Continuous Light Panel With Regulated Brightness From a Broken LCD Monitor

Intro: Continuous Light Panel With Regulated Brightness From a Broken LCD Monitor

I broke the glass panel in a high quality, 27 inch computer monitor. :( It was my wife’s favorite. Eh… Don’t ask me how; it’s sufficient to say I broke it with my utter stupidity. So what’s next - throw it away? Simple enough, but I really don’t like being wasteful. I have got an idea of making a light panel out of it - the 23 x 14 rectangle of continuous, evenly distributed bright white light seems perfect for photography and videography. Yes, an LCD monitor is made with an intense back light and no effort is spared to make its brightness distributed as evenly as possible through the use of advanced diffuser layers.

This idea is not new. 3 minutes of on-line search revealed at least two existing Instructables and a couple of other links on the subject:

https://www.instructables.com/id/Turn-a-broken-22-...

https://www.instructables.com/id/80cm-broken-LED-T...

https://lifehacker.com/turn-a-bunch-of-old-laptop-...

https://hackaday.com/2013/05/08/trashed-lcd-monito...

As such, this Instructable would not be worth publishing, if not for a few specific intricacies of the monitor’s electronics that let me use its own power supply and control brightness. Read on, if you are interested.

Standard disclaimer applies: by following this Instructable you acknowledge and assume all the risks, etc., etc.

Step 1: Opening the Monitor

It’s a DELL 2709 WFPb with fancy capacitive switches. I apologize for not having pictures/instructions on how to open it since I thought about publishing this Instructable halfway through this project. There exists, however, a video with instructions of dismantling a similar monitor:

When the bezel is out, be careful not to break the ribbon cable attached to it. It connects a small panel with proximity switches which is glued to the bezel with a double sided tape. The other end of the ribbon cable is simply inserted to the matching plug on the motherboard - you can just pull it out. The LCD matrix screen can be removed after the top 6 and side 4 screws are out of its holder and after the silver tape holding the LCD driver circuitry is either cut of peeled off. Make sure to dispose of it properly! After unscrewing the external shield in the back you can disconnect and remove the following boards:

  1. Motherboard
  2. USB board
  3. LCD driver
  4. Proximity switch panel

Keep these boards, you may sell them later on eBay. :) Leave the original mains filter and the power supply board - you’re going to use it. Also, do not open the inverter module! It contains high voltage transformers generating more than 500 V AC to power the fluorescent backlight.

Step 2: Hacking the Backlight Controls

I could not find any data sheets in public domain, but there is a thread in BadCaps forum with a ton of useful information:

https://www.badcaps.net/forum/showthread.php?t=318...

The motherboard connector marked "CN701" is of particular interest, since this is where all the action (regarding the lamp controls) is taking place. Using it as a starting point and doing a bit of experimentation, I have found out that:

  1. The PSU provides only a single rail of 23.5 V DC to both the motherboard and the inverter.
  2. Yellow and purple pins are internally connected to each other and to the electric ground.
  3. Orange, green, and blue pins are connected to each other and to the 23.5 V DC rail.
  4. Black and red pins are directly connected to the inverter (via CN1).
  5. The black pin turns the inverter on when the motherboard supplies 3.2 V DC on it. Otherwise, i.e. black at 0 V, the inverter is off.
  6. The red pin is for brightness control. Presumably, the motherboard can supply a value from 0 V DC (low brightness) all the way up to 3.2 V DC (full brightness). Yes, it's DC, not PWM, I checked it on my scope.
  7. The gray pin "switches the IC652 optocoupler on", but this does not seem to have any effect on the backlight.

I was tempted to actually accept the overkill of leaving the motherboard in solely for handling the on/off switch, but that would leave brightness stuck at the last setting. Normally, brightness is controlled via an on-screen menu, but, well, the screen is broken. ;)

Thus, I removed the motherboard and created a simple circuit that tricks the inverter into "thinking" that it's still controlled by it. First, I needed a source of stable 3.2 V DC. Since the PSU happily delivers 23.5 V DC, the simplest solution is to use a fixed linear voltage regulator. I even had one LD33V in my parts bin. But its maximum input voltage is only 15 V and, besides, even if I had a more capable regulator it would have to dissipate a lot of heat to drop 23.5 V down to 3.2 V. Makes little sense, right? Hence, I connected a 2.7 k-ohm resistor (R3, value found by experimentation) in series to drop the input voltage down to ~4.7 V. The rest of the circuit consists of a standard filter capacitors and a voltage divider for brightness control. The R1 resistor on top of the R2 potentiometer is optional, I put it in for extra safety.

List of parts:

  • R1 = 1 k-ohm, 0.25 W resistor
  • R2 = 100 k-ohm potentiometer.
  • R3 = 2.7 k-ohm, 0.25 W resistor.
  • C1 = 100 nF ceramic capacitor.
  • C2 = 10 uF electrolytic capacitor.
  • SW1 = a simple SPST switch.
  • U3 = 3.3 V linear voltage regulator LD33V (LD1117 series from ST Microelectronics).

Step 3: Connecting It All Together

Taking the motherboard out had an added bonus of leaving lots of holes in the chassis. I immediately used the one reserved for VGA plug to mount the switch and one dedicated for composite video as a mounting point for the potentiometer. Works like charm!

Ah, one more thing: my simple circuit uses only ~6.5 mA of current.

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