Car Dome Light Off-Delay

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15 February 2010 Author: Giorgos Lazaridis

Car Dome Light Off-Delay1Tw eet 0 2

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 "It does not operate properly" he the circuit indicated the positive, was actually a big sheet metal. said holding it in his hands.



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The CircuitThis is the circuit that i found:



Car Dome Light Off-Delay

HOT in heaven!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. When power is applied to the circuit at the connectors CON1 (positive +12V) and The circuit on a breadboard for CON2 (0V), the capacitor is fully charged. The test 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. You should find 3 connectors in the dome light shell

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TAGS cloudAuto & Moto

AutomationDC Light

Light DimmerController

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.

NEW in heaven!

The PCBActually, 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

New Circuit: Philips RC5 single LED Decoder and Transmitter circuit Random Circuits!2/8


Car Dome Light Off-Delay

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.

Light / Dark Activated Relay PWM 3-Wires Fan Controller with RPM feedback (Pulse Stretching Method) Testing the construction

The circuit was built on a predrilled prototype board.

Bill Of MaterialsResistors R1Resistor 2.2 KOhm 1/4 Watt 5% Carbon Film 2 R3 Resistor 33 KOhm 1/4 Watt 5% Carbon Film R4 47K trimmer Capacitors C1 47 uF 25 Volts electrolytic Capacitor Diodes D1 1N4002 General Purpose Diode Rectifier D2 1N4148 Switching Diode Transistors T1 BD243 NPN Silicon Power Transistor T2 BD140 Plastic Medium Power PNP Transistor T3BC547 Switching and Applications NPN Epitaxial Transistor 4

Relative pagesLearn 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

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Rate this article!16 votes total, rate is 4

CommentsAt 30 June 2012, 17:39:47 user Colin Mitchell wrote: [reply @ Colin Mitchell] 3/8


Car Dome Light Off-Delay

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, 15:48:40 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, 14:03:56 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, 13:43:27 user jerren liew wrote: [reply @ jerren liew] do you have the back of the pbc? you can send email to

At 31 January 2012, 1:51:33 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, 16:14:10 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 many thanks

At 23 November 2011, 10:37:53 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, 14:37:36 user MrFJ wrote: [reply @ MrFJ] Hi! Could upload a picture of the back of the pbc? Thanks!



Car Dome Light Off-Delay

At 21 November 2011, 3:03:05 user Ortega wrote: [reply @ Ortega] OK, i will.

At 21 November 2011, 0:43:29 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, 3:40:53 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, 2:34:27 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 19 November 2011, 19:19:00 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, 6:48:23 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, 14:18:34 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, 6:38:52 user Ivan wrote: [reply @ Ivan] Devinz and to others, I think best replacement for BD243 is the BD239

At 16 February 2011, 20:45:01 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, 12:52:57 user Kammenos wrote: [reply @ Kammenos] BD649 is darlington pair. it will not