How do blinking sequential Christmas lights work?

It's that time of the year when Christmas lights seem to be everywhere. They manage to catch our attention even captivate us by blinking in patterns. They can even blink in several patterns where the patterns can be changed by just a push of a  button. Now when I see those lights, the engineer in me tries to decipher how they work. Using my knowledge on basic electricals, I try to design a blinking light circuit but with no avail. 

 

A little bit of poking around & online research gave me answers that I was looking for. Let's start with the basics. The lights are usually made of 2.5V incandescent bulbs. But how can we plug a 2.5V bulb to a standard 240V supply? 240/2.5=96, Hence 96 bulbs  are connected in series to form a bunch of lights(Voltage adds up in series). If more bulbs are required, then another bunch of 96 lights are connected in parallel with the original bunch. So, if you purchase a Christmas light bunch, the voltage rating of each bulb can be calculated by dividing supply voltage by no. of bulbs in an individual loop . Eg: 240/96 = 2.5V. Before you do this ensure that you're able identify number of parallel loops. Typically the no. of bulbs in each loop is rounded off to 100. 

 

 

Now coming to the blinking. Let's consider a simple pattern where a bunch consisting of four loops where each loop goes on & off periodically. Each loop has a blinker bulb whose filament contains a bi-metallic strip. In simple layman terms, a bi-metallic strip can be said to bend in one direction on heating & in the opposite direction on cooling. Google would do a better job on explaining the working of a bi-metallic strip than me. This strip in the filament closes the circuit when cold & opens it when hot. This opening & closing of loops leads to the 'blinking' pattern of Christmas lights. 

 

 

And about the more sophisticated patterns. In this case the loops are interleaved than being connected one after another(Consider 4 loops). The controller box consists of integrated circuit having four triacs/transistors - One to control each loop. The IC simply turns on a triac to light one of the four loops. By sequencing the triacs appropriately, we can create all sorts of patterns. More on this here.