Overview

The YL-69 (also sold as FC-28) is a 2-prong resistive soil moisture sensor widely used in DIY plant watering, smart garden, and automated irrigation projects.

It consists of two exposed copper PCB traces inserted into soil and an LM393 voltage comparator driver PCB (YL-38). Moisture in the soil acts as a conductor: wet soil lowers electrical resistance between the prongs (producing a lower analog voltage output), whereas dry soil increases electrical resistance (producing a higher analog voltage output near VCCV_{CC}).

Quick reference

Operating Voltage (VCC)3.3 V to 5.0 V DC
Sensing PrincipleElectrical resistance / conductivity through soil
OutputsAnalog voltage (AO: 1.2V wet to 5.0V dry) / Digital threshold (DO via LM393)
Operating Current~15 mA
Probe Dimensions60×20 mm60 \times 20\text{ mm} (fork length 48 mm)

Pinout

Standard Driver PCB (YL-38 Driver Board Header)

PinNameTypeDescription
1VCCPowerSupply voltage (+3.3 V to +5.0 V DC)
2GNDPowerGround (0 V)
3DODigital OutputDigital threshold output (HIGH in dry soil, LOW in wet soil)
4AOAnalog OutputAnalog voltage output (proportional to soil electrical resistance)

2-Pin Probe Header (Connects Driver to YL-69 Fork Probe)

  • Connect the 2 pins on the probe fork to the 2-pin header on the driver PCB (non-polarized).

Specifications

ParameterSymbolMinTypMaxUnitConditions
Supply VoltageVCCV_{CC}3.35.05.5VDC
Operating CurrentICCI_{CC}—1520mAActive measurement
Dry Air Output VoltageVdryV_{dry}VCC−0.3V_{CC}-0.3VCCV_{CC}VCCV_{CC}VProbe in dry air
Water Immersion OutputVwetV_{wet}1.01.21.8VProbe submerged in water
Probe PlatingPlatePlate—Nickel / HASL——Standard PCB coating

Calibration & Voltage Response

Because soil composition, salt density, and mineral levels vary significantly:

  • Dry Air Baseline (0% Moisture): Analog output voltage reading ≈1023\approx 1023 (on 10-bit 5V ADC) / 100%100\% dry.
  • Submerged in Water Baseline (100% Moisture): Analog output voltage reading ≈250\approx 250 to 300300 / 100%100\% wet.

Soil Moisture (%)=100×(1−ADC Reading−ADCwaterADCair−ADCwater)\text{Soil Moisture (\%)} = 100 \times \left(1 - \frac{\text{ADC Reading} - \text{ADC}_{\text{water}}}{\text{ADC}_{\text{air}} - \text{ADC}_{\text{water}}}\right)

Wiring

YL-69 Driver Board Pin→Microcontroller (Arduino / ESP32)Notes
VCCGPIO Output Pin (or 5V/3.3V)Power via GPIO pin to prevent corrosion!
GNDGNDGround
AOAnalog Pin A0 (or ESP32 ADC)Reads voltage proportional to moisture
DODigital Pin (optional threshold trigger)Threshold adjusted via potentiometer
Warning

Electrolytic Corrosion Warning: Leaving constant DC voltage applied to the exposed copper prongs of the YL-69 probe causes rapid galvanic electrolysis corrosion. Copper plating dissolves into the soil, ruining the probe within days/weeks. Corrosion Prevention: Connect sensor VCC to a digital GPIO output pin on the microcontroller. Set GPIO HIGH for 50 ms only when taking a reading, then set GPIO LOW to turn off sensor power. For permanent outdoor soil monitoring, upgrade to a Capacitive Soil Moisture Sensor.

Common mistakes

  • Leaving power ON 24/7: Continuous DC current causes rapid probe corrosion.
  • Submerging driver PCB in soil/water: Only insert the 2-prong fork into soil up to the white line; the electronic driver board is NOT waterproof.
  • Assuming linear percentage without calibration: Raw ADC values vary based on soil compaction and salinity.

Notes

  • Obsolete for long-term deployments; replace with v1.2/v2.0 Capacitive Soil Moisture Sensors for corrosion resistance.

Assets & downloads