Interfacing LDR (Photoresistor) Light with STM32 Blue Pill

LDR (Photoresistor) practical reference

The LDR (Light Dependent Resistor), also called a photoresistor or photocell, is a passive component whose resistance drops sharply as light increases — typically from over 1MΩ in darkness down to a few hundred ohms in bright light. It has no polarity and no active circuitry, so it cannot be read directly by a microcontroller: you build a voltage divider with a fixed resistor (commonly 10kΩ) so the changing resistance becomes a changing voltage an ADC pin can measure.

It is the go-to sensor for automatic night lights, streetlight controllers, light-following robots, camera exposure triggers, and simple ambient-light dimmers. The catch that trips up beginners is response time: cheap CdS-based LDRs are slow (tens of milliseconds to react) and non-linear, so they suit on/off or slow-trend lighting decisions but not fast or precise lux measurements — for that, use a digital light sensor like BH1750 instead.

Key specifications

• Type: Cadmium Sulfide (CdS) photoresistor
• Package size: 5mm (also 4mm, 7mm, 12mm)
• Dark resistance: ~1MΩ (typical, 0 lux)
• Light resistance: ~200Ω–1kΩ (typical, 10 lux)
• Spectral peak response: ~540nm (green, close to human eye)
• Response time: ~20–30ms rise, ~30ms+ fall (slow, non-linear)
• Max power rating: 100mW (typical 5mm package)
• Max voltage: 150V DC (passive, not logic-level restricted)
• Interface: Analog only — needs a voltage divider and ADC pin, no library

Pinout and connection notes

Two-leg passive component, no polarity. Standard circuit: LDR from VCC to a sense node, fixed 10kΩ resistor from that sense node to GND, sense node to your ADC pin. Swap the LDR and resistor positions to invert whether brighter light reads higher or lower.

• Leg A: No polarity — connect to VCC or the divider midpoint, either leg works
• Leg B: No polarity — connect to the fixed resistor / ground side of the divider

Integration checklist

1. Confirm the exact part number and breakout-board revision.
2. Verify supply voltage, current demand, logic levels, and signal direction against the datasheet.
3. Check controller pin assignments and bus address or interface mode against the example code.
4. Power up with external loads disconnected, then test signals before full integration.
5. If readings or communication fail, check power and ground first, then pin mapping, interface settings, and library initialization.

Troubleshooting and reproducibility

Record the board used, wiring, library/version, supply arrangement, and measured behavior. Use the selected board-specific wiring example as the source of truth. Never assume visually similar modules share identical voltage tolerance or pin order.