Color to frequency

// A color sensor read by hand: the TCS3200 turns light into a frequency.
//
// There is no microcontroller here. Press Run and the simulator solves the
// circuit.
//
// WHAT IT IS. The TCS3200 has an 8 x 8 grid of photodiodes, a quarter of
// them behind red filters, a quarter green, a quarter blue and a quarter
// bare. S2 and S3 pick which group drives a light-to-frequency converter,
// and OUT is a square wave whose frequency is how much of that color it
// sees. The frequency counter on OUT reads it.
//
//   S2   S3   filter         S0   S1   scaling
//   LOW  LOW  red            LOW  LOW  power down
//   LOW  HIGH blue           LOW  HIGH 2%
//   HIGH LOW  clear          HIGH LOW  20%
//   HIGH HIGH green          HIGH HIGH 100%
//
// S0 is tied to 5 V and S1 to ground: 20%, what most sketches use. OE is
// tied to ground, which turns the output on.
//
// THE SWITCHES. The left one is S2, the right one S3. Each starts on
// ground, so the sensor starts on the red filter. Slide one over and its pin
// goes to 5 V.
//
// THINGS TO TRY. The card in front of the sensor is red (its color
// property). On the red filter the counter reads about 14 kHz; slide the
// right switch over for the blue filter and it falls to about 4 kHz. Set
// the card to white and the red filter reads about 18 kHz; set it to black
// and it falls to about a twentieth of that.
//
// IN A SKETCH. A board does the same with pulseIn(OUT, LOW), which times
// half a cycle: at 14 kHz that is about 36 microseconds. The lower the
// number, the more of that color.