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.

After disconnecting the soil sensor, the voltage goes from 1.1 to 0v (no load to mosfet)

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.

 

What voltage do you measure at A2? If it is non-zero, perhaps there is leakage between A2 and Vcc inside the IC (e.g. through an input protection diode).

 

What voltage do you measure at A2? If it is non-zero, perhaps there is leakage between A2 and Vcc inside the IC (e.g. through an input protection diode).

it's 0V and 3.4 v for short while to read the data from sensor.

 

After disconnecting the sensor's Vcc from the MOSFET, have you measured the Vcc pin then? If not, do that. You may be chasing the wrong problem.

 

The other thing you might do is replace the sensor with a resistor and adjust the resistance until you see the same 1.1 V. That would tell you how much current the MOSFET is providing when it is supposed to be off.

 

are you sure you are using correct footprint for that mosfet? your schematics pins match the mosfet datasheet, but chosen footprint may have them in different order...

 

Dear users,

I sincerely apologize for the confusion. In my schematic, I simplified things a bit—actually, the soil sensor is connected through an ADS1115, which in turn communicates with the Arduino via I2C.

When I removed the ADS1115 (as shown in the current schematic), the circuit works correctly—the MOSFET fully cuts off the power, and there is no leakage voltage.

However, when the ADS1115 was also connected to the MOSFET, I suspect there was some leakage through the SCL line or possibly through the ADS itself.

I wanted to use ads1115 but as I still have few analogue ports free - I will use them -

 

Dear users,

I sincerely apologize for the confusion. In my schematic, I simplified things a bit—actually, the soil sensor is connected through an ADS1115, which in turn communicates with the Arduino via I2C.

When I removed the ADS1115 (as shown in the current schematic), the circuit works correctly—the MOSFET fully cuts off the power, and there is no leakage voltage.

However, when the ADS1115 was also connected to the MOSFET, I suspect there was some leakage through the SCL line or possibly through the ADS itself.

I wanted to use ads1115 but as I still have few analogue ports free - I will use them -

The I2C pull-up resistors are providing the power, not the MOSFET.

 

The I2C pull-up resistors are providing the power, not the MOSFET.

Agreed. Disconnect just Data/A2 and re-measure voltage.