The circuit makes use of inexpensive TTL (Transistor-Transistor-Logic) chips to produce a pseudo random sequence of on-off states for the LED. The circuit is actually called a 127-state pseudo-random sequence generator, typically used to generate "white noise" or random numbers. A 555 Timer Chip is used as a self-oscillating clock circuit which "cycles" a 74164 8-bit shift register chip. The 6th and 7th register outputs are combined using a 7486 Exclusive-OR logic chip and the results fed back into the 1st register. This results in a continuing cycling of the shift register output so that 127 different states are repeated. In other words, the shift register output states are not repeated until 127 "clicks" of the clock. Sampling the output of any register will yield an apparently random length of on-off cycles. This is used to drive a LED, lighting the LED when the output is a 0 and turning the LED off when the output is 1. Since the 555 timer circuit is set to run at about 90 hertz (cycles/sec), the output sequence would repeat at about every 1.5 seconds (frequency of 1/127 of 90). This makes the LED on/off cycling appear random.