Smart blind stick on an Arduino Uno

// Smart blind stick: an HC-SR04 on the stick looks ahead, and a buzzer and a
// vibration motor in the handle pulse faster the closer the obstacle is. A
// water sensor at the tip gives its own alarm for a puddle or a wet floor.
//
//   HC-SR04          TRIG pin 9, ECHO pin 10, VCC 5V, GND
//   active buzzer    + on pin 8, - to GND
//   vibration motor  from 3.3 V to a 2N2222's collector; the base from pin 6
//                    through 1k, the emitter to GND, a 1N4007 across the motor
//   water sensor     DO on pin 7 (LOW when wet), AO on A0, VCC 5V, GND
//
// THE OBSTACLE: the stick measures every 70 ms, a little over the HC-SR04's
// 60 ms cycle: trigger it sooner and the module ignores the trigger, so every
// other reading comes back empty. Past 100 cm there is nothing
// to say, so it is quiet. From 100 cm down to 20 cm the buzzer and the motor
// pulse together, once a second at 100 cm and ten times a second at 20 cm,
// in a straight line between. Under 20 cm both stay on: stop.
//
//   distance   100 cm   80 cm   60 cm   40 cm   20 cm   under 20 cm
//   one pulse  1000 ms  775 ms  550 ms  325 ms  100 ms  continuous
//
// THE WATER: when the plate at the tip gets wet the buzzer switches to three
// short chirps and a pause, a sound the obstacle never makes. The motor keeps
// telling the obstacle, so a wet floor never hides a wall.
//
// The motor needs about 75 mA, more than a pin can give, so a transistor
// switches it and the diode takes the kick when it stops. The active buzzer
// is a small enough load for the pin. Things to change: FAR_CM and NEAR_CM
// for the range, and PULSE_MS for the length of each pulse.

const int TRIG = 9;
const int ECHO = 10;
const int BUZZER = 8;
const int MOTOR = 6;
const int WET = 7;
const int WATER_LEVEL = A0;

const int FAR_CM = 100;   // quiet past this
const int NEAR_CM = 20;   // continuous under this
const int PULSE_MS = 50;  // how long each pulse is on

long distanceCm = 999;
unsigned long lastMeasure = 0;
unsigned long lastPulse = 0;
unsigned long chirpStart = 0;
int zone = -1;  // 0 clear, 1 pulsing, 2 very close: printed when it changes
bool wasWet = false;

// One reading in centimeters, or 999 when no echo comes back in 25 ms.
long measure() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  unsigned long us = pulseIn(ECHO, HIGH, 25000);
  if (us == 0) return 999;
  return us / 58;
}

// The time between pulses for a distance in the pulsing band.
unsigned long intervalFor(long cm) {
  return map(cm, NEAR_CM, FAR_CM, 100, 1000);
}

void setup() {
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(BUZZER, OUTPUT);
  pinMode(MOTOR, OUTPUT);
  pinMode(WET, INPUT);
  Serial.begin(9600);
  Serial.println("Smart blind stick ready");
}

void loop() {
  unsigned long now = millis();

  if (now - lastMeasure >= 70) {
    lastMeasure = now;
    distanceCm = measure();
    int z = distanceCm > FAR_CM ? 0 : distanceCm >= NEAR_CM ? 1 : 2;
    if (z != zone) {
      zone = z;
      if (z == 0) {
        Serial.println("path clear");
      } else if (z == 1) {
        Serial.print("obstacle at ");
        Serial.print(distanceCm);
        Serial.print(" cm, a pulse every ");
        Serial.print(intervalFor(distanceCm));
        Serial.println(" ms");
      } else {
        Serial.print("STOP: obstacle at ");
        Serial.print(distanceCm);
        Serial.println(" cm");
      }
    }
  }

  // The obstacle's pulse, shared by the motor and (when dry) the buzzer.
  bool pulse = false;
  if (zone == 2) {
    pulse = true;
  } else if (zone == 1) {
    if (now - lastPulse >= intervalFor(distanceCm)) lastPulse = now;
    pulse = now - lastPulse < (unsigned long)PULSE_MS;
  }
  digitalWrite(MOTOR, pulse ? HIGH : LOW);

  bool wet = digitalRead(WET) == LOW;
  if (wet != wasWet) {
    wasWet = wet;
    chirpStart = now;
    if (wet) {
      Serial.print("WATER ahead (A0 reads ");
      Serial.print(analogRead(WATER_LEVEL));
      Serial.println(")");
    } else {
      Serial.println("dry again");
    }
  }

  if (wet) {
    // Three 100 ms chirps, 100 ms apart, then 700 ms of quiet: 1.2 s a round.
    unsigned long t = (now - chirpStart) % 1200;
    digitalWrite(BUZZER, (t < 500 && t % 200 < 100) ? HIGH : LOW);
  } else {
    digitalWrite(BUZZER, pulse ? HIGH : LOW);
  }
}