Published December 16, 2019

Build a Laser Harp with a LIDAR and Arduino

Let's pick magic chimes out of the air with a LIDAR scanner and an Arduino.

IntermediateProtip5 hours15,397

Build a Laser Harp with a LIDAR and Arduino

Things used in this project

Hardware components

RPLidar A1

Arduino Mega 2560

Texas Instruments General Purpose Dual Op-Amp

Software apps and online services

Microsoft Visual Studio Code Extension for Arduino

Story

There are many exciting things that you can do with laser. One of these is playing harp! In this project, a LIDAR scanner (LIDAR = Light Detection and Ranging) is used for that. LIDAR works like RADAR, but uses laser instead of microwaves.

Watch video to see it in action

The LIDAR sensor used here has a small DC Motor, that drives a turntable with a laser diode and a receiver diode. The Laser diode emits short pulses of Infrared light which are reflected by the neighbored objects around. The reflected light is then detected by the receiver diode. The electronics measures the time between the emission and detection of the reflected light and calculates the distance from the LIDAR. An accuracy of a few millimeters can be achieved with this method!

Open LIDAR Sensor

The LIDAR continuously sends the angle of the laser beam and the measured distance to an Arduino Mega over its UART interface.

The Arduino sketch then extracts the angles of up to two objects (i.e. hands) within a sector of 90 degrees above the scanner and in a distance between 10 cm and 1 meter from the LIDAR. According to these angles, up to two independent chimes are generated.

Instead of simply using the tone() function, the Arduino outputs a PWM signal that represents a sinusoidal wave with a decay. After low-pass-filtering with a 10k resistor and a 1 micro capacitor and amplification with an active speaker, we get a harp-like sound.

Recording of the output sound

Note: An Arduino UNO or similar ATmega328-powered device can also be used here. But in this case, debugging and uploading of sketches is not possible while the LIDAR sensor is connected, because it also uses the UART pins. So, Arduino Mega with its 4 UARTs is more convenient here.

Code

Arduino Sketch

#include <RPLidar.h> //Download from https://github.com/robopeak/rplidar_arduino.git

RPLidar lidar;
#define MOTOR_PIN 3
#define SAMPLE_LEN 156
#define NUM_STRINGS	24
#define OFFSET 36
#define DECAY 20

int16_t samples[SAMPLE_LEN];
const uint16_t max_d = 1000;
const uint16_t min_angle = 135;
const uint16_t max_angle = 225;

int16_t volume1, volume2;
uint32_t cnt1, cnt2; //Index counter

// the setup function runs once when you press reset or power the board
void setup() {
	Serial.begin(115200);
	for (int i = 0; i < SAMPLE_LEN; i++) {
		samples[i] = 10 * sin(20 * PI * i / SAMPLE_LEN);
	}
	// bind the RPLIDAR driver to the arduino hardware serial
	lidar.begin(Serial1);

	// set pin modes
	pinMode(MOTOR_PIN, OUTPUT);
	
	//PWM Signal generation
	DDRB |= (1 << PB5); //OC1A, Pin 11 
	TCCR1A = (1 << WGM11) + (0 << WGM10);  //Fast PWM 9 Bit
	TCCR1B |= (0 << WGM12);
	TCCR1A |= (1 << COM1A1); //Toggle OC1A on compare match
	TCCR1B = (0 << CS12) + (0 << CS11) + (1 << CS10); //No Prescaling
	TIMSK1 = (1 << TOIE1);

	//DDRB |= (1 << PB4); //OC2A, Pin 10
	//TCCR2A = (1 << COM2A0); //Toggle OC0A on Compare Match
	//TCCR2A |= (1 << WGM21) + (0 << WGM20); //CTC
	//TCCR2B = (0 << CS22) + (0 << CS21) + (1 << CS20); //Prescaling 256
	//TIMSK2 = (1 << OCIE2A);// +(1 << TOIE1);
}


// the loop function runs over and over again until power down or reset
float freq_1, freq_2;
int16_t string1, string2;
float freq;
void loop(){
	uint16_t pause;
	static long t1, t2;
	//return;
	if (IS_OK(lidar.waitPoint())) {
		float d = lidar.getCurrentPoint().distance; //distance value in mm unit
		float angle = lidar.getCurrentPoint().angle; //anglue value in degree
		bool  startBit = lidar.getCurrentPoint().startBit; //whether this point belongs to a new scan
		byte  quality = lidar.getCurrentPoint().quality; //quality of the current measurement
		if (quality && (angle >= min_angle) && (angle <= max_angle) && (d < max_d)) {
			if(!string1)
				string1 = NUM_STRINGS * (angle - min_angle) / (max_angle - min_angle);
			if (string1) {
				string2 = NUM_STRINGS * (angle - min_angle) / (max_angle - min_angle);
				if (abs(string2 - string1) <= 2) //Don't play adjacent strings. Sound odd
					string2 = 0;
			}
		}

		if (startBit) {
			if (string1 != 0) {
				freq_1 = 0.44 * pow(2.0, ((float)string1 + OFFSET - 49) / 12);
				volume1 = 10;
			}
			else if (volume1 > 0){
				volume1--;
				delay(DECAY);
			}
			string1 = 0;

			if (string2 != 0) {
				freq_2 = 0.44 * pow(2.0, ((float)string2 + OFFSET - 49) / 12);
				volume2 = 10;
			}
			else if (volume2 > 0) {
				volume2--;
				delay(DECAY);
			}
			string2 = 0;
		}
	}
	else {
		digitalWrite(MOTOR_PIN, 0); //stop the rplidar motor
		Serial.println("Stop Motor");

		// try to detect RPLIDAR... 
		rplidar_response_device_info_t info;
		if (IS_OK(lidar.getDeviceInfo(info, 100))) {
			Serial.println("Lidar detected...");
			lidar.startScan();

			// start motor rotating at max allowed speed
			digitalWrite(MOTOR_PIN, 1);
			delay(1000);
		}
		else {
			Serial.println("Lidar Error");
		}
	}
}

ISR(TIMER1_OVF_vect) {
	cnt1++;
	if(cnt1 % 2 == 0)
		OCR1A = 256 + volume1 * samples[(uint16_t)(cnt1 * freq_1) % SAMPLE_LEN];
	else
		OCR1A = 256 + volume2 * samples[(uint16_t)(cnt1 * freq_2) % SAMPLE_LEN];
}