Arduino Visual Robot (4/4): marker-aware robot challenge

The robot's line sensors look at the surface and return a reflectance
reading: an approximate number from 0 to 2000. Light surfaces give
low values (near 0) and dark surfaces give high values (near 2000). A
threshold is the cut-off number you use to decide "light" or "dark". The
challenge is not to follow a perfect line, but to measure light/dark and program a
reaction.

Choose one challenge. All are valid if they use a sensor, a decision, and an observable output.

- Marker detector: when a dark mark is detected, stop and turn the red LED on.
- Patrol with warning: drive slowly and warn when a dark marker is detected.
- Careful robot: stop if there is either an obstacle or a marker.
- Surface traffic light: blue on light surface, red on dark surface, without moving.

You are not asked to follow a whole line. You are asked to explain what the robot detects and how it reacts.

To see the reading you will use serial (a text channel from the robot to your
screen). If your lab does not show serial output, calibrate with the LED instead:
turn the red LED on when the reading is above your threshold and check whether it
tells a light surface apart from a dark one; adjust the threshold until it does.

Calibration block image and text reference:

Before using messages:

[Setup] Serial speed 9600

In the main loop:

[Read line sensors]

[print]
  center line-sensor value
  or
  center-left line-sensor value
  or
  center-right line-sensor value

[wait] 300 milliseconds

The read block must come before the individual sensor value. First measure light and dark; then choose your threshold.

Fill the table while you calibrate light and dark. Use at least two rows:
one light-surface row and one dark-surface or marker row. If you test an extra
marker position, add one more row; leave any remaining blank rows empty. In each
row, record the sensor position, surface, observed value, proposed threshold,
and your confidence.

Build a minimum version of your challenge:

1. Read sensors.
2. Compare the reading with your threshold.
3. Activate a visible output: LED, serial message, and motors if appropriate.
4. Use a practical low speed such as 125 or 150, keep the movement short, and end with a stop if the robot moves.

Example marker detector:

- If center sensor > threshold: motors 0,0, red LED, message marker.
- Otherwise: blue LED and, optionally, a short forward movement at 125 or 150.

Marker detector block image and text reference:

In the main loop:

[Read line sensors]

if [center line-sensor value > threshold]:
  [Change two motors speed]
    Left:  0
    Right: 0
  [Set] red [led to] HIGH
  [Set] blue [led to] LOW
  [print] "marker"

else:
  [Set] blue [led to] HIGH
  [Set] red [led to] LOW
  optional:
    [Change two motors speed]
      Left:  125
      Right: 125
    [wait] 300 milliseconds
    [Change two motors speed]
      Left:  0
      Right: 0

This structure detects a marker or surface; it does not try to follow the whole line.