Transforming a 2014 5K iMac into an external monitor

Unsurprisingly, at almost a decade old, my 2014 5K iMac is no longer as fast as it used to be. Replacing its fusion drive with an SSD helped squeeze a few extra years out of it, but the slow computer is no match for any of the new Apple Silicon computers.

I purchased a MacBook Air in 2020, and despite its numerous advantages, I missed the gorgeous 27” 5K monitor that was part of my old iMac. While Apple sells a comparable $1600 Studio Display, this is extraordinarily expensive for a monitor, especially considering the fact that the LCD panel in the 2014 iMac is essentially the same screen.

So, I set out to repurpose my 5K iMac as an external monitor to further extend its life. Prior to 2014, iMacs supported a feature called Target Display Mode, where any compatible iMac could be used as a monitor with the purchase of a simple cable. Unfortunately, this feature was discontinued with the 2014 5K iMac.

The R1811 driver board and accesories

In an attempt to overcome this limitation, I purchased a product called Luna Display, a small USB-C dongle designed to turn the iMac into a monitor without any modifications. However, I was not satisfied with this product because the image was compressed, it had a lot of latency, and it was frustrating to pair the laptop with the monitor every time I wanted to use it. These problems persisted whether I used it wirelessly or with a cord.

Thankfully, I found a far better solution in the form of an LCD driver board. The iMac’s original LCD panel lacks the electronics to power itself or receive standard video signals, tasks handled by the iMac’s logic board. The LCD driver board contains all the necessary electronics to power the LCD panel as an external monitor. Equipped with USB-C, HDMI, and Display Port connections, the driver board is similar to the circuit boards found inside traditional computer monitors.

Several driver board models are available for the 5K iMac, each of which has pros and cons. I found Aliexpress to be the best (and only) way to purchase the driver board, and I received it directly from China a few weeks later. I tested two models, the R1811 (about $350) and T18 (about $220).

To install the driver board, I first disassembled the iMac and removed all of its internal components, using a guide from ifixit.com. The only parts I reused were the iMac chassis and LCD panel. One of my goals was to complete this project non-destructively, so I saved and organized all of the components in case I ever wanted to revert the iMac to its original condition.

I first installed the R1811 driver board as it was touted as the best one available on various forums, and it supports both USB-C and 10-bit color depth. With one USB-C wire connected between the 5K iMac and M1 laptop, the R1811 receives video and charges the laptop simultaneously. I was unsure whether 10-bit color was noticeably better than 8-bit color, but at the time, I thought it was worth paying a little extra for this feature.

Beyond a few pictures on the Aliexpress website, there was very little documentation available about the R1811. I carefully plugged in the wires on my LCD panel so they matched the wires in the seller’s photos. Thankfully this worked with a little bit of experimentation! The 5K LCD panel sprung to life and displayed the incredibly sharp high-resolution video I was looking for.

The temporary testing setup

R1811 mounted in the iMac

I mounted the R1811 inside the iMac chassis using command strips, and I ran the power and video cables out of the RAM hatch, which ordinarily covers a large hole in the back of the iMac. I used packing tape to temporarily mount the LCD panel to the chassis. The R1811 included a sub-board for adjusting brightness, contrast, changing inputs, etc. I snaked the sub-board out through the RAM hatch and velcroed it to the back of the iMac.

The power supply got very hot during my testing, so I mounted it externally where it could receive maximum ventilation.

I used the iMac monitor this way for about a year and was reasonably satisfied, except that there were some annoying dim zones in the backlight towards the bottom of the iMac.

Backlight issue with the R1811

In an effort to fix this problem, I installed a supplementary LED backlight booster board (DZ-LP0818) intended for the R1811. This board did not fix the problem and the unevenness remained. And then, while removing this new board, disaster struck. A large spark jumped from the LED driver board and fried the circuitry that powered the backlight. The video-related circuitry (the expensive stuff) still functioned, and a very faint image appeared onscreen (only visible with a flashlight). So, I did some tests and discovered that the LCD backlight cabling was defective (hence the uneven backlight). I contacted the Aliexpress seller about this, and they asserted that my LCD panel was defective, but later tests proved that it was not.

So, I shelved the R1811 and purchased the Haijing T18 LCD driver board, which was $100 cheaper than the R1811. I slotted the T18 in its proper place, and not only did it eliminate many of the odd quirks I experienced with the R1811, but it also fixed the uneven backlight issue.

It remains unclear to me whether or not the T18 board supports 10-bit color. But, having used both the R1811 and the T18, I cannot see any difference in image quality between the two. The 5K image is fantastic!

Connecting my laptop to the T18 using a USB-C to USB-C cable only provided 4K resolution. I resolved this issue by purchasing a DisplayPort 1.4 to USB-C cable. The downside of using this cable was that I was no longer able to charge the laptop with a single cable, but this didn’t bother me as I prefer to use my laptop’s faster 96-watt Magsafe charger.

I removed the original iSight camera from the iMac and replaced it with an external Logitech webcam that had a higher resolution. I also opted not to reuse the existing iMac speakers, instead, setting up better-sounding external bookshelf speakers on my workspace behind the iMac.

Now that everything is working properly with the T18 driver board, I am very satisfied with my 5K monitor. It works well and the image quality looks excellent to my eyes. Despite frying the R1811 board, the project still cost less than $600, which is close to $1000 savings compared to buying a new Apple Studio Display.

While there were some technical hurdles to jump through, the end result of this project paid off, and I am glad to have a 5K monitor for much less money than any alternative product. If you are interested in attempting a similar project or have any questions, I highly recommend taking a look at the MacRumors forum, where many others have documented their projects.