A variety of humidity tester circuits are available, but this is a circuit that is as simple as possible. Using only a transistor, LED and resistor, this circuit can be used to check the humidity level of materials like soil, paper, etc. In this circuit, we have used an NPN transistor to detect soil moisture. This is a very simple and interesting circuit for electronics lovers. This Soil moisture sensor circuit can be used in many applications like Automatic plant irrigation system, Greenhouse projects, etc.

The circuit does not have a current-limiting resistor because the base resistor is very high and the current through the transistor is only 2mA.  Don’t change the supply voltage or the 220k as these two values are correct for this circuit.   

Parts List

  • Transistor BC547/2n2222/ or any general-purpose NPN Transistor
  • 220k ohm 1/4watt Resistor
  • LED
  • Switch
  • 3volt small battery
  • Insulated Copper Wire for circuit Legs ( The sensor probes can be anything like an iron nail, pin paper clip, etc )
Soil Moisture Circuit Diagram
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