Soft latching switch not working

Other Hardware General Electronics
1

Hi all,

First of all i will say i'm a novice circuit maker so please be kind.

I wonder if someone would be so kind as to offer some support in figuring out why my soft latching switch, which will be controlling an Arduino pro micro in the end, is not behaving as expected.

I'm following an example from EEVblog on YouTube but I am swapping the NPN and PNP transistors for N-channel mosfets and a P-channel mosfet respectfully.

this is my schematic

1920×1080 121 KB

And this is my board layout (there is 1 via for connecting GND)

1920×1080 181 KB

The Mosfets i'm using are: -
P channel Mosfet is a BSS84 (SOT23)
N channel Mosfet is a BSS138 (SOT23)

I have checked out the circuit to the best of my ability and cannot find any shorts and all traces have continuity.

Behaviour
The circuit is powered by a single 18650 cell, and when connected I get the full voltage on the output pins of the latching switch circuit without pressing the button.

Pressing the button, I see voltage drop to the mV range so I know the shutdown side of the circuit is good.

As soon as i release the button the voltage goes back to full output voltage.

It would seem that something is triggering the n channel mosfet and in turn triggering the main p channel output mosfet, but I cannot wrap my head around the logic of why this is happening.

I do wonder if my mosfet selection is the problem here and that the internal diode is breaking down, but this is me guessing as opposed to having any logic to back that up.

Thanks for any help and suggestions
Steve

2

My guess is current leakage through T1 and R2 is triggering T4, try a 10k bleeder from T1's drain to GND.

3

JCA34F:
My guess is current leakage through T1 and R2 is triggering T4, try a 10k bleeder from T1's drain to GND.

Firstly, thank you for your reply.

So this has improved things.
The circuit now powers up in an "off" state and pressing the button latches the circuit "on".
Pressing the button for a second time powers the circuit "off".

powering on for a second time in quick succession the circuit stays latched in the "off" state.
using a multi meter, i can see that the output voltage needs to drop below 8mV before Q5 disengages allowing the circuit to latch to the "on" state.

I may need to review the value of R4 so the capacitor charges and discharges at a faster rate.