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15 February 2010
Author: Giorgos Lazaridis
Car Dome Light Off-Delay

"It does not operate properly" he said holding it in his hands.

One day, a friend of mine came to me holding in his hand the dome light from his car. "Its does not work properly" he said. "When i close the door, the light turns off immediately". Obviously, the turn off delay circuit had gone bye bye.

Searching the Internet, i found tens of different dome light off delay circuits. Thus, i decided not to design a new circuit from a scratch. Again, i had to slightly modify it. The difference was the power supply. I had to invert it, as the position that the circuit indicated the positive, was actually a big sheet metal.

The Circuit

This is the circuit that i found:

The circuit on a breadboard for test

I will start explaining the circuit from right to left. The 3-resistors net (R1-R2 and R4) performs a voltage divider that controls the voltage on the base of T4. The T4 will control how fast will the C1 discharge through it's CE, that is parallel to the R3. The more the current through the T4, the faster the discharge.

You should find 3 connectors in the dome light shell

When power is applied to the circuit at the connectors CON1 (positive +12V) and CON2 (0V), the capacitor is fully charged. The transistor T1 will allow all the current to go through. The current is controlled from the transistor T2 that is attached to it's base. The transistor T2 is controlled by the transistor T3.

When the power is switched off (the car door is closed), the capacitor will start slowly to discharge. As the voltage across R3 (base of T3) is falling, the T1 will gradually allow less current to flow through. This is the main idea of this circuit.

This circuit is connected parallel to the switch of the dome light that is controlled by the door switches. This is how to connect the circuit:

In the dome light shell, you should find three connectors. One goes to the ground. The other one goes to the battery directly. The third one is connected to the door switches. These switches are connected in parallel directly to the ground. The circuit that i found had these switches connected in parallel to the positive of the batter (???). Anyway. As shown above, the circuit is connected in parallel to the negative (CON 2) and to the door switches (CON 1). So simple. No other wiring is required.


The circuit was built on a pre-drilled prototype board.

Testing the construction

Actually, it was already getting too late and my friend was in a hurry. I suggested a nice PCB construction, but he insisted on putting the things on a pre-drilled protoboard. What the heck. After all, a PCB would significantly increase the car's current value... its an old Ford for crying out loud.

With no further ado, i start soldering the parts on the PCB. The final size was around 3cm x 3cm. It could be much smaller, but there is plenty of space under the dome light.

Bill Of Materials
R1-2Resistor 2.2 KOhm 1/4 Watt 5% Carbon Film 
R3Resistor 33 KOhm 1/4 Watt 5% Carbon Film 
R447K trimmer
C147 uF 25 Volts electrolytic Capacitor
D11N4002 General Purpose Diode Rectifier 
D21N4148 Switching Diode 
T1BD243 NPN Silicon Power Transistor 
T2BD140 Plastic Medium Power PNP Transistor 
T3-4BC547 Switching and Applications NPN Epitaxial Transistor 

Relative pages
  • Learn how the capacitor works
  • The voltage divider theory
  • The transistor theory of operation
  • How to make a light / dark activated switch - 3 different circuits under the microscope
  • Dr.Calculus: Voltage divider calculator

  • Comments


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  • At 12 November 2015, 23:21:53 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Harry yes it could

  • At 4 November 2015, 19:24:53 user Harry wrote:   [reply @ Harry]
    • Hello. Csn this circuit be used for LED strips also?

  • At 27 February 2015, 8:37:09 user eugene wrote:   [reply @ eugene]
    • i am hoping that you could also show the under side as to the soldering and connection of each component.
      You see I am not a technician but I would like to execute this project, since my dimmer module is also busted.

  • At 27 January 2015, 9:09:01 user eugene wrote:   [reply @ eugene]
    • hello but my problem is the door light stop working but there is the door ajar warning light on my dashboard, meaning all the door switches are working perfectly it just no continuity to the dome light. but the manual switch is working fine.

  • At 13 October 2013, 12:46:53 user teguh wrote:   [reply @ teguh]
    • The light doesn't come off, from the firts time I connected con1 to the door switch and con2 to the ground, the light comes on, is that normal? What's wrong with my circuit? Thank you..

  • At 16 January 2013, 15:43:05 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Jim it is the r4

  • At 15 January 2013, 0:54:46 user Jim wrote:   [reply @ Jim]
    • You mentioned that the delay can be controlled with a potentiometer. Where in the circuit would this go?

  • At 20 November 2012, 7:50:35 user Jeff wrote:   [reply @ Jeff]
    • Would like to know what diodes D1 and D2 are for? Thanks

  • At 30 June 2012, 21:39:47 user Colin Mitchell wrote:   [reply @ Colin Mitchell]
    • dome light

      When a door or boot is closed, the Dome Light circuit takes over and keeps the light illuminated for an extended period of time.
      It does this by immediately seeing 12v across terminals CON1 and CON2.
      The electrolytic is initially uncharged and it gets charged very quickly via the base-emitter of transistor T3.
      This charging turns on T3, T2 and T1 and the voltage across CON1 and CON2 drops to a small voltage. This voltage is just enough to put a small voltage across the capacitor, plus the 0.6v on the base-emitter junction of T3. The capacitor charges a little bit more and T3 turns off a small amount. This turns off T2 and T1. This allows the voltage across CON1 and CON2 to rise.
      As the voltage across CON1 and 2 rises, it robs the globe of voltage and current and the globe dims.
      Eventually T3 turns OFF and so does T2 and T1.
      The capacitor is now fully charged.
      When the door switch is closed, it puts a "short" between CON1 and 2 and the capacitor is discharged via diode 2.
      T4 "robs current" from T3 to alter the delay time.

  • At 25 March 2012, 19:48:40 user Steve-0 wrote:   [reply @ Steve-0]
    • hi,

      my car only has 2 connections, live and switched ground (door pins) what is the best wat to wire this up?

      many thanks

  • At 18 March 2012, 18:03:56 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @jerren liew no unfortunately i haven't kept any photos of it, and the car is long ago sold.

  • At 18 March 2012, 17:43:27 user jerren liew wrote:   [reply @ jerren liew]
    • do you have the back of the pbc? you can send email to me...jerren_liew@yahoo.com

  • At 31 January 2012, 6:51:33 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @tristan i do not have it any more. The pcb is in a car that is sold.

  • At 30 January 2012, 21:14:10 user tristan wrote:   [reply @ tristan]
    • Hi mate have you got any pictures of the back of the PCB and the wiring diagrams if so my email address is pikey25@hotmail.co.UK many thanks

  • At 23 November 2011, 15:37:53 user Kammenos wrote:   [reply @ Kammenos]
    • @MrFJ i do not have it any more. The pcb is in a car that is sold.

  • At 21 November 2011, 19:37:36 user MrFJ wrote:   [reply @ MrFJ]
    • Hi!

      Could upload a picture of the back of the pbc?


  • At 21 November 2011, 8:03:05 user Ortega wrote:   [reply @ Ortega]
    • OK, i will.

  • At 21 November 2011, 5:43:29 user Kammenos wrote:   [reply @ Kammenos]
    • @Ortega when you build it, please upload the schematic to the forum because many people ask me for an LED dome light. thanks.

  • At 20 November 2011, 8:40:53 user Ortega wrote:   [reply @ Ortega]
    • Hi, Kammenos. I changed the connections, CON1 to the LED cathode, insted to door's pin, and now everything is OK. The circuit works fine and the side effect disappear. All the same, i'll try to apply circuit to control LEDs, maybe must to change some components values, don't know yet. When i build phisical model, i'll post the result.

  • At 20 November 2011, 7:34:27 user Kammenos wrote:   [reply @ Kammenos]
    • @Ortega no this is not for LED control. I designed it for incandescence light bulbs, so there may be indeed side effects.

  • At 20 November 2011, 0:19:00 user Ortega wrote:   [reply @ Ortega]
    • I have no time for soldering 'n searching required components, so i tested the circuit in Proteus(virtual simulation) instead. Well, it works, but i found some unexpected and unpleasant side effects. When the dome is OFF and switch over to AUTO, the LEDs lit for a while. Is this bug appears in the real model? And if exists, how to get rid of this? I'll think twice about this problem and how to fix it. Please, write here if you have any ideas. By the way, your site is very useful, regards!

  • At 19 November 2011, 11:48:23 user Ortega wrote:   [reply @ Ortega]
    • Is this circuit suitable for LED dome light (12pcs SMD3528)? I just now made modification of my dome light - from standard to LED. Because of negative controlled dome light circuit, i can't just put a capacitor before LEDs. Can i use the above circuit to add dimming effect on my dome? Thanks in advance.

  • At 9 October 2011, 18:18:34 user jrr wrote:   [reply @ jrr]
    • the light i have is only 2 wires doors and on it has no off button

  • At 14 June 2011, 10:38:52 user Ivan wrote:   [reply @ Ivan]
    • Devinz and to others, I think best replacement for BD243 is the BD239

  • At 17 February 2011, 1:45:01 user Devinz wrote:   [reply @ Devinz]
    • I just done the test. It still fade but not smooth fad in and fad out.
      I would get a BD139 later and try it out.
      It should work this time I reckon.
      Thx a lot for your replies.

  • At 16 February 2011, 17:52:57 user Kammenos wrote:   [reply @ Kammenos]
    • BD649 is darlington pair. it will not fade.

  • At 16 February 2011, 13:24:23 user Devinz wrote:   [reply @ Devinz]
    • i just wondering if the BD649 could do the job or not?

  • At 15 February 2011, 6:47:47 user Kammenos wrote:   [reply @ Kammenos]
    • Devinz, i am so sorry, i am not good at this. You should search for a transistor with same characteristics, but that would be a great deal of work. There are books that have transistor equivalents, but unfortunately i do not own one.

  • At 15 February 2011, 2:06:25 user Devinz wrote:   [reply @ Devinz]
    • What is the replacement for the DB243 when it is not available?
      I couldn't find anything in my local electronic shop.

  • At 10 November 2010, 15:10:29 user Michael wrote:   [reply @ Michael]
    • Can you provide me with a diagram that will allow for a timed delay followed by a gradual dimming?


  • At 25 June 2010, 3:24:15 user Tom Hargrave wrote:   [reply @ Tom Hargrave]
    • LED lights will tend to dim faster & go out at a higher voltage.

  • At 25 June 2010, 3:17:12 user Kammenos wrote:   [reply @ Kammenos]
    • I have not test it with LEDs. I cannot see a reason why not to work, but i am not sure.

  • At 25 June 2010, 1:41:17 user Jeetu Das wrote:   [reply @ Jeetu Das]
    • Can this circuit be used if i am using leds for the dome lights. Or what modification is required.


  • At 18 February 2010, 18:09:29 user Kammenos wrote:   [reply @ Kammenos]
    • Indeed, i did some research myself. I just think that, using the ground for doors is more practical, as you save one part of the wiring: that is from + to each door. The switch needs only to be ground, and only 1 wire departs from the 4 switches (in parallel) to arrive to the dome light.

      As for the beer thing, except the time that i consume for the site - that i do with pleasure FOR my fun - there are actually some expenses to keep this site. Not that much comparing to other yearly expenses, but is a respectable amount of money. Books, tools, components, and another $180 for the web hosting and name reservation. As i share all my the knowledge with all of you, i thought that sharing some expenses would be quite fair.

      P.S. Following the ads on the right side daily, can help a lot and it will cost nothing for you.

  • At 18 February 2010, 14:46:36 user Tom Hargrave wrote:   [reply @ Tom Hargrave]
    • There is a reason that one circuit you looked at had the door switches going to power. Older cars are wired different from your drawing and they are wired in one of two ways.

      With one version, power will come from the battery, through a common fuse, through a dimmer rheostat then through the door switches that are all wired in parallel. Then the ground side of the door switches will all connect to one side of the interior lights and the other side of the lights will go to ground.

      With another version, power will come from the battery, through a common fuse, through a dimmer rheostat then to the interior lights. Ground is provided through the door switches that are all wired in parallel.

      There is yet a third version where the dome light has one side tied to ground and power to the light is managed trough a lighting controller. With this version, the lightiong controller determines what needs to be sourced to the interior lights.

      The circuit you designed will work great for a "smart" dome light, one that is designed to be controlled externally. Also, I think it's a neat design.

      BTW, I like the "buy me a beer" pop-up and I will as soon as I have some extra money.

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