Photocell Ambient Light Sensor

Application

This circuit uses the Photocell as the light-sensitive leg of a voltage divider feeding a microcontroller-style ADC input. The same front end is common in automatic night lights, display auto-dimming circuits, simple daylight detectors, and data loggers that need a low-cost ambient-light signal.

Design Context

R1 pulls the sense node up to 5 V and LDR1 pulls it down toward ground. As more light reaches the photocell, its resistance falls and the ADC sense voltage decreases. This polarity is useful when firmware treats a lower ADC reading as brighter ambient light.

Expected Behavior

The table should show the Vsense output near the supply in darkness and lower as the Light value on LDR1 is increased. With the default 10 kOhm pullup and 50% light setting, the sense node should sit in a mid-to-high range rather than rail-to-rail, leaving room to observe changes.

Key Values

• Simulation mode: interactive operating point
• Supply: 5 V DC
• Pullup resistor: 10 kOhm
• Photocell bright resistance: 1 kOhm
• Photocell dark resistance: 1 Megohm
• Adjustable input: LDR1 Light from 0% to 100%

Suggested Experiments

• Edit the LDR1 Light label and watch Vsense update like an ADC input.
• Change R1 to 47 kOhm or 100 kOhm to shift the useful measurement range toward darker conditions.
• Swap the divider order if the firmware or comparator should read higher voltage in brighter light.

Featured Component

The Photocell is a two-terminal light dependent resistor. In this demo it models the sensor element that converts ambient light into a resistance change for a simple ADC-readable voltage divider.