Hello. I am using several sensors (Soil moisture, wind speed, direction etc) to get environmental data once in every 15 Minutes. As they are not measuring anything most of the time and consuming power, I wanted to use MOSFETs to power them up only when needed. But I am having problem finding correct P Mosfet with combination of N, as it's not getting completely turned on or off. Can you please suggest some combination and the whole diagram? Thanks
The sensors used:
Soil moisture: SMT100
Wind speed: PCE-WS P
Direction: PCE-WV A
Simple rule of thumb, P is for positive and N is for negative switching. To switch a MOSFET the reference point is the Source regardless of the power system. When the gate voltage is ~ to the source voltage it will be off. A P channel device needs the gate negative in reference to the source and a N channel devices needs the gate positive in reference to the source. If you look at the data sheet there is a term Vgs (Voltage gate source), that chart will tell you how much voltage you will need to switch it.
This is a High Side switch. The first N-Channel MOSFET is connected the same as the low side switch which it is. It allows the voltage to go to 24V referencing ground to put the gate and source of the P Channel device at 0. When it is turned on it switches the gate towards ground forcing the P Channel Source to Gate voltage to go - turning it on.
When switching MOSFETs it is always to put a resistor between gate and source to be sure it it turns off and stays that way until you switch it on. This is especially important when the Arduino is first turned on and until the processor gains control. The place to put this resistor is at the signal source. Placing it at the gate will form a voltage divider lowering the drive to the MOSFET.
Why use a MOSFET on a sensor? They draw trivial amounts of current; you would do much better to put the board to sleep. What board are you using?
BTW, the LED on the board is probably using way more power than all of the sensors combined
How long, after turning the sensor ON, does it take for the sensor to "boot up" and settle down to a reliable output?
Are you applying "pulled up" voltages to sensor while it is OFF?
The problem with removing power or ground to sensors is that the sensor still can get powered through it's data lines. And that could be harmful to the sensor or Arduino pin.
There is no standard recipe to power off a sensor.
Give us links to the exact sensors and we might be able to give the right advice.
Leo..
Thanks for the detailed diagrams. As the sensors should be connected to ground, I think I need the high side switching. But the MOSFETs are not logic level, right? As it'll be controlled via Arduino, can you please suggest some logic levels MOSFETs?
