Pulsed‑Light Sensor: Amplify LED Light Pulses into Audible Sound

Parts and Materials

• Two 6‑volt batteries
• One NPN transistor—2N2222 or 2N3403 (Radio Shack catalog # 276‑1617 offers a 15‑pack ideal for this and similar experiments)
• One light‑emitting diode (Radio Shack catalog # 276‑026 or equivalent)
• Audio detector with headphones

If you don’t already have an audio detector, you can build a sensitive one with a closed‑cup headphone set and a 120 V/6 V step‑down transformer. Connect the headphone stereo plug to the transformer’s 6 V secondary as shown:

Try both series and parallel configurations for maximum volume. A dedicated audio detector, built as described in the DC and AC experiment chapters, is a valuable addition to any hobbyist’s kit.

Cross‑References

Lessons In Electric Circuits, Vol. 3, ch. 4: “Bipolar Junction Transistors”.

Learning Objectives

• Use a transistor as a basic common‑emitter amplifier
• Employ an LED as a light sensor

Schematic Diagram

Illustration

Instructions

This circuit detects pulses of light striking the LED and converts them into audible signals via a transistor amplifier. LEDs generate current when illuminated—much like a miniature solar cell—so the transistor amplifies the weak LED signal enough to drive the headphones.

Expose the LED to pulsed light sources such as fluorescent or neon lamps that flicker at 60 Hz, or use a steady bright source and create moving shadows with your fingers. The resulting light pulses produce a distinct buzzing tone in the headphones. Because LEDs are narrow‑band devices, they respond best to wavelengths close to their own emission. Near‑infrared LEDs can also detect infrared remote‑control signals, yielding a recognizable audio cue.

With a transmitter that modulates an LED using an audio waveform, this receiver demonstrates how light pulses can carry music or speech—illustrating the fundamental concept behind optical audio transmission.