# A fast adder using minimum balls, and I've found a use for all those washers

#1

I finished the puzzle book. Now I’m back to making fast adders…

(5+7=12)

It adds the 3 bit purple register to the 4 bit blue register. Result goes in blue register.

Unlike my fast parallel adders, this is one ball at a time. The clever bit is that it uses as few balls as possible - normally 3 but it can finish early using only 1 or 2 for many inputs.

Also, it is only 4 parts wide :).

One challenge was having 3 independent pairs of gear bits with no gap. In the video I use an elastic band instead of a cog for the middle pair. Afterwards, I thought of a second way using only pieces in the box:

That’s three washers under the cog, raising it just enough to not interlock with gear bit above. The connected gear bits are raised in the same way.

#2

I’ve proved myself wrong - three isn’t the minimum number of balls. @nave figured out how to make an adder using only one ball posed the challenge in the new puzzles section. I managed to solve it: One ball adder - 4-bit + 4-bit