Variable fan speed timer?

I've recently switched to a 12/12 cycle which my lights and fan are both attached. My problem is that when the fan is off 12, obviously I loose negative pressure in the box and flower odor seeps back out. By the time the power returns the stench is a bit much.

My Phresh filter is great, but obviously can't do it's job when the fan is off.

So... I need to figure out how to keep my fan on during 12 off, yet at a different speed (slower) than when the lights are on. I don't know if this is some form of a double timer or how it can be setup.

Any ideas out there? I'm a pretty good DIY'er, but this has me stumped.

The reason I would like a different variable speed during off is to keep temps under control during dark but move enough air to maintain negative pressure and scrub the air. The ambient temps are just a bit too cool for the box to be the same fan speed as when lights are on.

-FragileBob

 

Yeah, I guess I was making it too difficult. Was hoping to not reconfigure physically and trying to work within the parts in there.

Probably the easiest solution.

Thanks

-FragileBob