Infrared Light Barrier: Analog Intrusion Alarm
Published:
March–April 2026 · ESET 350 · Two-person team
We built a Velleman MK120-based infrared light barrier that sounds a buzzer and lights an LED when the beam is interrupted.
Circuit
The transmitter used a BJT astable oscillator at approximately 6.2 kHz to switch an infrared LED pair. The receiver combined a phototransistor, op-amp high-pass amplifier, rectifier and smoothing stage, voltage reference, comparator, and alarm output stage.
My contribution
- Independently built a Multisim simulation to understand and debug the circuit beyond the assignment requirements.
- Used a multimeter and oscilloscope to diagnose faults, checking the circuit on a breadboard before final assembly.
- Soldered most of the final build.
Result
The completed alarm worked correctly. Moving between simulation, breadboard measurements, and the soldered assembly helped me isolate faults and verify each stage.
Tools: Multisim, oscilloscope, multimeter, soldering, op-amps, BJT circuits.