Author Topic: Help! How to fake-out monitor's signal present detection (to keep backlight on)  (Read 4645 times)

Offline alphageekyvr

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There was a great post a while back from "kwackers" detailing his project to build a light-box from a dead LCD monitor using the cold-cathode backlight (

I'm trying a silmilar project but I'm running up against the back-light turning off when the monitor goes into standby mode.  Now, Kwackers built a simple PWM with a 555 timer which is very elegant but for reasons I won't bother to detail here I'd like to avoid that and simply apply some kind of signal to the DB-15 VGA input connector.  Obviously, the monitor detects if there's an active video signal in some manner and goes into standby if it doesn't see one (i.e. computer is off or cable unplugged).  Does anyone know HOW the monitor does this detection?  I'm trying to find a simple way to fool the monitor into thinking it's connected so that the back-light will stay on.  The monitor's board doesn't have much in the way of labelling and I'm unable to find the ON and PWM pins that Kwakers found on his monitor so I'm hoping I can do something on the VGA input to get around this. 


Offline kwackers

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Usually these things switch based on a seeing sync pulses - in particular the vsync pulse (pin 14 if it's a 15 pin vga socket). So the first thing you can try is to try building something similar to the PWM I used but set to say 60 hz and see if that triggers the device to turn on.
TBH a good read about the VGA socket and the various modes would be useful, iirc they require various signals to figure out what's going on, you may need to also generate horizontal sync...

Presumably your board is all one unit, i.e. controller, psu and backlight on a single board?
Can you get a circuit diagram for it?

Offline Noitoen

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Usually, the power supply for the cold cathode lamp is a small board with a high voltage transformer, bolted to one of the other boards. You only need this one board and a small power supply.

Offline AdeV

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If I remember rightly, it might be as simple as putting a resistor across a couple of the VGA pins;  I seem to recall doing something similar in an somewhat complicated AV setup I put together for a client a few years back.

I'm not 100% sure it will work at the monitor end, it certainly worked at the computer end. The link below shows the resistor on a DVI plug; you would need to get the traditional VGA pinout & place the resistor(s) accordingly.

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