Photocell Night-Light BJT Indicator

Application

This circuit uses a Photocell to turn on a transistor-driven LED indicator when the light level falls. The same pattern is common in night lights, cabinet lights, alarm panels, outdoor markers, and simple dark-detect inputs that need a visible load instead of only an ADC reading.

Design Context

RPULL and LDR1 form a light-sensitive divider. LDR1 is the low-side sensor, so the sense node rises in darkness and falls in bright light. RB limits base current into Q1, while Q1 sinks LED current through RLED and DLED when the base drive is high enough.

Expected Behavior

With the default 0% Light setting, LDR1 is near its dark resistance, Vsense is high, Q1 turns on, and the LED indicator conducts. Increase the LDR1 Light value toward daylight and Vsense falls; the transistor turns off and Iled drops toward zero. The table updates to the current steady-state operating point as you edit the light value.

Key Values

• Simulation mode: interactive operating point
• Supply: 5 V DC
• Pullup resistor: 100 kOhm
• Base resistor: 47 kOhm
• LED current-limit resistor: 680 ohm
• Photocell bright resistance: 1 kOhm
• Photocell dark resistance: 1 Megohm
• Adjustable input: LDR1 Light from 0% to 100%

Suggested Experiments

• Edit the LDR1 Light value and watch the LED current fall as the simulated environment gets brighter.
• Change RPULL to shift the light threshold where Q1 begins to turn on.
• Change RB to see how base current margin affects the indicator near the threshold.
• Increase RLED to make the indicator current lower for battery-powered designs.

Featured Component

The Photocell is a two-terminal light dependent resistor. In this demo it controls a bias node that drives a practical transistor switch, showing how a passive light sensor can operate a visible load.