Atari 2600-like LFSR Random Noise [555 Timer Contest]

555/LFSR Random Noise Schematic

This project was conceived and completed over the course of one or two days, so it is not intended to be anything outstanding or innovative. Building on the concept of the Atari Punk Console, this circuit uses a 555 timer (and two other ICs) to generate random noise. When the random signal is played through a speaker, a distinctly “Atari 2600” sound is produced. This is not surprising, since the Atari 2600’s TIA chip allowed programmers to incorporate random noise into their games.

The circuit consists of a astable 555 timer circuit that produces a square wave to clock a shift register. The shift register, an SN74HC164, is configured as a 7-bit Linear Feedback Shift Register (LFSR), which produces a stream of pseudo-random bits. In order to act as an LFSR, the input to the shift register is the result of the 6th and 7th bits XORed* together. The feedback creates a repeating pseudorandom series of 127 states. In order to create audible random noise, the final bit of the shift register is coupled to a speaker through a 100uF capacitor. (*The XOR gate comes from an SN7486 Quad-XOR chip).

The frequency of the 555 timer circuit can be adjusted using a potentiometer, so the result is an amusing Atari-like sound effect.

555/LFSR Random Noise Schematic

LFSR random noise circuit, from the side

LFSR random noise circuit, from above

Pseudo-random pulse train produced by 7-bit LFSR

Square wave produced by 555 timer.

References
http://en.wikipedia.org/wiki/Linear_feedback_shift_register
http://www.kpsec.freeuk.com/555timer.htm

555/LFSR Random Noise Schematic