My ping pong ball computerHow does it work 
Computers use binary numbers when they calculate. In the binary number system we have only got the two symbols "0" and "1". A symbol in the binary number system can be represented mechanically by a tilting "arm" with two stable positions. The figures to the right show a tilting arm in each of the two positions. 


How large number can we represent by for example a 4 digit
binary number?

0000
0001 0002 0003 0004 0005 0006 0007 0008 0009 0010 0011 0012 .... 9998 9999 
0000 = 0
0001 = 1 0010 = 2 0011 = 3 0100 = 4 0101 = 5 0110 = 6 0111 = 7 1000 = 8 1001 = 9 1010 = 10 1011 = 11 1100 = 12 1101 = 13 1110 = 14 1111 = 15 
My calculator calculates with four digit numbers. Thus it can calculate with numbers between 0 and 15. To let the ping pong ball pass from digit to digit the digits are placed above one another instead of in a row. Theleast significant digit (right digit) is on the top.  1
0 1 0 = 5 
0
0 1 1 = 12 
A ping pong ball counterThe red lines show the possible trails the ball can follow between the digits in the counter. 

The counter shows:

One ball has passed through. The counter shows: '01' = 1 
Yet another ball. The counter shows: '10' = 2 
The last ball. The counter shows:: '11' = 3 
What happens if you add a ball when the counter shows
3? And still one more?
What happens if we add bore counter modules on top of each other? The answers to these questions illustrate a property of the way computers
usually calculate.
Calculation on a circular number line from 0 to 7 is called calculation
modulo
8.

A ping pong ball adder 

The
adder computes A + B = SUM. The two numbers to be added are first set one
in each coloumn by tilting the arms to the correct side. A is set to the
left as a binary number with the digits A2, A1, A0. B is set to the right
with the digits B2, B1, B0.
Add one ball on the top, and the sum modulo 8 can be read in the right coloumn. More numbers can be added to the sum by entering them in the left coloumn and adding balls on the top. 