Lo scenario
This circuit uses a soil moisture sensor (resistive) in a voltage divider, compared against a reference to drive a relay for irrigation control. The op-amp provides hysteresis-free threshold detection.
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Using an op-amp comparator for accurate threshold detection
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Flyback diode protects transistor from relay inductive kickback
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Resistive sensor with fixed resistor forms potential divider
Suitable for DIY automatic plant watering systems, smart garden projects, and educational demonstrations of sensor-based control.