add backlight switch to iMac

[SpatialZebra]

hi guys! I am new to this forum so sorry if I am posting in the wrong place, if there's already a thread similar to mine, and also if my English is bad!

I have a 2017 4K 21.5" iMac, and I would like to add a switch to turn the backlight on and off. I just need to know what pin can do that on the display connector, then I'll figure out the building part.
This is basic electronics. Literally just breaking a connection to add a switch in between. Before anyone warns me, I am not scared and I don't care about the exposed internal power supply, I know how to discharge a stupid capacitor with a power resistor or a light bulb.

I tried unplugging the backlight connector while the iMac is running, it turned the backlight off no problem. Only issue is when I plugged it back in I didn't get backlight until reboot.

 

[hg.wells]

Interesting idea. This may sound stupid, but what's the difference between using the screen brightness button on the keyboard? I was under the impression that the lowest option did turn the backlight off fully.

 

[DeltaMac]

Not on an iMac.
Mac laptops will turn brightness all the way down to black.
However, iMacs turn down to some minimum brightness, but do not go completely black.

 

[SpatialZebra]

True! This is why I want a switch, and also it's way faster than the keyboard and it makes sure your backlight is hard disabled and won't turn back on during a macOS install or whatever!

 

[B S Magnet]

True! This is why I want a switch, and also it's way faster than the keyboard and it makes sure your backlight is hard disabled and won't turn back on during a macOS install or whatever!

Don’t overthink this! You already have a “switch” of sorts.

Using High Sierra on my 2013 iMac as an example here, open the Desktop & Screen Saver prefPane (or similar, for whichever macOS iteration you’re running). Next, select the “Hot Corners…” button. Then, pick a corner (or corners) you want to use when you want to shut off your display without shutting off the system. Then, move your pointer to that corner and put your backlight to sleep instantly.

I personally don’t use a screen saver at all, so when I’m done for the day, I send my pointer off to the bottom-right (or “south-east”, if using cardinal description) corner, and the display backlight shuts off until I move the pointer again.

This feature has, more or less, existed in some form, ever since the early days of Mac OS X and has carried through onto macOS.

 

[Nguyen Duc Hieu]

hi guys! I am new to this forum so sorry if I am posting in the wrong place, if there's already a thread similar to mine, and also if my English is bad!

I have a 2017 4K 21.5" iMac, and I would like to add a switch to turn the backlight on and off. I just need to know what pin can do that on the display connector, then I'll figure out the building part.
This is basic electronics. Literally just breaking a connection to add a switch in between. Before anyone warns me, I am not scared and I don't care about the exposed internal power supply, I know how to discharge a stupid capacitor with a power resistor or a light bulb.

I tried unplugging the backlight connector while the iMac is running, it turned the backlight off no problem. Only issue is when I plugged it back in I didn't get backlight until reboot.

I really don't understand why you want to apply hardware solution to a pre-existing software solution, like @B S Magnet post above.

Anyway, if you want basic electronic, then start from the LCD panel rather than the logicboard.
Just look for the cables which supply power to the LED, and add a switch there. It will not be a simple 2 poles switch like normal household electric devices, but a 6 poles or 8 poles switch.

Just don't tell me how you are gonna wire that switch outside the iMac case.
I assume it will be ugly anyhow you do it.

 

[SpatialZebra]

@B S Magnet this seems pretty interesting, as long as:
-it can be done on Ventura
-I can do that very fast right from the menu bar (adding a shortcut)
-It only disables the backlight while every task is still running

There is one thing it won't do however, and that is keep the backlight disabled after reboot or during a macOS install, and most likely if I click the mouse it will restore backlight instantly.

This is why I would like a hardware solution better.

@Nguyen Duc Hieu because I want a hard switch that can disable backlight instantly and make sure it can't get enabled again unless I flip the switch back!
It may get a little ugly, however I just do not care.

I assume there is like an enable pin for the backlight, and if it does not get a certain voltage (usually 3.3V or maybe 5V), the backlight won't be on. That's where I need to add my switch!

 

[Nguyen Duc Hieu]

@Nguyen Duc Hieu because I want a hard switch that can disable backlight instantly and make sure it can't get enabled again unless I flip the switch back!
It may get a little ugly, however I just do not care.

I assume there is like an enable pin for the backlight, and if it does not get a certain voltage (usually 3.3V or maybe 5V), the backlight won't be on. That's where I need to add my switch!

It may seem easier for you as there is only 1 pin.
But I think it's better if your control switch nearer to the LED bar. It will be fully manual and not impacted by the logicboard (software), less factors to troubleshoot when fail.

 

[SpatialZebra]

Maybe. But that means I need to take my multimeter and look at every single pin, because nobody has the stupid pinout!

The enable pin is not software, it allows the chip to provide backlight, and there is simply no way you can get the backlight to light up without it no matter how good you are at hacking software.

 

[DeltaMac]

What is your plan, if you find out that turning off the backlight manually, always means a restart to start up the backlight again? (backlight will not come back on without an actual restart.)
I do not know if that will happen, but with the other software challenges that you already have about restarting the backlight, maybe that will always interfere, regardless of the hardware method that you choose to use for disabling the backlight.
Just a question (I'm curious!) Are you using external display(s), and need to completely turn off the internal screen while using that external screen, so internal screen is not a distraction?

 

[SpatialZebra]

@DeltaMac
After messing with the internal LCD connector, I eventually figured out that my plan was impossible without modding the actual logic board.
Even though it may be a very simple mod if I can just find one pin that enables/disables the backlight, I figured I would give up on doing this with this iMac and try it on my 2011 27" which has a lot more accessible and less complex backlight circuitry.
By the way I downgraded the GPU to a Radeon HD 4670 so it wouldn't fail like the 6000 series (I had my 6970 fail after two months of use where as this one didn't fail in almost a year).

I am not using an external display, I was previously using a Cinema Display, which I can turn off super fast just like I want, but it has no speakers, and the resolution is not the best to say the least. I also have a non retina 2017 iMac board lying around, so I might end up repurposing this to use with my Cinema Display if I can power it (I do not have the power supply it's supposed to come with but this should be no big deal).

 

[SpatialZebra]

As it turns out, you can in fact disconnect and reconnect the backlight driver while the system is running and that will disable/enable backlight. On the 2011 iMac, there is a cable between the power supply and that driver board. On the 2017 iMac, this is on the logic board and would be kinda hard to figure out and mod.

 

[B S Magnet]

@B S Magnet this seems pretty interesting, as long as:
-it can be done on Ventura

You should be able to.

-I can do that very fast right from the menu bar (adding a shortcut)

That’s sort of a cumbersome way to do it (versus just moving the pointer to a corner), but I’m sure a script can be made to do it.

-It only disables the backlight while every task is still running

Yes, moving the pointer to the corner, as I showed above, doesn’t affect anything else. The system continues to run with a pitch-black display. You can even be listening to music or compiling with the display off. To use the display again, move the pointer away from the corner.

There is one thing it won't do however, and that is keep the backlight disabled after reboot or during a macOS install, and most likely if I click the mouse it will restore backlight instantly.

At that point, what you’re describing is running your iMac in a headless mode. The feature I shared above is not related to running your system headless.

 

[SpatialZebra]

I'll be sure to try that on my iMac then. I got a screenless MacBook base to use when I want to run my system "headless".
I guess I misunderstood your comment, because if it's as easy as moving your mouse to a corner, I certainly don't want to add another shortcut to it! I read it again and noticed that's exactly what it said, but for some reason I thought you needed system preference open or whatever, I'm just stupid lol!

The only issue is I don't think screen saver disables your backlight entirely... and also I need to set it to a completely black screen!
Or again I could just be stupid as putting my display to sleep and screen saver are two different things...