# Car Interior Light Dimmer circuit - any recommendations?

#### Fyod

Joined Jun 2, 2019
11
Hi everyone!
I'm looking for a reliable car interior light dimmer/delay circuit (aka courtesy or theater light).
One where I could ideally set the length of off to fade on, the on delay, and the on to off fade. The circuit should be able to handle about 25W of power.
Could anyone recommend one or point me to where I could get started?
I have googled of course, but not sure which one would be suitable.
https://www.shawnville.com/2010/01/dome-light-dimmer/
https://www.electroschematics.com/car-interior-light-dimmer/
https://www.eeweb.com/luxury-car-interior-light-dimmer/

Thanks!

#### Fyod

Joined Jun 2, 2019
11
Btw I had a go at the below circuit a couple years ago, but the problem was that Q1 overheated with any load over 10W. The BJT (with the incorrect symbol) could be replaced with a mosfet, but I think even then it might overheat with 25W or so.
If anyone has any ideas on how to make this circuit more robust, it might be a starting point.

#### MisterBill2

Joined Jan 23, 2018
11,919
The circuit in post #1 is a linear dimmer and while it can be effective it will produce a lot of heat, that is inescapable with an analog dimmer.
Using the voltage across C2 to control a pulse with generator system to provide a PWM (pulse width modulated) control signal to a suitable switcing transistor can provide a similar control but with much better efficiency and much less heat.

#### AlbertHall

Joined Jun 4, 2014
12,026
If the dimming is done in a linear way, as in the circuit above, then the heat problem will be the same regardless of the rest of the circuit. It could work but woukd need a good heatsink on the transistor.

One possible solution is do the dimming using a PWM signal then the transistor would always be either fully on or fully off thus dramatically reducing the heat in the transistor. The third link in your post is of this kind and if the functions it provides suit your needs then that is one solution.

#### Fyod

Joined Jun 2, 2019
11
I like the one in the third link as well.
It seems that if R6 was replaced with a trimmer, the dimming delay could be adjusted like in post #2 circuit.
The only quibble I have is that it seems it doesn't include the ON->brightening. Would that just be a matter of copying everything left of IC1A into everything left of IC1A in this circuit?

#### eetech00

Joined Jun 8, 2013
3,162
Btw I had a go at the below circuit a couple years ago, but the problem was that Q1 overheated with any load over 10W. The BJT (with the incorrect symbol) could be replaced with a mosfet, but I think even then it might overheat with 25W or so.
If anyone has any ideas on how to make this circuit more robust, it might be a starting point.

View attachment 258664
Do you really need fade on?

#### Fyod

Joined Jun 2, 2019
11
It would definitely be nice to mimic it the same way new cars do, fade-on -> delay -> fade-off.

#### AlbertHall

Joined Jun 4, 2014
12,026
You come back to the car and open the door and have to wait while the light gets bright?
Is that a desirable thing?

#### Alec_t

Joined Sep 17, 2013
12,818
Modern cars will likely have the courtesy lamp controlled in some way, or at least monitored, by the car's body-management computer. Any add-on circuit would have to avoid conflict with that.

#### MrSalts

Joined Apr 2, 2020
1,762
I saw this post on another site earlier this week. Looks like you're getting the same types of answers here, too.

#### Fyod

Joined Jun 2, 2019
11
Modern cars will likely have the courtesy lamp controlled in some way, or at least monitored, by the car's body-management computer. Any add-on circuit would have to avoid conflict with that.
Definitely. This is for my old car. I want to mimic the way it works in modern cars.

I saw this post on another site earlier this week. Looks like you're getting the same types of answers here, too.
Indeed, and this community is much more helpful. I'm just here to learn, downvotes aren't educational.

AlbertHall said:
You come back to the car and open the door and have to wait while the light gets bright?
Is that a desirable thing?
Yes, but it is just a 1-2s fade-in, instead of just directly turning on. Usually happens before you open the door all the way.
The delay I mentioned is the time it then stays on before fading off. So sloped like this __/‾‾‾‾‾‾\__ instead of just on/off _|‾‾|_.

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#### MisterBill2

Joined Jan 23, 2018
11,919
I see the fade-on as a useless gimmick, and it has no value at all to me. I want that light on full instantly when I unlock the door, because it is handy to know what, or who, might be in the car that I did not leaver in it. And while it is included in that $500 body control computer, if it fails then that is$500 to fix it if I do the repair myself. Not a big deal for those individuals who get a new car every year, but certainly a nuisance for those of us who do not.

#### ericgibbs

Joined Jan 29, 2010
15,542
I see the fade-on as a useless gimmick, and it has no value at all to me. I want that light on full instantly when I unlock the door,
How does this address the TS's request for guidance with his project.???

E

#### Fyod

Joined Jun 2, 2019
11
@MisterBill2 I totally understand that concern, but I don't work for a car man'f and this will be a protoboard installed in my old car

#### Tonyr1084

Joined Sep 24, 2015
6,894
You come back to the car and open the door and have to wait while the light gets bright?
Is that a desirable thing?
One reason for having "fade to on" (FTO) is when you're IN the car. Who wants a full blast of light before you get out. I can understand wanting the light on quickly so you can see the mugger hiding in your back seat. That very scenario has happened to me exactly zero times in the 49 years I've been driving. Not even a friend playing a practical joke.

And FTO doesn't take 20 seconds to reach full brightness. Though I've never timed it on my truck, FTO would seem to take at the longest, just two seconds. I've never gotten in my car within two seconds of opening the door, even when the wind chill has been -40˚ (F or C). Or with pouring rain. Well, maybe in the pouring rain I've managed to get in the car within one second. Luckily there have been no Ninja's or Terrorists waiting for hours in my car just for that moment I come back to it.

Personally, I like the FTO and the FTo (Fade To oFF). Cool? Yeah. Necessary? My opinion is - not really.

#### shortbus

Joined Sep 30, 2009
9,375
Finally a question of circuit design that is made for and can be done with a micro controller and no one is even suggesting it.. And no I have no idea how to do it, I don't do micros. But that is how it would be done in a car that has this option in the micro controller called a body control module.

#### Fyod

Joined Jun 2, 2019
11
I have actually thought about it. I'm much more experienced in designing circuits around microcontrollers and programming. This would only require a regulator, some voltage dividing, a few pins for sensing, and a pwm output to the transistor. But I intentionally want to go old school with this.
New cars certainly use MCUs and dome lights are on the CAN bus.

#### shortbus

Joined Sep 30, 2009
9,375
My reply was somewhat tongue in cheek. Many times things that can be done in easier ways people here suggest it should be done with a micro.

#### Marley

Joined Apr 4, 2016
475
I don't want to boast but I could point you to a project of mine...