SensorsQUICK REFERENCE
LDR / Photoresistor
Build a light-sensitive divider and turn ADC readings into thresholds.
01Voltage divider
- Wiring
- VCC → LDR → ADC node → 10 kohm resistor → GND.
- Behavior
- More light usually lowers LDR resistance, raising voltage with this arrangement.
- Formula
- Vout = VCC × Rfixed / (RLDR + Rfixed).
- Reverse
- Swap LDR and resistor to make brighter light produce a lower reading.
02Choose & calibrate
- Resistor
- 10 kohm is a starting point; match it to LDR resistance near your target brightness.
- Sampling
- Average several ADC readings; measure dark and bright values in place.
- Threshold
- Add hysteresis: different turn-on and turn-off thresholds prevent flicker.
03ADC rules
- Voltage
- Power the divider from an ADC-safe rail. Uno usually 5 V; Pico 3.3 V.
- Meaning
- Raw ADC values are relative brightness, not lux without characterization.