I want to do a timer that will turn on and off mains powered light bulbs in predefined intervals using a triac. For added benefit I would like to add a soft start for incandescent lights. When incandesent light is cold it has low resistance, leading to high inrush current that is damaging the filament. Since I will have a MCU controlled triac I want to implement a slow turn on to give the filament time to heat up without the inrush. I have no idea how quickly the filament heats up. If it is fast enough switching at zero crossing is all that is needed. But if the filament needs more than one full mains cycle to heat up it is needed to slowly increase the conduction angle. I cannot find any info related to this or a similar project.
Are they special light bulbs of 100 years old ?
If not, then don't worry about it, what you propose might make it worse.
Unless its very high power with a thick filament mains bulb filaments heat and cool very fast so you can't use zero-cross switching, you need cycle-by-cycle triac operation to prevent flicker.
It doesn't heat-up that fast. An incandescent seems to come to full-brightness in about 1/10th of a second. I've made some sound activated lighting effects and if I don't hold it on for about 1/10th of a second (or more) I can see it's not reaching full-brightness. Of course, you could go more-slowly.
You should be able to find a dimmer project and then it's just a matter of fading-up the brightness...
A dimmer works by triggering a TRIAC at a known time after the zero crossing. Then a TRIAC latches-on and won't turn-off 'till the next zero crossing (when current goes to zero).
Of course, your zero -crossing detector (and your TRIAC) has to be isolated. Most people use an optical isolator for both, but when I built a dimmer* I also built the power supply so I used the transformer secondary to get an isolated signal for zero crossing.
*A long time ago, with a different microcontroller, I built a "sunrise dimmer" that fades-up a lamp over a 10 minute period. (My sound activated effects are just on/off with no dimming.)
Get some led bulbs !
Not suggesting you should or shouldn't slow start your incandescent lamps. However unless you are turning them on or off very very often the added life is negligible.
If you want to increase the life of an incandescent bulb you should reduce the voltage a little bit. The live of a bulb varies as the 12th power of the voltage ratio.
So a 5% reduction in voltage will yield nearly double the life.