If the monitor refresh rate is 56-90 Hz, your baud
rate would be a MINIMUM of 56-90 Hz, and would work
directly with LCD displays.

If you are looking at a patch of screen from a distance,
you can encode multiple bits per V scan because the horizontal
scan will provide you with a light flash every 1/H s, which
corresponds to a bit.
IIRC the Timex watch used full scan lines for this encoding,
rather than a patch.

Depending on phosphor persistence, a 1/2 screen wide data bar would
get you a LOT of bytes in a single refresh. 32 bars (easily
resolved) with a 1280 x 1024 x 60hz display would get you 1.96mbps.
Add a color filter to one of two phototransistors, and you can
easily reject ambient light imputs using differential methods.

If you use just a small patch of screen you'd
use the first two lines (bits) to establish the monitor scan rate,
then subsequent lines to encode your data and add some lines
for FEC (forward error correction). You should be able to
easily manage 60 BYTES per second with a 5% patch of the
screen. 5% of 480 lines is 48 lines which is 5 bytes
(one bit per scan line) with FEC.

However, this method WILL NOT WORK with LCD displays, since
they don't have a horizontal scan, they just update the
pixels once per V scan.

I would suggest using a serial port with the dead easy
diode clamp approach, but the serial port is becoming
exinct, particularly on laptops. Of course USB to serial
interfaces are available.

And though it's not as easy to do, you could use the
drive access LED found on most machines,
and some clever software, to encode the data at
low bit rates. Or the keyboard status LED's since
they are directly controllable with commands to the
keyboard scan processor. The baud rate would be limited
by the KB update rate, but I assume it is fairly high.

Robert

Jake Anderson wrote:
>
> have you looked at the diode protected serial xfer method?
> put a serial plug on your device a handfull of diodes and resistors etc
> might be ok because you are only recieving data
>
> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Kirk Lau
> Sent: Wednesday, March 31, 2004 11:52 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE:] Phototransistor as Serial Input?
>
> That's a pretty cool idea.... but.....
> As some folks pointed out, you might have to run in rather low baud..
> probably somewhere in 10 to 20 baud
> biggest problem u are facing is the refresh rate of the monitor...
> especially on a Win32 or any GUI platform
> some ppl runs at 56 hz, to something godly like 90Hz... and since
> phototransistor is quite fast (compare to 56Hz...) it will definately picks
> it up... so the signal from the output of the phototransistor collector will
> be "modulated" at a frequency of the refresh rate...
> but at the same time.. if u runs under DOS or any text base environment, the
> rate has yet to determine... and i can see there is a chance that it might
> be rather CPU dependent on some extreme cases.. even tho u can put delay
> loop in it..
> to get around with the refresh rate... probably a low pass filter (RC
> network?) might do the trick.. or may be a monostable oscillator (a simple
> 555?), or an integrator circuit(any opamp?), with a time constant around
> 30ms...  Or if u want to implement a software routine delay... that should
> be possible too..
> also, a CDS MIGHT be a better choice rather than a phototransistor, due to
> it's responses in terms of time as well as wavelength.  but that has to
> prove by experiment (I might actually give that a try later tonight and
> see...)
> another thing is you might consider putting a diffusive plate in front, to
> scatter the light, as well as "average" out the intensity of the area it
> looks at, cuz as the beam scanning down the monitor, the phototransistor
> should picks up the scan...i am not sure if that's needed.. but is also a
> possibility to look at.
> that's all so far.. i might do some test these few days to see how that goes
> and post more...
>
> BTW.. i am new to the list(just joined yesterday), nice to meet u all
>
> Kirk
>
> > I'm designing a little PIC-based gadget that will have 8-10 variable
> paramaters that I'll store on-board the PIC.
> > I'd like people to be able to configure this gadget by means of a PC
> program. The program will have the nice interface and do the
> > necessary calculations, but ultimately the 8-10 8-bit variables that will
> result need to be delivered to the PIC. I want to keep
> > cost and weight down, so my idea is to use a phototransistor on the PC
> board to watch a "flashing box" on the monitor that sends
> > serial data from the PC program. The user would hold the board up to the
> monitor, hit a key, and the data would be transferred.
> >
> > Has anyone done anything like this before? I think it would be relatively
> straight forward, but I'm not sure how to filter and
> > massage the input from the phototransistor. Any help would be appreciated.
> >
> > Thanks,
> >
> > Jeff
> >
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