// https://forum.arduino.cc/t/motor-is-stopping-because-of-delays-from-speaker/1420515

int MOTpwm  =  3; // PWM motor speed
int MOTdio  =  4; // DIO motor enable
int TRIGpin =  6; // HC-SR04 trigger
int ECHOpin =  7; // HC-SR04 ECHOpin
int REDled  =  8; // Red LED "danger"
int GRNled  = 10; // Green LED "near"
int SPKRpin = 11; // speaker

int stopspeed = 0, minimumspeed = 75, throttle, throttleOld;
float distance, speedratio = .45;
float echotime, echospeed = 58.2;

void setup() {
  Serial.begin(115200);
  pinMode (ECHOpin, INPUT);
  pinMode (TRIGpin, OUTPUT);
  pinMode (MOTpwm, OUTPUT);
  pinMode (MOTdio, OUTPUT);
  pinMode (REDled, OUTPUT);
  pinMode (GRNled, OUTPUT);
  pinMode (SPKRpin, OUTPUT);
}

void loop() {
  // measure distance to wall
  digitalWrite(TRIGpin, HIGH); // start transmit
  delayMicroseconds(10); // send for 10us
  digitalWrite(TRIGpin, LOW); // stop transmit
  echotime = pulseIn(ECHOpin, HIGH); // time "stop transmit" to "received echo" in microseconds
  distance = echotime / echospeed; // distance centimeters

  // calculate motor speed relative to distance
  throttle = minimumspeed + (distance * speedratio); // (400 * .45) + 75 = 255
  throttle = map(throttle, 75, 255, 1500, 2500);

  // display throttle
  if (throttle != throttleOld) {
    throttleOld = throttle;
    Serial.println(throttle);
  }

  // indicate distance is good
  if (distance > 3.0) {
    // update motor speed
    analogWriteMillisecons(MOTpwm, throttle);
    digitalWrite(MOTdio, HIGH);

    // indicate "running"
    digitalWrite(GRNled, HIGH); // GRN LED
    digitalWrite(REDled, LOW);
    digitalWrite(SPKRpin, LOW);
  }

  // stop and indicate distance is minimum
  if (distance <= 2.0) {
    digitalWrite(MOTdio, LOW); // disable motor
    digitalWrite(MOTpwm, stopspeed); // set speed to zero

    // indicate "stopping"
    digitalWrite(GRNled, LOW);
    digitalWrite(REDled, HIGH); // RED LED
    digitalWrite(SPKRpin, HIGH);
  }
  delay(100);
}