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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 PCB

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
 Resistors R1-2 Resistor 2.2 KOhm 1/4 Watt 5% Carbon Film 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 T3-4 BC547 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