RE #2 I think I was not clear. What I intended to say was to apply the PWM control signal to the base of a NPN transistor to turn the 5vdc on/off.
I had not though about using a capacitor to smooth out the control signal. Would also remove any spikes tough I would guess the fan could handle those.
If I read correctly there are two ways to control the PWM control signal output by the PCA9685 on the Adafruit HAT/Bonnet.
a) The :dim a LED example use one library to control the duty cycle 0 to 0xffff (which I was thinking to use to control the fan)
b) To control a servo, a 1-2ms pulse ever ~20ms is used
Although it is not clear, I think I can control one channel of the PCA9685 with duty-cycle and the other 15 to drive servos.
Even with a capacitor I don’t think the 5 to10% duty-cycle of the servo control signal will provide enough difference to turn the 5vdc on/off without critical design - which I am not up to.
Using one of the channels from the PWM HAT has the real advantage of pulling the 5vdc @200ms from a separate 5vdc source and doesn’t load down the Pi 5vdc. It also gets everything to the fan/mounting board with a single plug… If all else fails I can use one of the few GPIO pins I have left to turn the 5vdc on/off.
Thanks for your additional thoughts.