I’ve designed a signal protocol for the Arduino to convert phototransistor readings from the laptop screen into bytes of data. Here’s what I came up with today:

In the illustration, the white boxes in the black strip are light pulses on the screen which are shown as spread out across a time line. (Note that units of time have yet to be assigned.)

When the phototransistor signal goes high, it triggers (starts) the byte-reading cycle. By definition here, the start of the trigger pulse is t = 0. Thus the time line shown here is just for reading a single byte. (I’ll work on reading multiple bytes once I master reading a single byte.)

At time t = 1.5, the phototransistor voltage is read at Arduino pin A0. If the value is LOW, then the bit has a value of 0. If it is HIGH, you guessed it, the bit has a value of 1.

At time t = 2.5, the phototransistor is sampled again. If it is LOW, then the bit has a value of 0. If it is HIGH, now we’re assigning a value of 2.

And so on. After t = 7.5, all the bit values are added up to give a byte value from 0 to 127. And that tells me what my control code is.

After the last bit value is read, there is a delay of a quarter second, and then the trigger for the next byte can be sent. The reason for the delay is that I want each byte-reading to be started by a “clean” trigger because I don’t want to mess up the bit-reading frames, which I fear could rapidly get out of synch otherwise.

I’ve been having trouble with the Arduino IDE serial monitor (or maybe, it’s been having trouble with me), but I hope to test this protocol tomorrow.