distance-sensor.js

/* Define a .env file with the following properties

MAXIMUM_DISTANCE=7.08 <You can change this to whatever you like>
HOST_IP=<IP of HOST Machine that runs this code https://github.com/Pofay/parking-sensor-example-server>
PORT=3000

The distance is in centimeters(cm)
*/
const Gpio = require('pigpio').Gpio;
const axios = require('axios');
require('dotenv').config();

// The number of microseconds it takes sound to travel 1cm at 20 degrees celcius
const MICROSECDONDS_PER_CM = 1e6/34321;

const url = 'http://' + process.env.HOST_IP + ':' + process.env.PORT + '/api/parking_lot'

const trigger = new Gpio(23, {mode: Gpio.OUTPUT});
const echo = new Gpio(24, {mode: Gpio.INPUT, alert: true});

trigger.digitalWrite(0); // Make sure trigger is low

const watchHCSR04 = () => {
  let startTick;

  echo.on('alert', (level, tick) => {
    if (level == 1) {
      startTick = tick;
    } else {
      const endTick = tick;
      const diff = (endTick >> 0) - (startTick >> 0); // Unsigned 32 bit arithmetic
      const distance = diff / 2 / MICROSECDONDS_PER_CM;
      if(distance <= process.env.MAXIMUM_DISTANCE) {
		    axios.put(url, { status: 'occupied' })
             .then(res => console.log(res.status))
             .catch(console.err)
	    }
	    else {
		    axios.put(url, { status: 'vacant' })
             .then(res => console.log(res.status))
             .catch(console.err)
	    }
    }
  });
};

watchHCSR04();

// Trigger a distance measurement once per second
setInterval(() => {
  trigger.trigger(10, 1); // Set trigger high for 10 microseconds
}, 1000);