LED "laser" Alarm System

The final project of another analog electronics lab class was to build a "laser" alarm system, such that if the beam of light from the LED to the photo-diode is disturbed, the alarm will go off and must be manually reset.  Even further, the project objective was inspired by the scene in Harry Potter, where the three-headed dog Fluffy sleeps to the sound of music before it awakes due to any disturbance.

So in the "armed" state, the circuit transmitted music from an MP3 player across the light beam by modulating the LED voltage, and this signal was amplified and played on a speaker.  In the "tripped" state, the music stops playing, a piezo-buzzer rings for a few seconds, and a flashing light remains on until a manual switch is used to reset the circuit to the "armed" state.

A PDF of the circuit can be found here (I can only find a hand-drawn version now), and a picture and a video of the circuit operating are shown below.

The circuit uses a Schmitt Trigger with large hysteresis to decide when the alarm has been tripped.  The output of the trigger is sent to a logic IC to switch between the modes of playing the music on the speaker vs activating the buzzer and flashing light.  Two push-pull buffers are used in the speaker amplifier for current gain; we had the loudest speakers in the class by far!

This was actually the final project from my first analog electronics class, i.e. before I took the second class where I built the optical lock-in amplifier, so I'd say my circuit design was certainly more polished in that project.  Regardless, this was a really fun project and it was an awesome thing to build after one quarter of analog electronics.