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.