Soft On/Off switch

[LinkMTech]

Hi Audette,
Welcome to the forum.
Since you changed Q1 to a P-CH FET, then R1 and R2 values will need to be changed to much lower values than what is used it the original thread.
If the FET gate to source can handle a 6V difference, change R1 and R2 values to 1K for faster ON/OFF times. Consult the part's data sheet though.
The 12K on R1 may not be allowing the gate charge to drain fast enough and leaving enough residual "output" to turn the circuit back ON.

Or..

Something is leaking a little current at the R2/D2/Q2.C node.

[Charlie]

FET's are voltage operated devices, where BJTs are current operated devices. Also P-channel devices operate essentially backwards to N-channel devices, so when PIC/RA1 goes high, Q2 turns on hard, making it's collector effectively ground (about 0.6 V or so). This ground, turns the P-channel device ON hard. In other words, grounding the gate of a P-channel device will turn it full on, not full off.

Check your circuit carefully - if I had to guess what might cause the behaviour you describe, I'd check to make sure D1 is not in backwards.

Also, the absolute values of R1 and R2 are not important; only their ratio, despite what the previous poster said. A nice side benefit of using a FET is you can actually make these devices quite large and not waste as much bias current. He was correct that if you go too high, the gate capacitance will start to introduce a small time constant, but it's unlikely you would notice. Assuming Vgs is at least 12V on the device you've chosen, I'd be tempted to eliminate R2 completely for a FET circuit, and make R1 anything I've got lying around between 10K and 100K. That way you don't need to worry about threshold voltage of your device because you'd be pretty much guaranteed to cross it.