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The resistive soil moisture sensor measures the electrical conductivity between two probes inserted into soil. Higher moisture lowers resistance, increasing current flow. An onboard comparator outputs a digital threshold signal alongside the analog value.
| Supply voltage | 3.3 – 5 V |
| Output | Analog 0–VCC + digital threshold |
| ADC range | 0 – 1023 (10-bit) |
| Probe type | Resistive (bare metal) |
| Corrosion | High — use capacitive for longevity |
| Calibration | Needs wet/dry two-point cal |
Two metal probes act as a variable resistor in series with a fixed resistor forming a voltage divider. Wet soil conducts more current, increasing the voltage at the ADC pin. The digital output trips when the analog value crosses a preset threshold adjusted by a potentiometer.
Metal probes corrode within weeks in wet soil — use capacitive sensors for field deployment. Readings vary with soil type, salinity, and temperature. Requires per-location calibration. Applies DC current to soil (electrolysis).