Here’s the basic idea: I’ll send commands to the Arduino by flashing a rectangle on the screen and having a phototransistor on the Arduino read the light level.
The above photo shows the test bed set-up. This is very similar to the circuit found in Getting Started with Arduino, except that I’m using a phototransistor instead of an LDR. The optimal resistor appears to be 10K ohms, as in the book.
If I can communicate via phototransistor, then I don’t need the USB/serial cable for communications. And if I can get rid of that, then I don’t need a serial-ttl cable or the Arduino platform. I can just stick an Atmel microcontroller on my robot and have the phototransistor read user commands right off the screen. This will save a lot of money.
As for how well it works — well, that’s why I built a test bed, to find out.