Switching on/off the 3.3V pin


#1

I am using the 3.3V pin on the electron to power a handful of sensors. To extend battery life, I’d like to utilize standby mode so that the sensors only take readings and draw current when necessary. Two questions:

  1. Even when the particle electron goes into the standby mode, the 3.3V pin will continue to provide power and the sensors will continue to draw current, correct?
    1.1) If the above is correct, are there any way to control so that the power to the sensors can be turned on and off?

#2

You are correct, 3.3V is still active in sleep.

You should use an external FET to control power to your sensors.


#3

Ok, thanks! Is one NMOS enough? Like this?


#4

I believe you have drawn a P channel MOSFET. It would be in the correct location but you apply a 0v to the gate to turn it on and 3.3v to the gate to turn it off. Not sure how that works for you but you should probably apply a pull up resistor to the MOSFET gate so that when your Particle Device is asleep, the sensors are off. When the Particle device wakes up, it would have to set the GPIO attached to the gate as output and then drive it low in order to power the sensors. This is assuming your are using Enhancement-type MOSFETs.


#5

Can you use one digital pin on the particle to control turning the rest of the sensors/load on and off? I.e. Connect the Vin to the rest of the sensors to a digital pin on the particle. when the digital pin is high, it provides the 3.3volts to the rest of the sensors, and when the digital pin is turned low, the sensors are turned off?


#6

That’s an NMOS, where if pin D6 is +3.3, the mosfet is turned on, and if D6 is 0 volts, the mosfet is turned off and power to the load is turned off. In that case should I use a pull down?


#7

Sure you could attach the Vin of a sensor to a GPIO to control it’s on/off state… as long as the sensor doesn’t pull more than 20mA for the Photon/Electron and no more than 9mA on the Boron,Argon,Xenon. That is such a small amount of power that you will most likely need to use the MOSFET to supply more current. Make sure you select a MOSFET that will exceed your max current draw. MOSFETs are pretty awsome that the basically act like a switch and can sustain large current loads.


#8

If using an NMOS, then yes, you provide 3.3V to turn on and 0V to turn off… and use a pull down to keep it off during sleep. However, the NMOS should be placed on the other side of the load… the “low” side. Placement of the N vs P channel MOSFET is all about biasing the source and drain correctly.

http://www.learningaboutelectronics.com/Articles/N-Channel-MOSFETs

http://www.learningaboutelectronics.com/Articles/P-Channel-MOSFETs

And the arrow is not pointing the correct direction so that’s why I was questioning N vs P.


#9

By low side, you mean like this?

For the arrow I just referred to this picture
Capture


#10

This has a better graphic on N vs P. I always have to refer back to a reference when talking N vs P… not something that stick too well in my brain.

https://www.quora.com/When-is-it-good-to-use-N-MOSFET-and-when-do-you-use-P-MOSFET

And for the drawing:

image

Actually, just looked at a bunch of N vs P drawings and they can be drawn a bunch of different ways. So to each his own! Sorry.


#12

Sounds good! I’m thinking something like the following then. And it sounds like the pull down resistor (R1) needs to be a large value? 1Megaohm maybe?


#13

Size the pull down for what power consumption you are willing to incur. At 3.3V and 1MOhm, you will draw a constant 3.3uA. At 10KOhm, the constant draw is 330uA.