Adding extra backlight features to TV

[Th3_uN1Qu3]

My replacement LED strips got delivered today for this LG 32LF561V which had its backlight fail a third time, despite reducing the output current last time around, so I decided to just get a full set.

I left the back off for now since I want to hack in a couple of extra features I've grown rather fond of owning higher end sets.

Namely, backlight strobing with Vsync for better motion and an ambient light sensor.

Since backlight circuit operation isn't that different among different TVs there's no reason for this to not develop into a generic solution you can apply to any LED TV and even CCFL based ones.

I'll start with the strobe feature. First of all, we're going to need a sync signal. I have a service manual for my TV and looking around for a bit made it extremely obvious where I would get that: Pin 19 of the SCART socket, SYNC_OUT. Interfacing that to the backlight driver is probably a job for a basic 555 timer monostable circuit. Nothing fancy.

The light sensor otoh is going to be more interesting. I'll probably end up cutting open a metal can transistor to make it photosensitive as photoresistors or photodiodes are specialized parts that can't be found in your junk box. I thought about repurposing an old ball mouse, but iirc the sensors in those only respond to IR, which is probably not adequate.

[Th3_uN1Qu3]

Re: Adding extra backlight features to TV

Here's the first iteration, easier than I expected. I removed the dimmer wire from the mainboard side of the power connector and wired pin 19 of the SCART connector to it.

Of course, the results were less than optimal with the backlight going down to zero at times but on a stable bright scene it was obvious that I got the right idea. The LED driver goes full brightness with the dimmer input left floating, so to keep the strobe and content dependent backlight, all I needed was a DC offset on the dimmer signal so that the backlight doesn't go all the way down.

I did some brainstorming with a friend, namely Khron here, and he drew a couple of circuits for me to try, however I decided to exhaust all simple approaches first and went for a series diode with the cathode pointing towards the SCART connector. It was better, but still too dim at times. Two diodes in series and it would dim too little and not have the backlight strobe most times. I was ready to try a slightly more complicated circuit until I thought about trying a different kind of diode - a junction of a switching BJT, which tend to have a rather high voltage drop. I went for the B-C junction of a 13001 switching transistor from a CCFL driver board. So far, the results have been encouraging - I haven't had any scenes way too dim, backlight sync/strobe (which I nicknamed "CRT emulation") is functional at all picture levels and black levels are greatly improved. In fact, a full black screen looks like the backlights have turned off! A full black background with just a station logo on it is indeed quite dim, but I don't think anyone would miss a blindingly bright logo when that's the only thing in the picture. I've watched a couple of hours of varying content in both HD and SD and so far, so good.

Next up, the ambient light sensor!

[Th3_uN1Qu3]

Re: Adding extra backlight features to TV

Some results in photo and video form. Of course these don't do it justice, you have to try it on your own TV to see if you like it.

https://www.mediafire.com/download/uhnas57djvsyg5z

Edit: I should probably capture some action flick, the dimming will be a lot more obvious with such material.