Build Log 14 -- CCFL Dimming of the iMac 20" LCD
This was an important part of the project that I'm happy to report worked out very well. CCFL brightntess control works perfectly with the 20" G4. Using a Picaxe 08M2 microcontroller and two touch sensors, the backlights can be adjusted with easy-to-use touch control.
The only wire from the iMac neck that controls the brightness for this application is the YELLOW wire. The purple and orange wires were left floating without any negative effect. In summary, the inverter wiring is as follows --
1. Blue = 24V
2. Red = 24V
3. Black = GND
4. Green = GND
5. Yellow = Brightness Control via 25khz PWM signal (0.63V to 3.2V)
I mounted the touch sensors on the inside of the DVD door, out of sight, yet in an easy-to-find and unmistakable location for brightness control. They are cut from a thin piece of copper plating.
This is my first-ever attempt at soldering a strip board. With a fresh sharp tip for my cheapie soldering iron, it wasn't difficult at all. Watched a couple of videos first for technique. This board uses a very similar circuit to the brightness control in my 17" iMac G4's. The details are
here (different wire colors of course).
I tested the board first with a small LED on a breadboard. It worked great, the little bulb cycled up and down beautifully. The sensors must be installed in the iMac before the touch capacitance levels can be properly checked and adjusted. I used the Picaxe software to read the levels with the sensors wired from my iMac to my Picaxe programming board.
My 24V upconverter arrived just in time to to a real test on the iMac LCD. It just barely fit, I'm glad I ordered the smaller cased version. I also installed my picaxe dimming control board into the iMac. I covered the components with JB Weld rather than electrical tape to prevent shorts from contact with other components; its going to be VERY tight in the dome when this is all done.
Using a Mavericks install and one of my other Intel NUC computers, I lit the LCD up for the first time with full control of the brightness via the two tiny copper touch sensors. It works great.
After the success of this, I ordered my new Intel NUC D54250WYK. I'm confident that this project can be successfully completed now; so I put my money down on one today.
Ersterhernd