Fast optical to electrical converter design solutions ?

[ynwia]

optical to electrical converter

The circuit is to detect the onset/offset of pictures on a computer monitor (CRT or LCD). Monitor starts with a totally dark background, then a 300x300 picture appears for a few hundred milliseconds then switches to darkness, then another 300x300 picture, then dark background again...

Once the circuit detects optical change(darkness to pic, pic to darkness), it either immediately outputs a TTL pulse or changes it current status like a flipflop. The response time should be as small as possible and 1 millisecond is definitely the upper limit. (microseconds or even nanoseconds would be ideal)

I already made a simple circuit with TAOS TSL230R. It kind of works except its output is frequency (not a single pulse or constant voltage) and I have no idea of its response time. :|

The options in my mind :idea: are

1.:arrow: Go analog. For example, :idea: Photodiode->op amps->voltage regulator (7805:?: ).

2.:arrow: Digital. For example, :idea: Photodiode->op amps->microcontroller (pic 12f675:?: )

Which one do you suggest :?:
Any specific parts/circuit solutions to recommend :?:

Any input is highly appreciated.

 

[R00KIE]

Well .... you could use an LDR with a comparator and a flip-flop, it would change state every time a new picture come on the screen. I don't have a clue of how fast it would detect the picture, I guess you would have to find some datasheets and sum the delay of the LDR, comparator and flip-flop.
This approach will be very sensitive to ambient light so either you control the ambient light carefully or you find a way to adjust your comparison levels accordingly (probably using another LDR).

 

[FvM]

Regarding resposnse time, you should consider the video frame rate, may be 70 or 100 Hz with most monitors. There's no video imformation present with higher frequency, except of line scan with a CRT monitor (a low pass filter is advisable to remove the several 10 kHz horizontal frequency with a CRT). Thus 10 ms response time is basically fast enough. A photo transistor would have a sufficient output without an amplifier, but a photodiode with OP is is suitable as well. The best design of detection depends on your application, obviously a 8-pin uP with analog input, a phototransistor and a load resistor (+ optional low pass C) makes a versatile detection circuit, that is hard to compete in part count.

 

[pixel]

I think that you should use some integrated solution, because currents for dark colour could be extremely small.