From: gsulliva@enuxha.eas.asu.edu (Glenn A Sullivan)
Subject: Re: Need help with video detection circuit
Summary: Build a cheap video detector
Organization: Arizona State University
Lines: 19

verity@jack.sns.com (Steve Verity) writes:
> I am trying to build a circuit that detects the presence of video (Vs.
> a blank screen) by monitoring the R,G, and B outputs of a graphics
> card.  It should be able to detect the presence of a single pixel at
> 65 MHz, which would mean detecting a 15 NS pulse.  It should also be
> able to tell the difference between a blank screen (about 300 mv)
> and a dim screen (say, around 310 mv).  Oh yes, it also needs to be
> cheap.  ............... In 
> fact the difference between completely black and completely white is
> only 650 mv.  I am wondering if I am going to have to amplify the
> video signals to make this work.  

Try a differential amplifier. Put back-to-back diodes between the collectors
restrict the voltage swing. Have 10 mA flow, with 500 ohm collector Rs, and
keep the stray C below 10 pF. Try 2N5179s. Run collector to your logic.
Allen Sullivan