Display brightness of my old iMac

[SubDriver]

It is another step down module from 12V to 5V and on the LV terminal the 3.3V come from the first step

That’s exactly the same setup I had. Glad it worked for you.

Hopefully my next attempt with one of the buck converters I mentioned a few posts above will work. It would be good to have more than one method that works so people have a choice for their conversions.

If it doesn’t work, I will go back and try another logic board.

 

[iMacConversionEnthusias]

I can report success with the bi-directional logic board. The screen went to sleep after I switched off the Chromecast with Google TV. Initially it showed no signal then going to power saving and then off. I am attaching a very "dirty" drawing. Will prepare something better later or tomorrow (east European time) because my wife is ready to kill me 🙂

Hi Guys. I created a drawing with the boards and connections that I have currently of course based on all I have been reading and learning in this form. Thanks to everybody involved. It is attached to this message. I will appreciate it if somebody can check if I have messed up something on it. Also, a little picture of the result - screen with beautiful colours.

Some observations though:
1. Screen goes to sleep and back from sleep based on the attached HDMI device being switched off or switched on.
2. The colours I believe are better with the PWM module than without it.
3. The internal temperature is running slightly lower, but not by much so my next step will be to install the fans, but this is going to happen when the temperature sensor board arrives in about two weeks.
4. The duty cycle settings with the PWM board seem to be reversed - 18% is higher brightness and 100% no brightness
5. PWM below 15% makes the screen flicker rapidly.
6. In my setup all the boards stay powered because they draw power directly from the external PSU (I have an idea how to fix it but it involves management of the 220V to the external PSU which will work for me).
7. The power consumption at the moment when the screen has gone to sleep is about 1.2W and when the screen is on it is about 75-77W which is lower that the 85-87W before I installed the PWM module.
8. I added to my configuration a Temperature monitoring WiFi module just because I can, and I am very curious what's the temperature story inside the box 🙂

EDIT: As I understand this forum is to share experiences and learn from what other people have done. I am not a professional electronics technician, and I created the attached diagram based on the information I gathered in this forum. If you use it, it is your responsibility and at your own risk.

EDIT: Please read the newer posts in this forum as there is further development in the setup. This worked for me, but couple of other people reported problems and are unable to get the screen going. I would have removed the drawing, but I do not know how.

 

[iMacConversionEnthusias]

That’s exactly the same setup I had. Glad it worked for you.

Hopefully my next attempt with one of the buck converters I mentioned a few posts above will work. It would be good to have more than one method that works so people have a choice for their conversions.

If it doesn’t work, I will go back and try another logic board.

Thanks. I was also thinking about a buck converter before I got the idea for the bi-directional module from a friend of mine who has done lots of hobby type of work with electronics. He mentioned that the buck converter needs power for its own operation (we know that). Although the power needed is not too much as the efficiency of these modules is around 90% but it may damage the board supplying the power if the board can't handle it - just an observation.

 

[iMacConversionEnthusias]

I may be speculating with this, but I just realised why we are having all these issues with our 27" iMacs. I noticed that the two boards that I have seen for the 27" models year 2010-2011 are the same boards as for the 21.5" iMacs 2011-2013. So, I think they just not designed to work 100% with the 27" model. I checked some images on Aliexpress of the 21.5 conversion and the brightness looks good with the Chinese board alone 🙂

 

[SubDriver]

Hi Guys. I created a drawing with the boards and connections that I have currently of course based on all I have been reading and learning in this form. Thanks to everybody involved. It is attached to this message. I will appreciate it if somebody can check if I have messed up something on it. Also, a little picture of the result - screen with beautiful colours.

Some observations though:
1. Screen goes to sleep and back from sleep based on the attached HDMI device being switched off or switched on.
2. The colours I believe are better with the PWM module than without it.
3. The internal temperature is running slightly lower, but not by much so my next step will be to install the fans, but this is going to happen when the temperature sensor board arrives in about two weeks.
4. The duty cycle settings with the PWM board seem to be reversed - 18% is higher brightness and 100% no brightness
5. PWM below 15% makes the screen flicker rapidly.
6. In my setup all the boards stay powered because they draw power directly from the external PSU (I have an idea how to fix it but it involves management of the 220V to the external PSU which will work for me).
7. The power consumption at the moment when the screen has gone to sleep is about 1.2W and when the screen is on it is about 75-77W which is lower that the 85-87W before I installed the PWM module.
8. I added to my configuration a Temperature monitoring WiFi module just because I can, and I am very curious what's the story inside the box 🙂

Your drawing looks correct to me. Your display looks great too!

Your observation #4 and #5 are strange. My display doesn’t respond to brightness change in the same manner. How are you controlling the brightness?

Observation #6 - If u are able to figure out a way to turn off the power to your boards from the external power supply many others will benefit from that. For those using the iMac power supply unit they should not have the same problem as the iMac power supply can be turned on/off using pins 6 and 7.

Observation #7 - Great info to have. From what I have seen in the schematics, the normal power draw of the display is in the mid-70s. Your results give confidence that we are not overpowering the display!

Observation #8 - Definitely interested in what the temperature readings are once you have the data.

 

[iMacConversionEnthusias]

Your drawing looks correct to me. Your display looks great too!

Your observation #4 and #5 are strange. My display doesn’t respond to brightness change in the same manner. How are you controlling the brightness?

Observation #6 - If u are able to figure out a way to turn off the power to your boards from the external power supply many others will benefit from that. For those using the iMac power supply unit they should not have the same problem as the iMac power supply can be turned on/off using pins 6 and 7.

Observation #7 - Great info to have. From what I have seen in the schematics, the normal power draw of the display is in the mid-70s. Your results give confidence that we are not overpowering the display!

Observation #8 - Definitely interested in what the temperature readings are once you have the data.

#4 and #5 : I am controlling the brightness with two non-latching buttons. Will investigate further, but these two buttons can move the duty settings on the PWM module very fast if I don't release the knob quick enough. Is your PWM behaving the same way ?

#6 : I think I may have found a way. Of course, it needs to be tested and proved working. I am trying to get a relay module locally so hopefully we don't have to wait 2-3 weeks for it to arrive.

#6 : But the iMac and the original PSU do not have a remote control - I think I am thinking in a different direction than you as I am planning to use my modified screen as a TV Screen for Google Chromecast with Google TV attached to it via HDMI.

#6 : Another way which I have tested already is using a WIFI Smart Plug 220V (110V for the US) and I managed to trigger it to go On/Off and therefore power the entire setup On/Off using the remote control of Google Chromecast with Google TV by creating an automation scene in the Google Home App. So basically, when the Google Chromecast with Google TV is switched on the screen is powered on and the other way works too. All this goes over the Internet, so it has a slight delay of 2-3 seconds, but it works. The smart plug I have can also be switched off automatically (by itself) when the power consumption goes below 3W, but it cannot switch back on when the power goes higher than 3W because the entire setup is switched off from the main power source.

 

[iMacConversionEnthusias]

Hi. Do pins 6 and 7 on the original iMac PSU have to stay permanently shorted for the PSU to output power or is it only a momentary trigger to wake it up?

 

[SubDriver]

Hi. Do pins 6 and 7 on the original iMac PSU have to stay permanently shorted for the PSU to output power or is it only a momentary trigger to wake it up?

They must be continually shorted. I used a latching pushbutton to achieve this.

You could use a momentary pushbutton but it would need to activate a holding circuit that maintains the shorted connection after the pushbutton is released.

 

[csdesign3]

They must be continually shorted. I used a latching pushbutton to achieve this.

You could use a momentary pushbutton but it would need to activate a holding circuit that maintains the shorted connection after the pushbutton is released.

I've found this on Ali... https://www.aliexpress.com/item/100....order_list.order_list_main.16.2f331802KINRK3

i will try to install on the iMac power button place, this will keep the Apple PSU pins 6 and 7 locked until we want to turn off. This button is very small.. right now im waiting delivery..

 

[SubDriver]

I've found this on Ali... https://www.aliexpress.com/item/100....order_list.order_list_main.16.2f331802KINRK3

i will try to install on the iMac power button place, this will keep the Apple PSU pins 6 and 7 locked until we want to turn off. This button is very small.. right now im waiting delivery..

View attachment 2479967

The buttons I used are in the pdf guide I posted a few days ago.

 

[iMacConversionEnthusias]

The buttons I used are in the pdf guide I posted a few days ago.

I used the same buttons. Thanks. One can add remote control functionality. The image below is from Ali. https://a.aliexpress.com/_Euo4Tt2 but I am not sure that it is going to work with the original PSU as it is not clear to me if pins 6 & 7 can be shorted by COM & NO.

 

[iMacConversionEnthusias]

#4 and #5 : I am controlling the brightness with two non-latching buttons. Will investigate further, but these two buttons can move the duty settings on the PWM module very fast if I don't release the knob quick enough. Is your PWM behaving the same way ?

#6 : I think I may have found a way. Of course, it needs to be tested and proved working. I am trying to get a relay module locally so hopefully we don't have to wait 2-3 weeks for it to arrive.

#6 : But the iMac and the original PSU do not have a remote control - I think I am thinking in a different direction than you as I am planning to use my modified screen as a TV Screen for Google Chromecast with Google TV attached to it via HDMI.

#6 : Another way which I have tested already is using a WIFI Smart Plug 220V (110V for the US) and I managed to trigger it to go On/Off and therefore power the entire setup On/Off using the remote control of Google Chromecast with Google TV by creating an automation scene in the Google Home App. So basically, when the Google Chromecast with Google TV is switched on the screen is powered on and the other way works too. All this goes over the Internet, so it has a slight delay of 2-3 seconds, but it works. The smart plug I have can also be switched off automatically (by itself) when the power consumption goes below 3W, but it cannot switch back on when the power goes higher than 3W because the entire setup is switched off from the main power source.

The classic relay triggered from the BLO did not work. It looks like the BLO signal is very week and it could not trigger the relay. I am still searching....

 

[iMacConversionEnthusias]

Your drawing looks correct to me. Your display looks great too!

Your observation #4 and #5 are strange. My display doesn’t respond to brightness change in the same manner. How are you controlling the brightness?

Observation #6 - If u are able to figure out a way to turn off the power to your boards from the external power supply many others will benefit from that. For those using the iMac power supply unit they should not have the same problem as the iMac power supply can be turned on/off using pins 6 and 7.

Observation #7 - Great info to have. From what I have seen in the schematics, the normal power draw of the display is in the mid-70s. Your results give confidence that we are not overpowering the display!

Observation #8 - Definitely interested in what the temperature readings are once you have the data.

My PWM behaves weird. I confirmed the reversed behaviour and also found out that every time I disconnect the power it changes both setting with one unit. The frequency goes up and the duty goes down.

 

[SubDriver]

My PWM behaves weird. I confirmed the reversed behaviour and also found out that every time I disconnect the power it changes both setting with one unit. The frequency gous up and the duty goes down.

Very strange behavior on power removal…. Mine stays locked in at 13.3kHz and whatever the last duty cycle setting was. It doesn’t change. Maybe a faulty board or something is getting shorted unintentionally?

With respect to the speed of the duty cycle adjustment, that is normal. A short press of the button changes the duty cycle by one unit. A long press will change it rapidly.

 

[iMacConversionEnthusias]

Very strange behavior on power removal…. Mine stays locked in at 13.3kHz and whatever the last duty cycle setting was. It doesn’t change. Maybe a faulty board or something is getting shorted unintentionally?

With respect to the speed of the duty cycle adjustment, that is normal. A short press of the button changes the duty cycle by one unit. A long press will change it rapidly.

Can you send me a picture of the board you have. No need to be powered. Or a link to check it online. Thanks

 

[SubDriver]

Can you send me a picture of the board you have. No need to be powered. Or a link to check it online. Thanks

Here is link to the board I bought. It has three channels - I did that on purpose in the event I ruined one of the channels with my poor soldering skills! 🙂

It looks similar to the one in your drawing, just with two extra channels.

 

[iMacConversionEnthusias]

Here is link to the board I bought. It has three channels - I did that on purpose in the event I ruined one of the channels with my poor soldering skills! 🙂

It looks similar to the one in your drawing, just with two extra channels.

Thanks. The GND on the exit is not connected to anything, correct?

 

[SubDriver]

Thanks. The GND on the exit is not connected to anything, correct?

Correct. Just one line from the PWM board to the inverter.

 

[iMacConversionEnthusias]

Hi guys, does somebody know what type of plastic they use for the black sheets that are stuck under the original PSU.

 

[SubDriver]

Hi guys, does somebody know what type of plastic they use for the black sheets that are stuck under the original PSU.

I do not know. Tried a quick Google search but nothing comes up. Maybe someone else will chime in.

 

[iMacConversionEnthusias]

I do not know. Tried a quick Google search but nothing comes up. Maybe someone else will chime in.

I found some info on ChatGPT, hope it is correct :

Apple primarily uses polycarbonate (PC) and polycarbonate-blended plastics for internal structural components in iMacs, including areas near the power supply unit (PSU). These plastics are chosen for their:

1. Heat Resistance – The PSU generates heat, so the plastic must withstand high temperatures without deforming or degrading.

2. Electrical Insulation – Prevents electrical shorts and enhances safety.

3. Durability & Impact Resistance – Protects internal components from physical damage.

In some models, Apple may also use polyphenylene sulfide (PPS) or glass-filled nylon (PA-GF) for additional thermal and structural benefits.

 

[iMacConversionEnthusias]

I do not know. Tried a quick Google search but nothing comes up. Maybe someone else will chime in.

I am asking because I want to reposition the boards and make it nice and tidy. It's like a dog's breakfast at the moment and I don't like it. I ordered plastic screws for the boards with spacers and also velcro cable ties to keep the wires together but all this needs a good insulating surface that's not going to become curly with the higher temperatures. So, I dismenteled the broken screen I have to salvage the LEDs and also the sheets to become the surface for the boards🙂

 

[iMacConversionEnthusias]

Deleted

 

[iMacConversionEnthusias]

With that crossovers on speakers can you ear them working? Where did you buy them?

Nice job with the ports! I may try something similar with mine in the future, but not right now.

Regarding the power supply, if you have access to an original PSU, I highly recommend you give it a shot. Although it is large and takes up a lot of space, it provides a significant amount of flexibility due to having numerous 12V power lines.

My final layout is shown in the images below. I haven't yet connected the audio to the crossovers (waiting on my small amplifier to arrive). Also, I haven't converted the SD card reader yet. I will complete both of those within the next couple of weeks.

The pictures below show the following:
1. Overall layout with everything installed in the iMac shell
2. Picture of the power button
3. Picture of the brightness control buttons
4. Picture of the brightness control button wires soldered to the PWM signal generator board (not great soldering, but it works!). 🙂
5. Short video showing the brightness control buttons changing the duty cycle of the PWM signal

View attachment 2476644
View attachment 2476645
View attachment 2476646
View attachment 2476647


View attachment 2476648

Hi. How is the sound with the crossovers? Are you using an amplifier?

 

[iMacConversionEnthusias]

So, I have been trying various combinations to achieve the switch on/off of all the additional boards when they are not needed (screen's gone to sleep). Initially with no luck, but today I think I have succeeded. So, I have taken the impulse cable for the relay from the green/red LED indicator on the Chinese little OSD keyboard and for my surprise it actually worked. By switching off the HDMI device I can make the relay disconnect the power supply on COM/NO and vice versa. I have to underline that the tests are not completed in full as I have not applied this setup in real life yet. It is just voltage measurements and observation of the behaviour, but so far so good.

While snooping around the Chinese board I actually discovered 5V supply and also 3.3V supply as roughly shown on the attached picture with arrow #1. Both exists in the 4-pin plug next to the keyboard plug and in the 2-pin left out from the keyboard interface plug. Will have do more investigation as at the moment I have no clue what current they can handle. I am sharing this because if they have enough power, we can get rid of the step-down modules. I already removed the one as I realised, I can take 5V from the build in USB on the Chinese board.

But can somebody tell me what type of plugs these are (arrow #1 & #2)? They look to me like JST PH2.0 connector with 4-pin, but are they? Thanks.