the MOSFET not fully turning off? Unexpected 1.1V instead of 0V

[adamskyyy]

I'm working on a circuit using an **IRLML6402 P-channel MOSFET** to control power to a **Capacitive Soil Sensor V2.0**. The schematic is attached.

### **Expected Behavior:**
When the MOSFET is supposed to be off, I expect the **Vcc** pin of the soil sensor to be **0V**.

### **Observed Issue:**
Instead of 0V, I measure **1.1V** at the **Vcc** pin when the MOSFET should be off.

### **Key Circuit Details:**
- **Source (Pin 2)** is connected to **+3.3V**.
- **Gate (Pin 1)** is controlled via a **2.2kΩ pull-down resistor (R3)** and a **4.7µF capacitor (C4)**.
- **Gate-Source voltage should be 0V when off**, but the MOSFET seems to still conduct slightly.

### **Questions:**
1. **Why is the MOSFET not fully turning off?** Could there be leakage current causing this?
2. **What could be the reason for the unexpected 1.1V at the sensor's Vcc?**
3. **Is the gate pull-down resistor (2.2kΩ) too weak, or is the capacitor affecting turn-off time?**
4. **Should I try a stronger pull-down resistor or another MOSFET with a lower threshold voltage?**

Any insights would be greatly appreciated! Thanks in advance.

 

[Diver300]

How much current does the soil sensor take at 1.1 V? At very low voltages like 1.1 V the sensor may take so little current that the slightest bit of leakage could cause that voltage.

I suggest that you try with 10 kOhm between Vcc of the sensor and ground.

 

[Tony Stewart]

The condition for Vgs =0 is called Idss.
Your sheet says this max leakage -1 to -25 uA @ -20V from 25'C to 70'C
Using V/I=R this is equivalent to 20 M to 1 Meg resistance.

Yet the schematic says capacitance sensor and the above is resistance.

So where is you AC pulse current measurement? This will not do it and if the FET capacitance is more than dry soil, that is a bad design. A CMOS Schmitt inverter Osc. would be adequate with a C analog bridge.

 

[Tony Stewart]

Please learn how to read datasheets and define how the design should work with specs.