(Design review requested) GPIO and manually controlled switch

Hi all

I’m trying to take what I’ve learned so far and design a 2 way switch that can be operated by both a GPIO pin AND a manual switch where toggling either one will turn the load on or off. The operation should be similar to those hall ways/landings with a light which can be controlled by flicking a switch at either end.

I’ve attached a simple schematic of what I’ve come up with so far and would like those more knowledgeable to OK it or spot the flaws in my logic if there are any.

The idea is that I have the GPIO pin connected to the gate of both an N-channel and P-channel MOSFETs so that whether the GPIO is HIGH or LOW only one of the MOSFETs is active. This configuration in conjunction with a SPDT switch this should control the flow to complete or break the circuit.

Assuming there are no glaring issues with my design, I’d like some help deciphering the spec sheets for P-channel MOSFETS (PMOS?).
For N-channels I understand that when the Gate voltage reaches Source voltage the MOSFET should close and allow the flow from Source to Drain. I also understand that for a P-channel MOSFET that when the gate reaches source voltage the MOSFET should open and stop the flow from Source to Drain.

What I don’t seem to understand is how to read the spec sheet for a P-channel MOSFET (as they seem to give the threshold in -ve V). I’m after one that is closed when 0v supplied at gate and open when 3.3v is applied across both the gate and source.

Many thanks for your help.

Edit: The actual load has a fairly low current draw. Will be in the ma range so high spec MOSFETS are not required.

Edit: Thanks DrAzzy, I’ve now realised my misunderstanding and hence realised that in the schematic I have the N and P MOSFETs the wrong way.


N-channel mosfets conduct when Vgs (voltage of gate relative to source) is positive. Source is usually ground.

P-channel mosfets conduct when Vgs is negative (ie, gate is below source voltage). Source is usually supply voltage.

Thanks DrAzzy.

Can you clarify if P-channels conducts when Vgs is the same as source? I.e. if I connect the source to ground, if the GPIO is low will it conduct?

Yes. If you are switching a 12V supply, then often a NPN is used to pull the gate low to turn on, and a resistor from the gate to 12V supply is used to turn off.