The Laser Harp

My intensive project is a laser harp. The harp consists of six laser modules and six photodiodes that are pointed towards each other. Once a laser beam is broken, a specific note plays via an Arduino. The harp plays six different notes at various pitches and sounds. Modifications to the project include adding a wave shield and speaker to the Arduino and making a PVC pipe frame for the harp.


Alex U.

Area of Interest

Mechanical Engineering

Genetic Engineering



The Urban School of San Francisco


Incoming Senior

Final Milestone


Third Milestone

My final milestone is the increased reliability and accuracy of my robot. I ameliorated the sagging and fixed the reliability of the finger. As discussed in my second milestone, the arm sags because of weight. I put in a block of wood at the base to hold up the upper arm; this has reverberating positive effects throughout the arm. I also realized that the forearm was getting disconnected from the elbow servo’s horn because of the weight stress on the joint. Now, I make sure to constantly tighten the screws at that joint.

Second Milestone

First Milestone


For my intensive project, I am making a laser harp. The basic function of the project is to play a note when a laser beam is broken. For my first milestone, the harp is made up of 6 lasers and 6 light sensing photodiodes. Each laser is pointed directly at the center of each tip of their corresponding photodiode. Positioned on the same breadboard, both photodiodes and laser modules are in series with 100Ω resistors. Since photodiodes more effectively convert light energy into current when they are in a reverse biased orientation, I made sure to put the longer lead (usually the positive lead) into the ground power supply and the shorter lead into the positive 5V position. I would like to scale the project up by moving the lasers to a separate breadboard that is parallel to the breadboard with photodiodes and the Arduino. I connected each photodiode to the 6 analog pins on the Arduino (A0-A5). This allowed me to retrieve serial monitor readings from each photodiode. Since photodiodes sense light, the serial monitor readings were an expression of how much light each photodiode is picking up from the lasers. These readings were represented in values from 0 to 1024. In order to get the serial monitor to show when a note is being played, I made a threshold value in my code of 35. When the lasers were on target with the sensor inside of the photodiodes, the serial monitor value for each laser-photodiode pair was ~300. If the serial monitor value dropped below the threshold then the laser beam was broken/blocked. If the laser is broken/blocked, then a laser has been “strummed” and a note will play. The hardest part about achieving this first milestone was keeping the lasers pointed consistently and perfectly at the tip of the photodiodes. If the laser module was touched or moved, even by a millimeter, the beam could skew away from the photodiode sensor. The serial monitor value could accidentally drop below the threshold value, falsely playing a note at the wrong time. Going forward, I would like to add a speaker and eventually build a frame for the electrical components so that my project can take the shape of an actual harp.


Serial Monitor

Starter Project: The Useless Machine

The Useless Machine is indeed very useless, as its only function is to flip a switch to its original position after the user has primarily flipped it. Besides from the plastic casing and corner posts, the machine is made up of a motor, a switch, an acrylic arm, a printed circuit board (PCB), a bicolor LED, a 100Ω resistor, a 220Ω resistor, and a snap switch. It is powered by a battery pack with three AA batteries, giving the machine the power of 4.5 volts. The purpose of this machine is useless, yet the concept of making something so simple through an intriguing and complex process gives it more meaning than may be perceivable.


The batteries power a mechanical arm that flips the protruding switch back to its original position. Once the switch is flipped by the user the circuit becomes closed, inducing a current that flows in the direction from the battery pack to the PCB. Once the current hits the PCB, it goes across the bicolor LED in one direction, making the LED green (one of its two colors). Subsequently, the current flows into the motor, which is attached to the mechanical arm, pushing the arm out of the box and in the direction of the protruding switch to flip it back. When the arm begins to move up, the snap switch is released, signifying that the arm is out of its original position. Once the arm has flipped the switch, the current has reached the end of the circuit and begins to flow back towards the battery pack. As the current passes back through the circuit, the motor turns in the opposite direction, bringing the arm back down, and the bicolor LED turns from green to red. While the arm is coming back down, the snap switch slowly becomes closed, communicating to the motor that it must stop in order to return the arm to its original position. This opens the circuit once again, awaiting to be uselessly flipped until the end of time.

Schematic Design

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