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Many circuits I have seen that use a 555 to drive an N channel MOSFET often have an inverter stage after them. This is because the 555 wants to start its timing cycle high which would turn the MOSFET on for most of the time. Changing the discharge resistor for a higher value still reults in a more on than off circuit. There is a finite limit to the charge/discharge resistor ratio and often that will not fit in the scheme of on / off time required. A simple fix that requires no inverter stage is to put a common signal diode in parallel with the discharge resistor so that the charging current goes through it and the discharge current can only go through the discharge resistor.
regards
bugwhiskers
regards
bugwhiskers