Rub the Atmel writing off an Attiny85 and add something like NE555.
I predict that it will have trouble driving 200mA through an output from a 12V supply...
The AVR reset circuit discussed recently isn't totally out of the running, though. A bit on the simple side.
How else might a 555 be used together with a microcontroller? This would at least be an area that hasn't already had hundreds of 555 circuits already published!