A 9540, P Channel, versus 540, n channel, are both a bit marginal, worst case spec, for turn on, 540 -
Then you have, as drawn, load is switched to supply, not to ground.
Start with 540, place load in drain.
+12 >> LED STRIP >> 540 DRAIN, 540 source to ground, 540 GATE to PIR.
If this is a one off look at 540 heat. To see if its on the marginal side, not turning on
hard enough with 3.3V. If a production item get a logic level MOSFET that will run off 3.3V
Is strip rating of 300 mA a hard limit or does strip have its own current regulated circuitry ?
If former you need to drop V to strip, use a zener, it wants to be (12V - 5V) = 7V. Its power
(50% margin) = 7V x .3A x 1.5 (margin) = > 3W, small heatsink on it. Note make these
calculations with worst case 12V value (high), same for Vz tolerance. So connections -
+12 >> ZENER >> LED STRIP >> 540 DRAIN, 540 source to ground, 540 GATE to PIR.
And fuse the the circuit.
Now not to confuse, if stirp is rated such that user must supply minimum 5V, with current regulated
to .3A, then a simple 3 terminal regulator, like LM317 operated in constant current mode
would be a clean easy solution to that problem.
Input diode for reverse polarity input connection. Not needed if you dont have that as a
potential issue.
So connections would be
+12 >> LM317 Input >> LEM317 Output >> R1 >> LED Strip >> 540 DRAIN, 540 source to ground, 540 GATE to PIR.
And the connection of LM317 ref pin to right side of R1.
And fuse in 12 V line....
Regards, Dana.