And I wanted a small window decoration instead.
So I cut the 20 bulbs (originally connected in series, 220V) into pieces and connected 9 of them in parallel to 7.5V adaptor. One decided not to work but I also ran out of preferred colours of bulbs so that's no big loss. It looks really messy ducttape thingy on one side but the other side is more decent and main point is to show the lights throught the window. Mission completed.