Adding Full Array Local Dimming LED Backlighting to an Existing LCD TV

LCD panels do not emit any light themselves. In a completely dark room, you wouldn’t be able to see anything shown on an LCD screen unless there is an added light source — a fact that frustrated many Game Boy owners in the ‘90s. To make LCD panels bright, TV and monitor manufacturers add backlights that shine white light through the screens. But that results in uniform brightness, which rarely matches the onscreen content. To solve this problem, Mousa built his own full array local dimming LED backlight to add to his existing LCD TV.

This idea isn’t new and lots of high-end TVs have been sold with this technology for many years. By putting an array of LEDs behind the screen, it is possible to locally adjust the backlight brightness to suit the content. Black areas can have their backlight LEDs turned off entirely, while white areas can receive full brightness at the same time. However, we’ve never heard of anyone retrofitting a TV with this feature. It is complex, doesn’t fit into a standard enclosure, and requires the backlight LEDs to react to the onscreen content.

Mousa pulled this off by first building a new enclosure with more space to accommodate the LED array. That enclosure is wood with the TV’s original plastic bezel frame glued on.

That gave him room for the array of 98 CoB (Chip-on-Board) LEDs, arranged in a 14×7. When they’re all on, they consume a similar amount of power as the original backlight.

From there, Mousa faced his biggest challenge: controlling the LEDs based on the incoming video content. One might envision hacking the TV’s control board to tap into a video signal or something, but the real solution is both jankier and cleverer.

Mousa started with an HDMI splitter, with one side going to the TV’s input. The other side goes to the input of a control board pulled from a cheap LCD projector. With the projector’s LCD still attached, Mousa had a duplicated view of the video content. He then placed a 14×7 array of LDRs (light-dependent resistors) on a PCB that mounts over the projector’s LCD. That array matches the LED backlight array and the LDR at position A1 controls the power going to the LED at the same A1 position. The LDR controls the LED power by feeding into the gate of a transistor.

So, a very bright area of the LCD projector screen will cause the LDR to send full power to the corresponding LED via the transistor. The opposite is true for very dark or entirely black areas of the screen.

This actually works very well, but it isn’t perfect. The biggest problem is resolution, as 98 LEDs don’t come anywhere close to matching the 2,073,600 pixels of a full HD TV. That results in a lot of light leaks, so bright spots produce a kind of “halo” that spreads over into dark areas. Even so, Mousa’s work here is incredibly impressive and the implementation is ingenious.