches - but AFTER or without affecting any alarm or other triggers.

    Traditional dome light wiring:


    • #3
      Hi, I would like the dome light comming on to send a 12v to the PC to turn on, so the pc starts to power up remotely with the key fob. I tried wiring a wire into the dome light unit onto the positive of the bulb but the wire is constantly putting out 10v and when the light is elluminated it goes up to 12v. A question I've thought is would this 10v do nothing to power up the pc until it becomes 12v? Or would it just drain the battery this way?



      • #4
        I'm not sure you want to wire this to your dome light. When you hit the key fob, it will signal the PC to turn on alright. However, when you close the doors and start the car, the dome light goes out. That will signal the PC to shutdown.
        Originally posted by ghettocruzer
        I was gung ho on building a PC [until] just recently. However, between my new phone having internet and GPS and all...and this kit...Im starting to have trouble justfiying it haha.
        Want to:
        -Find out about the new iBug iPad install?
        -Find out about carPC's in just 5 minutes? View the Car PC 101 video


        • #5
          Hi, I've thought about that already, I've got another 12v that will take over once the ignition is on.


          • #6
            In that case it sounds like you have some kind of dimmer circuit on your dome light. If it is 10 volts when the light is out and 12 when it is one, you'll need a relay that closes at 12 volts an opens at 12 volts.

            The other alternative is to wire it to your door locks, unless they also have 10 volts on them at all times.

            You can also buy a wireless relay controller but would have to carry an extra fob with you.
            Originally posted by ghettocruzer
            I was gung ho on building a PC [until] just recently. However, between my new phone having internet and GPS and all...and this kit...Im starting to have trouble justfiying it haha.
            Want to:
            -Find out about the new iBug iPad install?
            -Find out about carPC's in just 5 minutes? View the Car PC 101 video


            • #7
              Why not use a relay and hook the + side of the coil to a power source and the light will trigger the - side of the coil which will activate the relay and connect the computer on the NO terminals of the relay. You can send anything though the relay -/+ because all it does is connect/disconnect two terminals. In your case you can take the switch off the computer and connect it to the NO terminals of the relay. Then you have to hook another relay to the ACC + and – of the bat and splice the one wire going to the computer on the NC terminals so it won’t turn off when you open the door (light come back on)Now it will cut the computer off when you turn your car off and the light comes on.


              • #8
                A 12V relay will close at 10V and probably lower. They usually hold on down to 6V or lower (eg 3V).
                But your dome wiring is sus - it is not a straight switch wiring as depicted above. (Where - if you measured at the switches relative to GND - it would measure 12V when OFF and 0V when on.)

                Something must be triggering your dome light. Find that and use it.
                EG - if it's a door switch as depicted above, a hot relay coil is grounded thru that switch (maybe through a diode to prevent effecting other dome light circuits) to switch power to the PC.

                [ The PC then turns off if the switch opens before your IGN power takes over - unless you latch the dome-switch relay. That's easy if it's a 12V powered switch or if the relay can ground the PC instead, but otherwise another SPST relay is required. ]

                If central locking or an alarm triggers the dome, an equivalent circuit should work.

                You could also use a current sensor (reed switch) or light sensor, but that requires the dome light to actually work. Again, finding the source of the dome light's signal is the best option.

                PS - I missed jfxwave reply. Won't the relay coil be in series with the dome light, hence severely dimming it? If it's a LED dome, they may not dim, but the relay ma not pull operate....
                Last edited by OldSpark; 02-21-2011, 04:02 PM. Reason: PS...


                • #9
                  OldSpark might be right and it would dim the lights with normal sized relays but i have used them before with no problems, maybe he can get those mini relays?

                  Something like these
                  Last edited by jfxwave; 02-21-2011, 04:31 PM. Reason: More info


                  • #10
                    No - mini relays will make it worse. They have a higher resistance so the lights will be dimmer. If 1,000 Ohm coils, I doubt an incandescent bulb would light - eg, a 10W 12V bulb is near 1 Ohm so it will have 1/1,000th the voltage of the coil!

                    The solution is to put the relay an parallel with the bulb.
                    And that is what I described. (IE - between +12V and the grounding door switch; else gnd and the +12V switch if that is the case. A +12V switch would make the relay-latching easy!)

                    Bingo... the coils for the SRD-S-112D range consume 0.36W - negligible compared to a 5W or 10W dome. There will be negligible voltage across the dome light.
                    Last edited by OldSpark; 02-21-2011, 04:35 PM. Reason: Bingo...


                    • #11
                      Oh i guess i didn't understand the above post but yes I'm taking about running it parallel with the light. Is his light negatively switched on and has a content 12V? or the other way? I had a ford winstar the had all lights 12V all time and switched on with a negative switch.

                      I do know who I'm asking for help if i have a electrical problem ---> OldSpark he knows his wires LOL
                      Last edited by jfxwave; 02-21-2011, 04:53 PM.


                      • #12
                        I was gonna reply with a me, but your diagram said "to light" - so it's a instead. (LOL!)

                        Cool - so your intent was CORRECT. (My interpretation was relay coil in SERIES with the light, not in parallel. You might be surprised how many suggest doing that....)

                        Alas I ain't too sure what sam1990rhodes' switching is....
                        But if using a parallel relay, it should not matter - unless a dimmer exists.

                        But your ford winstar is "typically traditional" being "hot" and ground switched - ie, "had all lights 12V all time and switched on with a negative switch".
                        And that's the diagram I posted above (whether it had the 30way selector switch or other or not).

                        Oh shucks - I just saw your last line....
                        But REMEMBER - I can be wrong, I can misinterpret, and others may know better. (But that's one important way that I learn!)


                        • #13
                          Thanks for all of your time and replies, I'm still confused as to witch solution is best, my dome light seems to be switched on when a 12v flicks to 0v, I'd ideally like to find a solution on how to get the dome light to power it as I've allready ran the wiring to it and the light comes on when the key fob is pressed. I have a separate 12v that will take over the place of the dome light once the ignition is turned on so it cutting out is no problem.

                          On a separate note and not trying to confuse the situation does the bulb light up only when the 10v going to it increases to 12v also causing a dim affect?


                          • #14
                            All I have to say is you are working on a Dub so be very thorough in your wire investgation before using one. VW REALLY likes to do things different in the electrical department. I put a stereo in my brothers '03 Jetta last year - no ignition trigger wire to the stereo..... I had to tear the steering column plastic off and relay the key sense wire to give me an accessory circuit to work with.
                            My 2007 Ford F350 Work Log located HERE


                            • #15
                              Originally posted by sam1990rhodes View Post
                              my dome light seems to be switched on when a 12v flicks to 0v
                              That sounds as if you are sensing the "door" wire in my diagram (with the "dome selector switch" in the "door" position).

                              IE - you have a hot (+12V) dome light hence it measures +12V at the "door" position (above the door switch) unless the door switches is/are closed in which case you measure 0V and the dome light is on.

                              If that is the case there is no point tapping into the dome light "+12V power" because it is +12V ALL the time.

                              Instead you need to control a relay with the door switch(es) - ie, #85 to the door switcher (maybe thru a diode) & #86 to +12V or the relay's #30.
                              #30 is fused heavy power from the battery, and #87 powers your whatever (PC).
                              (#30 & #87 can be interchanged, but #86 goes to whatever terminal is the relay's fused battery +12V power - unless you have another +12V source in mind.)

                              The above is a repeat of earlier replies.