How do you make a machine unpredictable?
This is a good question because anyone who's worked with electronics knows that the devices are generally designed to produce precise intervals. There's more to dancing than just counting. 

This machine has a circle of 16 leds driven by a one-of-sixteen decoder, fed by a four bit binary up/down counter. When the counter is clocked, the lighted led appears to move around the circle in the direction determined by the state of the up/down line:
 

I wanted the led to appear to 'search' the perimeter of the jar, in an unpredictable, arhythmic, back-and-forth motion. What I needed was a  4-bit binary number that could be pre-loaded into the counter to give a visually random count each time the led reversed direction (count up 6, count down three,count up 14,...etc.). The answer was a circuit described in Don Lancaster's  CMOS Cookbook. It's a psuedorandom sequence generator:
This is simply a four-stage shift register with a data input that is an xor function of  the two most significant bits. Note that an all-zero state is disallowed.  It produces a sequence of fifteen numbers that are visually unpredictable, even though they repeat every fifteen clock cycles. I enhanced the effect by including an audio pickup and circuitry to change the starting point.

This was 1977, and my home made CP/M computer was running well enough to tempt me to test the feedback connections in software. The xor function in the above diagram looked like a parity check on those two bits, so I guessed that the following Z80 assembly language routine would generate some interesting sequences:

The routine was used in many loops over the next few days and became really rich when I found that any logical operation would produce weird sequences. It became richer yet when the routine was put into a loop that would print out the results in various ways.
This printout, for example, shows that a seed (hl) of 10000000b and a mask of 0000001b will produce a sequence that is eight bytes long before it begins to repeat. The whole printout is 256 lines and tests all combinations of seeds and masks to give a maximal length for each combination. Other printouts sorted the lines by maximal length and satisfied my need to know that I had a source for interesting visual sequences. It is important to remember that every configuration of the routine will have one disallowed state. 

This circuit is a hard-wired version of the routine. It continuously plots sequences, at various plotting speeds, changing the mask upon sensing a sound. In this case, an AND logical operation is used. click on it to zoom. 

If you've got it in you to wire up a 16x16 matrix of leds, the following circuitry will display its output.


I am aware that this is probably the re-invention of some wheel. If you have seen anything similar in the literature, please send me a note.