Here’s a little noise-making circuit I’ve been working on. It has a sound reminiscent of vintage sound chips, such as the AY-3-8910.
It works by using a D flip flop clocked by a 555 timer to sample and hold the signal from a ring oscillator. The high speed and frequency instability of the ring oscillator (close to 4 MHz) makes an ideal noise source, and the variable sampling frequency affects the frequency content of the noise.
This is a good example of the Nyquist theorem in action. As the clock rate of the 555 is lowered the high frequency content of the noise disappears. There is naturally a lot of aliasing, as this is a 1-bit signal, and there is no reconstruction filter on the output, but the frequency roll-off is still clearly audible.
CD4070 XOR Gate (IC1)
NE555 Timer (IC2)
CD4013 D Flip Flop (IC3)
LM386 Audio Amplifier (IC4)
100 kΩ Log. Potentiometer
10 nF (x2)