Beam-Break Detector LED Driver

Application

Use this circuit as a simple optical interrupter or beam-break indicator, similar to a slotted sensor driving a visible status LED.

Design Context

A pull-up resistor biases the NPN transistor base high so the LED turns on when the photodiode is dark. When light reaches the photodiode, photocurrent pulls the base node low and turns the transistor and LED off.

Expected Behavior

At the default 80% light level, the beam is present, Vbase stays low, Q1 is off, and the LED is dark. Set D1 light to 0% to emulate a broken beam; Vbase rises, Q1 turns on, and LED current flows.

Key Values

• V1 is 5 V.
• RLED limits LED current.
• RBASE provides the base pull-up.
• D1 starts at 80% light to represent an unbroken beam.

Suggested Experiments

• Set D1 light to 0% to emulate blocking the beam.
• Increase RBASE to make the detector easier for the photodiode to pull low.
• Change IphotoMax to model a stronger or weaker optical sensor.

Featured Component

The photodiode component converts light level into photocurrent so the table and LED indicator update live in interactive operating point mode.