Electronic – MOSFET IRF740 and 3,6 V


I'm looking for a way to completely shutdown power supply to two Modules (UltraSound HC-SR04 and a 433 MHz transmitter unit) while the ESP8266 Module is in deep sleep mode.
I bought some MOSFET IRF740 units because they were recommended by the clerk in the electronics store for that kind of undertaking.
I wired everything the way I think It's supposed to:
Gate = GPIO 14
Drain = Ground of HC-SR04 and 433 MHz
Source = Minus From the Circuit
The Whole system is powered via 3 1,2 V Batteries so there's 3,6 Volt

I defined the ping as output and set it to low to test the whole thing but there was still a reading from the HC-SR04 Module although this should only be possible if the ping is HIGH?

Is my setup wrong… Did I misread the purposes of a MOSFET? Is 3,6 V not enough to switch the system from Power On to Power Off?

I edited the code a bit and found out that it works if I putting the Pin on LOW twice … ?
So like this:

const int GATE_PIN = 14;
digitalWrite(GATE_PIN,LOW); //SensorPower --> OFF
digitalWrite(GATE_PIN,LOW); //SensorPower --> OFF   

this sets the Pin to low and the MOSFET switches

Best Answer

The threshold voltage is the minimum voltage required so that the transistor starts to conduct. And you need more than this to put it into saturation.

The Vth (threshold voltage) of the IRF 740 is min=2v, max=4v so definitely you're in unchartered territory.

Moreover let's look at this graphic: enter image description here

You need at least 4V to have 100mA (although with a 50V drain voltage).

To power off, specially since you might have ground planes I strongly believe it's better to use a P mosfet to cut the (+) side of the supply. The schematic would look something like this:

enter image description here

Control input is your microcontroller. This turns on/off a logic NMOS. When it's on the current passing is very little because it's limited by a 1Mohm resistor.

While turned on it forces ground on the gate of the PMOS, thus, turning it on and allowing power.

When the control input is low, the NMOS is off. The 1Mohm resistor pulls up the gate of the PMOS and it's off. With this simple circuit you can control power to your loads. If your microcontroller will shutdown the pin and turn it off, make sure you put a pulldown to the pin so the NMOS stays off when the microcontroller pin is in High-Z while asleep.

Hope it helps