DIYbanter - Interfacing a power mosfet to a pic running on 3 volts

I wanted to use a power mosfet with a gate threshold of 5 volts in a 3
volt pic circuit. 3 volt nominal - the pic only sees about 2.7 volts
so the output drive is rather low.

Plan A was to just use the pwm output of the pic to toggle a voltage
doubler or tripler. Doesn't work, the 4 diode drops in the doubler
eat up 2.4 volts and the output is only ~3 volts (the pic eats another
1.2 or so)

Plan B is to use a blocking oscillator and step up the voltage. Works
very well. While the oscillator only requires about 5 milliwatts, the
low voltage of the output pin won't drive it directly, so a switch to
ground is called for. Running the pic on 4.5 volts will drive the
blocking oscillator directly from the pic without the transistor.

If the low side switch is left in, to allow 3-4.5 volt operation (the
two forward biased 1N4148's have to be replaced with one 1N4148 and
one reverse bias zener to prevent the mosfet from staying in its
linear range when it should be "off.")

I wound a 15 turn center tapped coil on a tiny ferrite core for the
blocking oscillator transformer - the open circuit output under no
load is on the order of 80 volts (and may eat the 2N3904 - so it needs
a load). The red LED and Zener diode regulate the voltage to a more
reasonable 10 VDC gate drive - and indicate the circuit is working.

Another, isolated 15 turn winding on the oscillator, would give an
isolated 10 volt power supply and allow the use of an N channel mosfet
as a high side switch.

Plan C was a return to the pwm approach, and just use it to switch an
NPN transistor and use inductor flyback pulses to provide a high
voltage power source. The pwm drive is 10 KHZ at 50% duty cycle. For
3-4.5 volt operation the diode/zener is necessary to prevent the
mosfet from staying biased on when the drive signal isn't present.
Works well and uses fewer parts.

I originally used the blocking oscillator toroid for the inductor -
that worked. Any inductor in the 1-3 mhy range works, but a 3 mhy
gives a better output voltage margin as the batteries deplete.
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